Author: WaterTank Guides Editorial Team

Water pressure is measured in three units depending on where you are: PSI (pounds per square inch) in the US, bar in Europe and Australia, and kPa (kilopascals) in countries using strict SI units. They all measure the same thing — the force that water exerts per unit area — but with different numeric scales. Knowing how to convert between them and what values mean in practice determines whether your gravity-fed tank will run a shower, whether your pump is sized correctly, and whether your pipe fittings are rated for the load.

The quick answer

The three units relate to each other as follows: 1 bar = 100 kPa = 14.5 PSI. Domestic water systems typically run at 2–4 bar (200–400 kPa / 29–58 PSI). A gravity-fed rooftop tank at 3 metres of head delivers roughly 0.3 bar (30 kPa / 4.3 PSI) — enough for a shower only if flow rate requirements are modest.

Use the water pressure calculator to convert between units and check whether your system pressure falls within safe operating ranges.

How the calculation works

Pressure from a static water column — which is what a gravity-fed tank produces — follows the hydrostatic formula: P = ρ × g × h, where P is pressure in pascals, ρ is water density (1,000 kg/m³ at 20°C), g is gravitational acceleration (9.81 m/s²), and h is the vertical height of the water column in metres.

Worked example: A rooftop tank with its base 5 metres above the shower head.

P = 1,000 × 9.81 × 5 = 49,050 Pa = 49.05 kPa = 0.49 bar = 7.1 PSI

Most showers require at least 1.0 bar (100 kPa / 14.5 PSI) for comfortable flow. At 0.49 bar, that installation will run a low-flow shower head only. Adding a booster pump or raising the tank to 10 metres would double the pressure to 0.98 bar — just below the 1.0 bar threshold. At 11 metres: 1.08 bar, which clears it. Use the minimum tank height for shower pressure calculator to find the exact height required for your fixture.

What these numbers mean in real systems

Municipal mains supply in the UK is typically 1.0–3.0 bar at the meter. Australia’s water utilities target 500 kPa (5 bar) at the boundary — well above what residential fixtures need. US systems run at 40–80 PSI (2.8–5.5 bar), with 60 PSI considered optimal. India’s urban mains often deliver below 1 bar for much of the day, which is why overhead tanks are standard.

Pressure reducers are required when mains pressure exceeds 5 bar (500 kPa / 72.5 PSI) — above this, standard fittings risk failure, and appliances like washing machines void their warranties. Pressure below 0.7 bar (70 kPa / 10 PSI) at the fixture is insufficient for most combi boilers and tankless water heaters, which will shut off on under-pressure protection.

For tank-fed systems, pressure at the outlet depends on both the height of the tank and the friction losses in the pipe run. Longer or narrower pipes reduce effective pressure. Use the gravity feed flow rate calculator to account for pipe friction and get the actual delivered flow rather than theoretical head pressure.

Converting between PSI, bar and kPa

The conversion factors are exact or near-exact:

The water column pressure calculator handles all unit conversions automatically and shows pressure at any depth or height in a water column.

Common mistakes

Confusing gauge pressure with absolute pressure. Pressure gauges in plumbing read gauge pressure — the pressure above atmospheric (101.3 kPa). If a fitting is rated to 600 kPa, that is gauge pressure. Absolute pressure is gauge + atmospheric. When specifying tank equipment, always confirm whether a rating is gauge (g) or absolute (a). Mixing them up can lead to choosing fittings that fail at operational pressure.

Treating head height as the full story. A tank 8 metres above a tap theoretically delivers 0.78 bar. But a 20-metre run of 15mm pipe with two elbows can cut that to 0.45 bar at full flow. Engineers account for friction losses using the Darcy-Weisbach equation or simplified tables. For most residential installations, pipe runs over 15 metres should be upsized to 25mm to preserve pressure.

Using PSI specifications on a bar-rated system. A pump rated to “60 PSI” is not a 60-bar pump — it is a 4.1 bar pump. Misreading this when selecting a booster pump for a high-pressure system is a common and expensive error. Double-check the unit on every specification sheet, especially on imported equipment where labels may be inconsistent.

Ignoring pressure variation through the day. Municipal pressure is not constant. It peaks during low-demand hours (early morning) and drops during peak usage. In parts of South Asia and Sub-Saharan Africa, daytime pressure can drop to zero even when supply is nominally present. Size storage tanks based on nighttime refill assumptions and verify with a pressure logger rather than a single reading.

