Category: Concepts & Explainers

Pump head is the total height, expressed in metres of water, that a pump can raise water against gravity and system resistance. It determines whether a pump can move water from a borehole to a rooftop tank, from a storage tank up a hill, or through a long pipe run with significant friction. Selecting a pump on flow rate alone — without checking head — is the most common cause of a pump underperforming once installed. A pump rated at 50 metres of head can lift water to 50 metres of elevation at near-zero flow, but delivers its rated flow at a lower effective head. Getting this relationship right is the whole job of pump selection.

The quick answer

Total Dynamic Head (TDH) is the sum of three components: static head (vertical lift), friction head (pipe resistance), and pressure head (required outlet pressure converted to metres of water).

TDH = Static head + Friction head + Pressure head

Component	Definition	Typical value
Static head	Vertical lift from pump to delivery point	5–40 m for most installations
Friction head	Resistance of pipes, fittings, valves	10–30% of static head
Pressure head	Required outlet pressure (1 bar = 10.2 m)	5–15 m for residential fixtures

Use the pump head pressure calculator to work out TDH for your specific installation, including pipe sizing, elevation change, and outlet pressure requirements.

How the calculation works

Worked example: a pump at ground level filling a rooftop tank on a 3-storey building, with the tank inlet 10 metres above grade. The pipe run is 25 metres of 25mm pipe with 4 elbows. Required delivery pressure is 0.5 bar.

1. Static head. The vertical lift from pump to tank inlet: 10 m.
2. Friction head from pipe run. Using Darcy-Weisbach, 25mm pipe at 15 L/min generates approximately 0.8 m of friction per 10 m of pipe. For 25 m of pipe: 2.0 m.
3. Friction head from fittings. Each 90° elbow adds roughly 0.6 m of equivalent pipe length. Four elbows: 2.4 m. Combined with the pipe run, total friction head is 4.4 m.
4. Pressure head. 0.5 bar × 10.2 = 5.1 m — see how pressure units convert to metres of head if you’re working from a different unit.
5. Total. TDH = 10 + 4.4 + 5.1 = 19.5 m.

A pump rated to 25 m head at 15 L/min would handle this comfortably. A pump rated to only 15 m head would fail to deliver adequate flow — not because of motor power alone, but because it can’t overcome the total resistance. The pump curve (a head-vs-flow graph supplied by the manufacturer) shows exactly how much flow the pump delivers at 19.5 m head.

Understanding the pump curve

Every centrifugal pump has a characteristic performance curve showing how head and flow interact. At zero flow, the pump reaches its maximum head (shut-off head); as flow increases, achievable head falls. The intersection of the pump curve with the system curve — TDH plotted across flow rates — is the operating point.

A pump running to the right of its best efficiency point (BEP) is working harder than designed, heating up, and wearing faster. One running far to the left is deadheading or close to it, generating heat without doing useful work. Calculating TDH correctly keeps the operating point within 80–110% of BEP flow rate.

For installations with variable demand — a farm tank filling overnight during low demand and delivering during high-demand irrigation, for instance — the system curve shifts. The pump horsepower and flow rate calculator helps confirm the motor is sized correctly for both conditions. If you’re weighing a pump against a gravity-fed alternative entirely, it’s worth reading through how pump systems compare to gravity feed before committing to either.

Key variables that change total dynamic head

Pipe diameter

Friction head is highly sensitive to pipe diameter — it scales roughly with the inverse fifth power of diameter (velocity falls with the square of diameter, and the equivalent-length term shrinks too). Doubling pipe diameter from 20mm to 40mm at the same flow rate cuts friction head by roughly 32 times, not just the 4× reduction in velocity alone would suggest. For long pipe runs, upsizing the pipe is almost always cheaper than buying a higher-head pump. As a rule of thumb, for runs over 30 metres, go one standard pipe size above the calculated minimum.

Number of fittings

Elbows, tees, gate valves, and check valves all add equivalent pipe length, and the difference between fitting types is large enough to matter in the total:

Fitting	Equivalent length
Ball valve	0.3 – 0.5 m
Check valve (non-return)	5 – 10 m
Globe or angle valve	10 – 20 m

A check valve is required in most pump installations, so budget for its equivalent length even in a system that otherwise looks simple. Account for every fitting in the calculation — it’s easy to under-count these on a short run and end up short on delivered flow.

Suction lift

Centrifugal pumps have a maximum practical suction lift of around 7–8 metres under ideal conditions (atmospheric pressure minus the vapour pressure of water). In practice, once you account for leaks, turbulence, and site elevation, 5–6 metres is the reliable limit. Exceeding it causes cavitation — vapour bubbles imploding rapidly enough to erode impellers and casings. For borehole depths beyond 6 metres, install a submersible pump instead of relying on suction lift.

Elevation above sea level

Atmospheric pressure drops with altitude, which reduces the net positive suction head available. At around 1,500 m above sea level, effective suction lift falls to roughly 4.5 metres; at 3,000 m, closer to 3 metres. This matters for installations in highland agricultural regions and mountain communities, where a suction-lift design that works at sea level can cavitate at altitude.

Common mistakes

Most pump underperformance traces back to one of a handful of selection errors — the same pattern that shows up when a tank is taking far longer than expected to refill.

Mistake	Consequence	Fix
Selecting a pump on flow rate alone	Delivers far less flow at real-world head than the spec sheet implies	Cross-reference required flow against TDH on the manufacturer’s pump curve
Ignoring friction on short pipe runs	A 10 m run of 20mm pipe at 20 L/min can still add ~4 m of head, rising to over 8 m at 30 L/min	Calculate friction head for every run, regardless of length
Reading max flow and max head as one spec	These are the two endpoints of the curve, not simultaneous performance	Match pump selection to the specific flow/head combination the system needs
Leaving no margin for expansion	Pump exactly meets today’s TDH and can’t absorb an added fixture, longer run, or age-related wear	Size for 120–130% of current TDH

Related calculators you might need

The water pressure calculator converts between pressure units and head so you can work consistently in metres of water throughout the TDH calculation. If you’re designing a gravity-fed system and comparing it to a pumped one, the gravity feed flow rate calculator shows how much flow a tank at a given height can deliver without a pump. For sizing the pipe diameter in the pump delivery line, the pipe size and flow rate calculator gives friction loss per metre for any pipe diameter and flow combination. Once the pump is selected and installed, the tank refill time calculator confirms how long it will take to fill the storage tank at the actual delivered flow rate.

Frequently asked questions

What does pump head mean in simple terms? Pump head is how high a pump can push water, measured in metres. A pump with 20 m of head can raise water 20 metres against gravity with no flow. In a real installation, the effective head available for lift is reduced by pipe friction and required outlet pressure. Total dynamic head (TDH) is the true measure of what a pump must overcome — the number to match against the pump’s performance curve.

How do I calculate total dynamic head for my pump? Add three components: static head (vertical height from pump inlet to delivery point), friction head (from pipe diameter, length, and fittings, using a friction loss table or formula), and pressure head (required outlet pressure converted to metres, where 1 bar = 10.2 m). Use the pump head pressure calculator for a step-by-step calculation without manual arithmetic.

What happens if my pump head is too low? If TDH exceeds the pump’s capacity at the required flow rate, it will deliver less flow than needed — or none at all if TDH exceeds shut-off head. The pump runs continuously, heats up, and eventually fails. Common symptoms include the pump running without water reaching the tank, the tank filling slowly or only partially, or pressure cutting out at the fixtures during high-demand periods even with a full tank.

Is more pump head always better? Not necessarily. Over-specifying head pushes the pump’s operating point to the left of its best efficiency point — low flow at unnecessarily high energy use, plus mechanical stress and the risk of pipe pressure exceeding fitting ratings. Match pump head to TDH within a 15–20% margin rather than buying the highest-head pump in the range.

Can I use a submersible pump to fill a rooftop tank? Yes — submersible pumps are commonly used in boreholes and underground cisterns to deliver water to elevated tanks. The head rating must account for the full depth of submergence plus the height of the rooftop tank above grade. A borehole 20 m deep feeding a tank 10 m above grade needs a pump rated to at least 30 m static head, plus friction and pressure head on top.

Hydrostatic pressure is the pressure that still water puts on any surface it touches, caused by the weight of the water above that point. In a tank it is zero at the water surface and greatest at the floor, and it rises by about 9.8 kPa (0.098 bar, 1.42 psi) for every metre of depth. At the bottom of a tank filled to 2 metres it is 19.6 kPa, whether the tank holds 500 litres or 50,000.

Those are small pressures. A car tyre holds ten times as much. What makes them matter in tank design is the area they act on: the same 19.6 kPa spread over a wall adds up to tonnes of force. This guide covers the formula, the forces on walls and floors, why round tanks cope better than flat-sided ones, and the three places where hydrostatic pressure causes real failures: unvented tanks, buried tanks and base fittings.

The hydrostatic pressure formula

P = ρ × g × h
P is pressure in pascals, ρ (rho) is the density of the liquid in kg/m³, g is gravity (9.81 m/s²) and h is the depth below the surface in metres.

Fresh water has a density of 1,000 kg/m³ at 4°C and about 998 kg/m³ at room temperature (USGS), so each metre of depth adds very nearly 9.8 kPa.

Example: a tank is filled to a depth of 2.5 m. Pressure at the floor = 1,000 × 9.81 × 2.5 = 24,525 Pa, which is 24.5 kPa, 0.245 bar or 3.56 psi. Halfway down, at 1.25 m, it is half that.

Divers know the same rule from the other end. In the sea, NOAA notes, “for every 33 feet (10.06 meters) you go down, the pressure increases by one atmosphere” (NOAA). A household tank a metre or two deep works in the first tenth or two of that.

This is gauge pressure, the amount above the surrounding air, which is what matters for tank walls because the air pushes equally on the outside. For the units, see water pressure explained in psi, bar and kPa.

Hydrostatic pressure at different tank depths

Water depth	kPa	bar	psi	Load on the base
0.5 m (1.6 ft)	4.9	0.049	0.71	500 kg/m²
1.0 m (3.3 ft)	9.8	0.098	1.42	1,000 kg/m²
1.5 m (4.9 ft)	14.7	0.147	2.13	1,500 kg/m²
2.0 m (6.6 ft)	19.6	0.196	2.84	2,000 kg/m²
2.5 m (8.2 ft)	24.5	0.245	3.56	2,500 kg/m²
3.0 m (9.8 ft)	29.4	0.294	4.27	3,000 kg/m²
4.0 m (13.1 ft)	39.2	0.392	5.69	4,000 kg/m²
5.0 m (16.4 ft)	49.0	0.490	7.11	5,000 kg/m²

If you know the volume of your tank but not its depth, the cylindrical and rectangular tank volume calculators work from the dimensions.

Why depth matters and volume does not

Pressure at a point depends only on the height of water straight above it. A narrow pipe filled to 2 m and a reservoir filled to 2 m press on their floors with exactly the same 19.6 kPa. Physicists call this the hydrostatic paradox, and it has three practical consequences for tanks:

• A bigger tank is not a higher-pressure tank. A 10,000 L tank filled to 1.5 m has less pressure at its floor (14.7 kPa) than a 500 L tank filled to 2 m (19.6 kPa).
• The load on the floor follows the depth. Each metre of water puts one tonne on every square metre beneath it. A tank 1.5 m deep loads its footprint at 1,500 kg/m², large or small. That is why a rooftop tank is a structural question: see will my roof hold a full water tank and the Rooftop Load Bearing Calculator.
• The same formula gives the pressure at your taps. There, h is the height from the water surface down to the tap. A tap 3 m below the surface gets 0.29 bar. How gravity feed water systems work covers that side.

From pressure to force: what the walls feel

Pressure is force per unit of area, so the force on a wall is the pressure multiplied by the wall’s area. Because pressure rises steadily from zero at the surface to its maximum at the floor, the average over the whole wall is half the floor value. For a flat wall, that gives:

Force on a flat wall, per metre of width = ½ × ρ × g × h²

Water depth	Pressure at the floor	Total push on each metre of wall width
1.0 m	9.8 kPa	4.9 kN (about 500 kg)
1.5 m	14.7 kPa	11.0 kN (about 1,125 kg)
2.0 m	19.6 kPa	19.6 kN (about 2,000 kg)
3.0 m	29.4 kPa	44.1 kN (about 4,500 kg)

Two things stand out. The force grows with the square of the depth: going from 1 m to 2 m doubles the pressure but quadruples the push. And the push is not centred. It acts one-third of the way up from the floor, because that is where most of the pressure is. The lower part of any tank wall does most of the work, and it is where bulges, cracks and leaks start.

Why round tanks handle pressure better than flat-sided ones

A flat wall resists water by bending, like a shelf under a load. A curved wall resists it by stretching, like the skin of a balloon, and a thin wall is far stronger when stretched than when bent. In a cylinder the stretch runs around the circumference and is called hoop stress:

Hoop stress = pressure × radius ÷ wall thickness

This is the standard thin-walled cylinder formula (Roylance, MIT). Since pressure depends on depth, the load in the wall depends on depth and on radius together:

Tank diameter	Water depth	Pull in the wall at the base, per metre of height
2 m	2 m	19.6 kN (about 2,000 kg)
4 m	2 m	39.2 kN (about 4,000 kg)
4 m	3 m	58.8 kN (about 6,000 kg)
8 m	3 m	117.7 kN (about 12,000 kg)

Double the diameter at the same depth and the wall has to carry twice the pull. This is why large tanks need thicker walls, more steel or more reinforcement even when they are shallow, and why the wall of a well-designed tank is strongest near the bottom.

Flat-sided tanks need help that round ones do not: thicker walls, moulded ribs, internal tie rods or an external frame. Round vs rectangular water tanks compares the two shapes on cost, space and cleaning.

How each tank material deals with it

Material	How it carries the load	Where it goes wrong
Polyethylene	Flexible wall in hoop tension, made thicker in the lower third to match the pressure	Slow bulging under constant load, worse when warm; damage from a base that does not support the whole floor
Fibreglass (GRP)	Stiff shell with glass fibres carrying the tension	Cracking from impact or from high points in an uneven base
Concrete	Strong when squeezed, weak when pulled, so steel reinforcement carries the hoop tension	Too little steel, or cracks that let water reach the steel and corrode it
Steel	Thin wall carries tension well; corrugations add stiffness	Joints, bolts and seals between panels; corrosion thinning the wall

Polyethylene shows the principle most clearly. Tank maker Poly Processing explains that the standard wall profile is thicker in the lower third and thinner near the top, which “matches the pressure distribution” inside (Poly Processing). The plastic also creeps: India’s standard for moulded water tanks tests for it by filling a tank for three days and allowing the circumference to grow by no more than 2% (IS 12701). A slight barrel shape in a full poly tank is expected. A bulge that keeps growing, or one low on a single side, is not.

Denser liquids raise the pressure. The formula includes density, so brine, liquid fertiliser or any liquid heavier than water presses harder at the same depth. Chemical tanks are sold with a specific gravity rating for this reason. A tank made for water should hold water.

For how the materials compare on lifespan and cost, see polyethylene vs fibreglass water tanks and plastic vs steel vs concrete water tanks.

A storage tank is not a pressure vessel

Everything above assumes the water surface inside the tank is open to the air. Storage tanks are designed on that assumption. Polyethylene tanks are rated for atmospheric pressure, not for internal pressure, and the American standard for fibreglass water tanks covers only vented tanks at atmospheric pressure. The lid and roof of a tank are built to keep dirt out, not to hold pressure in.

The danger is that hydrostatic pressure follows the highest water level connected to the tank, not the level inside it. If water can stand in a pipe above the tank, the whole tank feels that extra height:

Example: a sealed tank is fed from another tank 3 m higher, and its float valve sticks open. Water backs up the feed pipe and the tank is now under 3 m of extra head, which is 0.29 bar on every surface, including the roof. On a flat tank roof 1.5 m across, that is an upward force of more than 5 tonnes on a panel designed for none.

Three rules prevent it:

• Keep the vent clear. Air must leave as the tank fills and enter as it drains. A blocked vent lets a fast fill pressurise the tank and a fast drain pull the walls inward.
• Keep the overflow clear and at least as large as the inlet. It is the relief path if the float valve fails. Water tank overflowing covers the causes.
• Never extend a standpipe above a tank to gain pressure. It raises the pressure in the tank itself. To get more pressure at the taps, raise the whole tank or add a pump.

A vessel that is meant to hold pressure is a different product. See what a pressure tank is and how it differs from a storage tank.

Underground tanks: pressure from the outside

Bury a tank and the problem turns around. Soil presses inward on the walls, and if groundwater rises around the tank, hydrostatic pressure acts on the outside as well. A full tank balances it. An empty one does not, and two things can happen: the walls can buckle inward, or the whole tank can float.

Flotation follows Archimedes: “The buoyant force is equal to the weight of the displaced water” (Lentz, Septic Tank Buoyancy Control 101). That is about 1 kg of lift for every litre of tank below the water table.

Example: an empty 5,000 L tank sits fully below the water table after heavy rain. Uplift is about 5,000 kg. The same guide calls for a safety factor of at least 1.5, so the soil cover, anchors and tank weight together must hold down 7,500 kg. A plastic shell weighing a couple of hundred kilograms contributes almost nothing.

Makers deal with this in their installation rules. Norwesco tells installers to fill its polyethylene cisterns with water as the backfill rises, to leave them a quarter full afterwards, and not to install them in a high water table at all (Norwesco installation guide). Fibreglass tanks for wet ground are strapped to concrete anchors. Concrete tanks are heavy, which helps, but an empty one can still float.

So the rule for a buried tank is the opposite of the rule for a rooftop one: the risky moment is when it is empty. Do not pump one out in wet ground without checking the maker’s instructions. Overhead vs underground water tanks covers the wider choice, and the Underground Water Tank Volume Calculator gives the capacity of a buried tank from its dimensions.

Outlets and fittings at the base

The outlet is usually near the floor, where pressure is highest. Even so, pressure rating is almost never the problem. The floor of a 2.5 m tank sees 0.245 bar, and ordinary plumbing fittings are rated for many times that. Base fittings fail for other reasons:

• The seal. On plastic tanks the fitting passes through the wall and seals with a gasket. The Texas Manual on Rainwater Harvesting warns that such bulkhead fittings “might be subject to leakage” (TWDB). The lowest fitting is under pressure all day, every day, so a poor seal there weeps first.
• Rigid pipework. A plastic tank wall moves a little as the tank fills and empties. A rigid pipe fixed to the outlet turns that movement into stress on the fitting. Bushmans voids its guarantee unless a flexible hose is fitted at the outlet (Bushmans).
• Unsupported weight. A heavy valve or a long pipe hanging from the outlet levers on the wall. Support the pipe on its own bracket.
• New holes. Cutting an extra outlet removes wall material exactly where the load is highest. Use the flat mounting pads the maker moulded in, and ask the maker before cutting anywhere else.

Mistakes to avoid

• Thinking a bigger tank means more pressure. Only a deeper one does.
• Setting a tank on an uneven base. The floor carries a tonne per square metre for each metre of depth. Stones, steps or an undersized slab concentrate that load. IS 12701 says the base should be “fully supported over its whole bottom area”.
• Blocking the vent or sealing the lid airtight. A storage tank needs to breathe.
• Extending a pipe above the tank to gain head. It pressurises a tank that was never built for it.
• Emptying a buried tank in wet ground. That is when it can float or collapse.
• Storing something denser than water in a water tank. Pressure rises in proportion to density.
• Bolting rigid pipe straight to a plastic tank outlet. Use a flexible connection.

More calculators for this topic

• Water Column Pressure Calculator: pressure from any height of water, in psi, bar, kPa and head.
• Water Pressure Calculator: supply pressure at a tap from the heights of the tank and the outlet.
• Water Tank Weight Calculator: the full weight of a tank, shell included.
• Tank Weight per Square Foot Calculator: the load on the base in imperial units.
• Pump Head Pressure Calculator: the head a pump must overcome, explained in how pump head works.

Frequently asked questions

How do you calculate hydrostatic pressure?

Multiply the liquid’s density by gravity and by the depth: P = ρ × g × h. For water, that is 9.8 kPa for each metre of depth, or 0.433 psi for each foot. Water 3 m deep gives 29.4 kPa at the bottom, and water 10 ft deep gives 4.33 psi.

Does a bigger tank mean more hydrostatic pressure on the walls?

Not more pressure, unless it is also deeper. Pressure depends on depth alone. What a bigger tank does increase is the total force: the same pressure acts on more wall, and in a round tank the pull in the wall rises in proportion to the diameter. That is why large tanks need stronger walls even when they are shallow.

What is the hydrostatic pressure at the bottom of a 1,000-litre tank?

It depends on how tall the tank is, not on the 1,000 litres. A squat tank filled to 0.9 m has 8.8 kPa (0.088 bar, 1.28 psi) at the floor. A tall one filled to 1.5 m has 14.7 kPa (0.147 bar, 2.13 psi). Measure the water depth and multiply by 9.8 for kPa.

Does the shape of the tank change the pressure?

No. At a given depth the pressure is the same in a round tank, a square tank or a tapered one. Shape changes how well the walls carry that pressure: curved walls work in tension, and flat walls have to resist bending, so flat-sided tanks need ribs, ties or thicker walls.

Can hydrostatic pressure crack a concrete tank?

Yes, if the wall has too little reinforcement. Water pressure puts a round concrete wall into tension, and concrete is weak in tension, so the steel inside has to carry it. Once a crack lets water reach the steel, corrosion widens it. Regular inspection catches this early. Illinois EPA, for example, says public water storage tanks should be inspected “at least every five years” (Illinois EPA).

Does hydrostatic pressure change when the water is moving?

Strictly, hydrostatic pressure describes water at rest. When water flows, part of the pressure is converted into motion and part is lost to friction, so the pressure in a flowing pipe is lower than the static figure. For a tank wall the worst case is the simple one: full and still. Tanks are designed for that.

How do I know if my tank can take the pressure?

A tank is designed for its own full depth of water, open to the air, on a flat and fully supported base. Used that way, the pressure is what it was built for. You move outside the design when you add height above the tank, seal it, store a denser liquid, bury a tank not made for burial or set it on an uneven base. For anything unusual, ask the manufacturer in writing.

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Sources

• US Geological Survey, Water Density. Density of water by temperature.
• NOAA National Ocean Service, How does pressure change with ocean depth? One atmosphere for every 33 feet (10.06 m).
• Roylance D, Pressure Vessels, Massachusetts Institute of Technology, 2001. Hoop stress in a thin-walled cylinder: pressure × radius ÷ wall thickness.
• Poly Processing, How to make sure your tank’s walls are thick enough and the importance of venting. ASTM D1998 wall profile, specific gravity ratings, atmospheric pressure rating.
• Bureau of Indian Standards, IS 12701:1996, Rotational Moulded Polyethylene Water Storage Tanks. Deformation test and base support.
• American Water Works Association, AWWA D120, Thermosetting Fiberglass-Reinforced Plastic Tanks. Scope limited to vented tanks at atmospheric pressure.
• Lentz D, Septic Tank Buoyancy Control 101, 2019. Buoyant force and the 1.5 safety factor.
• Norwesco, underground tank installation guide. Filling during backfill; no installation in a high water table.
• Texas Water Development Board, The Texas Manual on Rainwater Harvesting, 3rd edition. Bulkhead fittings on plastic tanks.
• Bushmans, warranty terms. Flexible hose at the outlet; sound base.
• Illinois EPA, Storage Tanks. Inspection at least every five years.

The runoff coefficient is the fraction of rainfall that actually reaches your tank after losses from absorption, evaporation, splashing, and wetting of the catchment surface. It’s expressed as a decimal between 0 and 1: a coefficient of 0.85 means 85% of the rain falling on your roof ends up as collectible water, and the remaining 15% never makes it to the tank.

Every step of a rainwater harvesting system depends on getting this number right. Once you have it, the annual rainwater collection calculator combines it with roof area and local rainfall to give a realistic annual yield figure.

The Calculation: How Runoff Coefficient Is Applied

The core formula is: Collectible volume = Rainfall depth × Catchment area × Runoff coefficient. For metric units: 50 mm of rain on a 100 m² roof with a coefficient of 0.85 gives 0.050 m × 100 m² × 0.85 = 4.25 m³ (4,250 litres). Skip the coefficient and you’d overestimate yield by 15% — which means a tank sized to run dry more often than the model predicts.

The coefficient isn’t a fixed property of the roof material alone. It shifts with rainfall intensity, roof condition, how dry the surface was beforehand, and slope. A light drizzle on a dry tile roof loses more to initial wetting and evaporation than a heavy downpour on the same roof. The long-run average coefficients used for annual sizing already blend this variation across event types.

Runoff Coefficients by Roof Material

Roof Material	Typical Runoff Coefficient	Notes
Metal roofing (Colorbond, Zincalume, tin)	0.90 – 0.95	Best performer; minimal absorption
Glazed ceramic tiles	0.85 – 0.90	Good; some absorption at joints
Concrete tiles	0.75 – 0.85	Absorbs water when dry; lower initial coefficient
Fibreglass / polycarbonate	0.90 – 0.95	Similar to metal
Painted or sealed concrete	0.70 – 0.80	Depends on surface condition and age
Asphalt shingles	0.70 – 0.80	Absorbs and leaches more than metal
Green / living roof	0.10 – 0.30	High absorption by growing medium
Gravel-topped flat roof	0.40 – 0.60	High losses to absorption and evaporation

These ranges are long-run averages. If you’re sizing a tank conservatively — where running out is the thing to avoid — use the lower bound for your roof material. If you’re estimating maximum potential yield, use the upper bound.

Factors That Adjust the Coefficient

Roof slope

Steeper roofs shed water faster, cutting down evaporation time during and after rain. A roof pitched at 30° or more performs closer to the upper end of its material’s coefficient range. Flat roofs (under 5° slope) hold water longer and lose more to evaporation, pulling performance toward the lower end.

Roof condition and age

Cracked, porous, or moss-covered roofing absorbs substantially more water than the same material in good condition. A 20-year-old concrete tile roof may perform 10 to 15 percentage points below a new one, and lichen or moss on any surface can knock 0.05 to 0.15 off the effective coefficient.

First-flush losses

If your system includes a first-flush diverter, the volume it diverts reduces effective yield and is sometimes folded into an adjusted coefficient. A diverter losing 4 litres per event across 80 annual events works out to 320 litres of annual loss — subtract this from gross yield when sizing storage.

Gutter and downpipe losses

Water left in gutters after rain, overflow during high-intensity events, and evaporation from hot metal gutters between events all chip away at effective yield. General industry guidance in Australia applies a system efficiency factor of 0.85 to 0.90 on top of the roof coefficient to account for these distribution losses — worth checking if you’re comparing figures against a rainwater harvesting setup in Australia, where this kind of adjustment is common practice.

Worked Example: Sizing a Tank Using Runoff Coefficient

A homeowner in a semi-arid region has a 120 m² metal roof (coefficient 0.90) and 450 mm average annual rainfall. Annual collectible volume = 0.450 m × 120 m² × 0.90 = 48.6 m³ (48,600 litres). Applying a 0.87 system efficiency factor for gutter losses brings that down to roughly 42,300 litres of usable annual yield.

That figure feeds directly into tank sizing — the rainwater harvesting calculator matches it against household demand and seasonal rainfall distribution to give a minimum tank size that avoids running dry. From there it’s worth checking whether that yield actually beats buying from municipal supply once installation and maintenance costs are factored in.

Common Mistakes

1. Using a coefficient of 1.0, or ignoring it entirely, for metal roofs. Even the best metal roofing loses 5 to 10% to wetting, evaporation, and splashback. Assume 100% collection and you’ll consistently overestimate yield. Use 0.90 to 0.95 as the realistic range.
2. Using one coefficient year-round in climates with sharp wet and dry seasons. A dry-season roof is hotter and drier, so initial wetting losses and evaporation are higher for every event. An annual average of 0.80 can mask a dry-season effective coefficient of 0.65 and a wet-season figure of 0.88. If you’re sizing for dry-season survival, use dry-season numbers for that calculation.
3. Applying the coefficient to roof surface area instead of catchment area. Catchment area is the horizontal projection of the roof, not its surface area — rainfall is measured vertically, so a steeply pitched roof covers more surface than footprint. The roof catchment area calculator converts roof dimensions to the correct horizontal figure.
4. Applying a textbook coefficient to a roof that hasn’t been inspected. A roof listed as concrete tile gets the standard concrete-tile coefficient in most calculations — but if it’s covered in moss with cracked or missing tiles, actual performance can run 20 points below that figure. Inspect the roof before using any table value in a sizing calculation.

Related Calculators You Might Need

Once you have your runoff coefficient and roof area, the annual rainwater collection calculator is the natural next step — it applies your coefficient to local rainfall data to give an annual yield. From yield, the rainwater harvesting calculator sizes the storage tank to your demand profile. If your system includes a first-flush diverter and you want to factor its volume loss into yield, the first flush diverter size calculator gives the diverter capacity, which you can subtract from gross yield. Finally, the rainwater savings calculator translates your adjusted yield into annual cost savings against mains supply.

Frequently Asked Questions

What runoff coefficient should I use for my roof?

Start with the material type from the table above, then adjust for slope, age, and condition. For a clean metal roof at moderate pitch, 0.90 is a reliable working figure. For concrete tiles in average condition, 0.80. If you’re unsure of your roof’s condition, use the lower end of the range — it produces a more conservative, safer tank size.

Does the runoff coefficient change with rainfall intensity?

Yes. During heavy rainfall (over 25 mm/hr), coefficients approach the upper limit of the range — there’s little time for evaporation and surface absorption is overwhelmed by volume. During light rain (under 5 mm/hr), initial wetting losses are proportionally larger and coefficients drop toward the lower end. Annual yield calculations using average rainfall implicitly blend both, which is why the long-run average coefficient is the right value for sizing.

How do I measure my actual runoff coefficient?

Know your roof area precisely, measure rainfall with a gauge, measure how much actually enters the tank, then divide tank inflow by (rainfall × roof area). Do this across 10 or more events of varying intensity and average the results. It’s more work than reading a table, but it accounts for your specific roof, gutters, and downpipes in a way no standard figure can.

Does roof colour affect the runoff coefficient?

Colour affects surface temperature, not porosity. A dark roof in full sun evaporates water faster during and right after light rain, which can shave 0.02 to 0.05 off the effective coefficient compared to a light-coloured roof of the same material. For heavy rainfall events the effect is negligible. It’s worth accounting for in hot climates with frequent light drizzle; elsewhere it’s not a meaningful factor.

Can I increase my runoff coefficient?

The main levers are roof material and condition. Replacing cracked concrete tiles with metal roofing typically raises the effective coefficient by 0.10 to 0.20. Cleaning moss, lichen, and debris off any roof helps noticeably, and improving gutter capacity to prevent overflow during heavy rain retains water that would otherwise be lost. None of this changes how the roof catchment area calculator computes your catchment size, but it directly increases the yield that coefficient delivers.

Total Dissolved Solids (TDS) is the combined concentration of all inorganic and organic substances dissolved in water — measured in milligrams per litre (mg/L) or parts per million (ppm), where 1 mg/L = 1 ppm. It includes calcium, magnesium, sodium, potassium, carbonates, chlorides, sulfates, and trace amounts of metals and organics. TDS does not measure bacteria or suspended particles — it specifically captures what is chemically dissolved and invisible to the naked eye.

Use the TDS water calculator to assess your stored water quality against WHO and national drinking standards.

What TDS Levels Mean in Practice

TDS Level (ppm)	Classification	Taste Profile	Suitable For
< 50	Very low	Flat, bland	Not recommended for prolonged drinking
50–150	Low	Clean, slightly bland	Drinking, cooking, RO permeate
150–300	Acceptable	Neutral to pleasant	Ideal for drinking
300–600	Moderate	Slightly mineral	Acceptable for drinking
600–900	High	Noticeably mineral	Marginal for drinking; taste complaints common
900–1,200	Very high	Salty or metallic	Not recommended for drinking
> 1,200	Unsafe	Strongly saline	Unsuitable for human consumption

WHO does not set a single enforceable TDS limit; instead it classifies palatability, with water rated “good” up to roughly 600 ppm and quality declining noticeably above 300 ppm. Most national standards cap acceptable TDS between 500 and 1,000 ppm. India’s BIS standard sets 500 ppm as desirable and 2,000 ppm as the maximum permissible limit. The US EPA sets a secondary standard of 500 ppm (non-enforceable, taste-based). If stored water is developing an off-taste alongside a rising TDS reading, our guide on why water tastes or smells bad after storage walks through the likely causes.

Why TDS Changes During Tank Storage

Water sitting in a tank does not stay chemically static. Four mechanisms typically drive TDS up over time:

1. Evaporation. Open or poorly sealed tanks lose pure water vapour while dissolved solids stay behind, concentrating what remains. As a rough rule, a tank losing 5% of its volume to evaporation will see TDS rise by roughly a similar margin — the relationship isn’t perfectly linear, but it’s close enough to use as a planning estimate.
2. Material leaching. Freshly cured concrete tanks add calcium hydroxide and other alkalis, raising both TDS and pH. Corroding iron or galvanised steel tanks contribute iron and zinc ions. Plastic tanks are largely inert but can leach additives if they’re low-grade or UV-degraded.
3. Biological activity. In warm, light-exposed, or poorly treated tanks, algae and biofilm growth produce metabolic by-products that add to organic TDS.
4. Disinfection by-products. Residual chlorine reacts with organic matter to form by-products that also register as TDS. For the chemistry behind this, see how chlorination of water tanks actually works, and use the chlorine dosage calculator to dose accurately and avoid over-chlorination.

TDS and Storage Safety: What the Number Does Not Tell You

TDS is a proxy measure, not a safety guarantee. Low TDS water is not automatically safe — pathogens, pesticides, and industrial contaminants may be present at dangerous levels while TDS reads below 100 ppm. Conversely, high-TDS water from a mineral-rich aquifer may be microbiologically safe but taste unpleasant and cause scaling in appliances.

TDS testing should always be paired with pH testing and, where there is any doubt about the source, microbiological testing. The water pH adjustment calculator helps correct pH imbalances that often accompany abnormal TDS readings.

For long-term tank storage, TDS alone cannot confirm water is safe to drink after several weeks. Pair TDS readings with a residual chlorine check and a visual inspection — cloudiness, colour change, odour, or a layer of sediment at the bottom of the tank are all signs worth investigating on their own, regardless of what the TDS meter reads.

TDS Thresholds for Specific Uses

Application	Maximum TDS (ppm)	Notes
Drinking water (WHO)	600	Taste acceptable up to ~300 ppm
Baby formula preparation	< 100	Low mineral content required
Aquarium (freshwater)	< 500	Species-dependent
Hydroponics / irrigation	< 1,500	Depends on crop type
Livestock (cattle)	< 3,000	Short-term tolerance higher
Reverse osmosis membrane	< 2,000 feed	Higher TDS accelerates membrane fouling
Boiler/hot water system	< 200	Scaling risk above this level

Common Mistakes

Treating TDS as a pass/fail safety test

A TDS reading of 250 ppm tells you the total dissolved load but nothing about what’s dissolved. Arsenic contamination, nitrates, and E. coli are invisible to a TDS meter. Treat TDS as a screening tool and first-line quality indicator — not a definitive safety assessment.

Ignoring TDS rise in sealed tanks during extended storage

Even sealed tanks concentrate dissolved solids slowly over weeks, through minor evaporation at vents and leaching from tank materials. Water stored longer than 30 days should be re-tested. The safe water storage duration calculator estimates how long treated water remains safe under your conditions.

Assuming very low TDS means superior quality

RO-purified or distilled water with near-zero TDS is demineralised. Drinking very low-TDS water over long periods can leach minerals from body tissue and is mildly corrosive to metal plumbing. WHO recommends a minimum of 100 ppm for regular drinking water.

Testing TDS once and assuming it stays constant

TDS shifts with season, supply-source variation, tank condition, and treatment. Test stored water at least quarterly, and again after any change to the supply source or tank.

Related Calculators You Might Need

TDS is one part of a broader water quality picture. If elevated TDS is coming from water hardness, the water hardness calculator will tell you whether scaling is a risk for your appliances and pipes — our guide on how water hardness affects your tank covers the mechanism in more depth. If the storage itself is the source of contamination, the water tank disinfection calculator guides you through a cleaning protocol. For water that requires UV treatment rather than chemical dosing, the UV disinfection tank calculator sizes the system correctly for your flow rate. And if you’re buying a filtration unit to reduce TDS, confirm it can handle your demand with the water filter flow rate calculator.

Frequently Asked Questions

What is a good TDS level for drinking water?

Between 150 and 300 ppm is considered ideal — enough minerals for taste and health, below the threshold where most people notice off-flavours. The 300–600 ppm range is acceptable but may taste noticeably mineral. Above 600 ppm, palatability drops sharply. Below 50 ppm is demineralised and not recommended for regular long-term consumption.

Can high TDS make you sick?

High TDS alone is not acutely toxic for most people — it depends entirely on what’s causing the reading. Elevated TDS from calcium and magnesium is generally harmless. TDS elevated by nitrates, arsenic, heavy metals, or industrial contaminants is a direct health hazard at much lower concentrations. A TDS meter can’t tell these apart; lab analysis is needed to identify specific ions.

How do I reduce TDS in my storage tank?

Treat or replace the water — there’s no shortcut. Reverse osmosis reduces TDS by 90–98%. Distillation removes nearly all dissolved solids. Ion exchange softeners reduce hardness-related TDS. Boiling does not reduce TDS — it concentrates it by driving off pure water as steam. If TDS is rising because the tank itself is leaching, the surface needs cleaning or lining.

Does chlorination affect TDS?

Yes. Chlorine or bleach introduces sodium, chloride ions, and disinfection by-products, all of which raise TDS slightly. A standard shock dose for tank disinfection typically adds 5 to 15 ppm — negligible for most applications, but dosing accuracy still matters. The bleach to water ratio calculator prevents over-dosing that would unnecessarily spike TDS and create off-flavours.

Why does my TDS reading spike after heavy rainfall fills the tank?

Roof runoff carries atmospheric dust, bird droppings, leaf decomposition products, and surface contaminants, all of which dissolve into the collected water and raise TDS. First-flush events — the first few minutes of rainfall — carry the highest contamination load. A first-flush diverter removes the most contaminated initial runoff before it reaches the tank, protecting baseline TDS. After major rain events, re-test TDS before using the stored water for drinking.

Chlorination kills or inactivates bacteria, viruses, and protozoa by oxidising their cell membranes and disrupting metabolic processes. For stored water, the effective dose is 2–5 mg/L of free chlorine (also expressed as 2–5 ppm), held in contact for a minimum of 30 minutes before the water is used. The key variables — tank volume, initial contamination level, pH, and water temperature — all affect how much chlorine you need and whether a residual remains after treatment. This article explains the chemistry, the dose calculation, and the practical steps for disinfecting a storage tank. If you’re weighing chlorination against other disinfection methods, our guide on chlorine vs UV vs filtration covers the trade-offs.

The quick answer

To disinfect a tank, you need to know its volume and whether you are doing routine maintenance or treating a contaminated tank. WHO guidelines recommend 0.5 mg/L free chlorine residual after 30 minutes of contact time for routine disinfection. For a contaminated or newly installed tank, dose to achieve 2 mg/L residual after contact.

Scenario	Target residual	Sodium hypochlorite (5%)	Contact time
Routine maintenance	0.5 mg/L	10 mL per 1,000 L	30 min
Post-contamination shock	2 mg/L	40 mL per 1,000 L	30–60 min
Algae or heavy turbidity	5 mg/L	100 mL per 1,000 L	60 min minimum

Use the chlorine dosage calculator to compute the exact amount of chlorine product required for your tank volume and target residual.

How the calculation works

The dose formula is: Chlorine product volume = (Tank volume in litres × Target dose in mg/L) ÷ (Product concentration % × 10,000). For a 5,000-litre tank dosed to 2 mg/L using 5% sodium hypochlorite (household bleach):

Volume = (5,000 × 2) ÷ (5 × 10,000) = 10,000 ÷ 50,000 = 0.2 litres = 200 mL

This is the amount of bleach to add, not the amount of active chlorine. Bleach concentration varies by brand — always check the label. Granular calcium hypochlorite (65–70% available chlorine) requires a very different quantity: for the same scenario, approximately 15 grams for a 5,000-litre tank at 2 mg/L. The bleach to water ratio calculator handles both liquid and granular forms.

The chemistry of chlorine disinfection

When sodium hypochlorite (NaOCl) dissolves in water, it produces hypochlorous acid (HOCl) and hypochlorite ion (OCl-). HOCl is the active disinfecting species — it is substantially more effective than OCl-. The ratio between the two depends on pH. At pH 7.5, roughly 50% of free chlorine is in the HOCl form. At pH 8.5, only about 10% is HOCl — meaning at high pH you need significantly more chlorine to achieve the same kill rate (EPA Guidance Manual for Compliance, 1999). For the full mechanism behind why this matters, see how chlorination of water tanks actually works.

Temperature also matters. Chlorine reacts faster at higher temperatures but dissipates more quickly. At 25°C, a well-dosed tank may lose half its free chlorine residual within 48–72 hours through oxidation of organic matter, UV degradation (in uncovered tanks), and off-gassing. An enclosed, dark tank at moderate temperature retains residual for considerably longer. The safe water storage duration calculator models this decay based on your storage conditions.

Key variables that change the required dose

Water pH. As described above, high pH (above 8.0) dramatically reduces chlorine effectiveness. If your source water is alkaline — common with borehole or hard water supplies — test pH before chlorinating and consider acidifying to pH 6.5–7.5 with citric acid or CO₂ injection before adding chlorine. Use the water pH adjustment calculator to determine the adjustment dose.

Turbidity and organic load. Chlorine reacts with organic matter — soil particles, algae, biofilm — to form disinfection by-products (DBPs) and is consumed in the process without providing any disinfection. WHO recommends water be clarified to below 1 NTU before chlorination. A visibly turbid tank may consume 5–10 mg/L of chlorine just in DBP formation before any residual remains. Always settle and filter heavily turbid water before chlorinating.

Tank condition and biofilm. A tank that has never been cleaned accumulates biofilm — a bacterial community embedded in a protective polysaccharide matrix. Chlorine at normal doses does not penetrate mature biofilm effectively. Tanks must be drained, physically scrubbed, and rinsed before chemical disinfection. Shock chlorination alone of a fouled tank produces a false sense of safety. Sediment buildup is often a sign this hasn’t been done recently — see why is there sediment at the bottom of my water tank.

Chlorine product age and storage. Liquid bleach degrades at roughly 20% per month at room temperature. A bottle stored for 6 months at 30°C may have lost 50–60% of its stated concentration. Always check the manufacture date and factor in degradation — or use fresh product. Granular calcium hypochlorite is more stable if kept dry and sealed, retaining potency for 2–5 years.

Common mistakes

Dosing by tank size alone without accounting for concentration. Adding “one capful” of bleach to a 10,000-litre tank based on a rough estimate delivers an unknown dose. Calculate using the exact product concentration and target residual. Under-dosing leaves pathogens alive. Over-dosing (above 5 mg/L free chlorine) creates taste and odour problems and may form excessive trihalomethanes.

Not testing for residual after contact time. The dose calculation gives you the starting concentration, not the end result. Chlorine demand — the amount consumed by organic matter and reactions in the water — reduces the residual. After 30 minutes, test with a DPD test kit or strip to confirm at least 0.2 mg/L free chlorine remains. If residual is zero, re-dose and wait. If it never holds residual, the tank requires physical cleaning first.

Chlorinating without isolating the tank. If the supply line feeds directly into the distribution system, chlorinating the tank without isolation will push high-chlorine water into pipes serving sinks and appliances. Isolate the tank outlet valve before dosing. Only reconnect after testing confirms the residual has dropped to below 0.5 mg/L — or flush the tank before reconnecting.

Ignoring pH when treating borehole water. Borehole water commonly has pH 7.8–8.5 and high hardness. At these pH levels, chlorination is inefficient — you may dose correctly and still leave viable pathogens because HOCl is too low a fraction of the free chlorine. Test and correct pH before dosing. This single step is the most commonly skipped in rural and peri-urban settings.

Related calculators you might need

If you are treating a contaminated tank, start with the water tank disinfection calculator, which walks through the full disinfection sequence including drain-down, rinsing, and re-dosing phases. For tanks where biofilm or iron bacteria are the problem, the potassium permanganate dosage calculator provides an alternative oxidative treatment — see potassium permanganate water treatment for how this method works. If you need to assess your water’s overall quality before deciding on a treatment approach, the TDS water calculator gives a baseline reading of dissolved solids. For long-term water quality monitoring, the water hardness calculator helps assess scaling risk that affects chlorine contact efficiency in pipes and fittings.

Frequently asked questions

How much bleach do I add to a 1,000-litre water tank? For routine disinfection using 5% sodium hypochlorite (household bleach), add approximately 10 mL (two teaspoons) per 1,000 litres. For shock disinfection after contamination, use 40 mL per 1,000 litres. Wait 30 minutes, then test with a chlorine test strip to confirm a residual of at least 0.2 mg/L before use. Adjust if your bleach concentration differs — check the label.

How long does chlorine last in a water tank? In a covered, dark, well-maintained tank with low organic load, a 0.5 mg/L residual typically falls below detectable levels within 3–5 days at 25°C. In an open or algae-affected tank, this may happen within hours. WHO recommends re-testing residual every 24–48 hours for stored drinking water and re-dosing if it drops below 0.2 mg/L. Use the safe water storage duration calculator to estimate based on your conditions.

Is it safe to drink water after chlorinating a tank? Yes, provided the free chlorine residual is between 0.2 and 0.5 mg/L at the point of consumption (WHO Guidelines for Drinking-water Quality, 2022). Above 5 mg/L, the water should not be consumed without dilution or further treatment. Always test with a DPD kit or test strip before resuming use. If residual exceeds 1 mg/L after contact time, flush the tank until it drops to an acceptable level.

Why does my chlorinated tank still smell bad? Persistent odour in a chlorinated tank usually indicates chloramines — compounds formed when chlorine reacts with ammonia or organic nitrogen. Chloramines are a sign of under-treatment (not enough chlorine to complete oxidation) or high organic load in the water. The fix is to shock-dose to breakpoint chlorination — typically 7–10 mg/L — which fully oxidises the nitrogen compounds. After breakpoint, the odour disappears and free chlorine residual is restored. Persistent bad taste or smell that doesn’t resolve this way may have a different cause — see why does my water taste or smell bad after storage.

Can I use swimming pool chlorine for drinking water tank treatment? Only if it is pure calcium hypochlorite without stabilisers. Many pool products contain cyanuric acid (a stabiliser) which is not approved for drinking water use. Look for food-grade or NSF-certified calcium hypochlorite at 65–70% available chlorine. Trichlor and dichlor tablets used in pools are not suitable for potable water treatment.