Related calculators you might need

If your pressure analysis points to a pump-fed system, the pump head pressure calculator will tell you the total dynamic head your pump needs to overcome — accounting for static lift, pipe friction, and required outlet pressure. For installations where the tank is on a rooftop, check structural implications with the rooftop load bearing calculator before adding water weight. If you are retrofitting an existing system and need to size replacement pipework, the pipe size and flow rate calculator matches pipe diameter to required flow at a given pressure. For emergency or off-grid contexts where you are relying solely on stored water with no boost pump, the hydrostatic pressure calculator confirms what pressure you can realistically expect from a fixed tank height.

Frequently asked questions

What is a good water pressure for a house? Most residential plumbing is designed for 2–3 bar (200–300 kPa / 29–43 PSI) at the highest fixture. Below 1.5 bar, some appliances will not function correctly. Above 5 bar, you should install a pressure reducer to protect fittings and appliances. The ideal range for most households is 2.5–3.5 bar — enough for good shower pressure without stressing the pipework.

How much pressure does a rooftop tank produce? Every 1 metre of water height above the outlet produces approximately 9.81 kPa (0.098 bar / 1.42 PSI). A tank base at 5 metres above the lowest fixture delivers about 49 kPa (0.49 bar / 7.1 PSI). This is marginal for a conventional shower and insufficient for a power shower or combi boiler. Use the minimum tank height for shower pressure calculator to find the exact height you need for your specific fixtures.

Is 40 PSI enough water pressure? 40 PSI (2.76 bar / 276 kPa) is within the acceptable range for residential use. Most fixtures are rated to work from 25–80 PSI. At 40 PSI you will get adequate flow from taps, showers, and most appliances. If pressure drops below 40 PSI during peak use — indicating insufficient supply or pipe losses — you may need a booster pump or larger pipework.

Why does my water pressure drop when multiple taps are open? Simultaneous demand draws more flow through a fixed pipe diameter, which increases friction losses and reduces pressure at each outlet. This is a pipe sizing and pump capacity problem, not a pressure unit issue. Upsizing the supply main from 15mm to 22mm or 28mm, or adding a pressure-boosting pump with a buffer tank, typically resolves the problem. The water flow rate calculator can help quantify the flow demand.

What is the difference between static and dynamic pressure? Static pressure is measured when no water is flowing — it represents the maximum available head from the tank or main. Dynamic pressure (residual pressure) is what remains at the outlet when flow is occurring. For system design, dynamic pressure is what matters. A tank at 8 metres static head might deliver only 5 metres of dynamic head at full flow once pipe friction is subtracted. Always design for dynamic conditions, not static.

A first-flush diverter is a device fitted in a rainwater harvesting downpipe that captures and discards the initial portion of roof runoff — the water most contaminated with dust, bird droppings, leaf debris, and atmospheric pollutants — before allowing cleaner water to flow into the storage tank. The principle is that the first rainfall washes the roof surface, and that wash water should not enter your tank.

The volume of water to divert depends on your roof area and local contamination levels. Use the first flush diverter size calculator to work out the correct diverter capacity for your catchment.

How a First-Flush Diverter Works

The device sits in the downpipe between the roof gutter and the tank inlet. When rain starts, water flows into a standpipe or chamber within the diverter. This chamber fills first, holding the contaminated flush water. Once the chamber is full, subsequent runoff overflows or redirects through a bypass pipe into the storage tank. A slow-release valve (typically a small orifice of 2 to 4 mm) at the base of the chamber drains the captured flush water between rain events, resetting the device for the next storm.

The slow-drain orifice is critical. Without it, the diverter chamber stays full after the first event and passes all subsequent runoff directly to the tank — defeating the purpose. The orifice must drain slowly enough that it empties between rain events but does not drain so fast it empties during a prolonged storm, allowing a second contamination flush to enter the tank.

How Much Water Does a First Flush Actually Contain?

The standard guideline is 1 litre of diverter capacity per 25 m² of roof catchment area, though some authorities specify 1 L per 10 m² in heavily polluted urban areas or near busy roads, and 1 L per 40 m² in rural areas with clean air and low bird activity. A 100 m² roof in a typical suburban area needs approximately 4 litres of first-flush diverter capacity.

These are starting points. The actual contamination load depends on how long since the last rainfall, whether trees overhang the roof, proximity to roads, and local wildlife. In areas where rooftop water is used for drinking, err toward a larger diverter — the cost is a few litres of water per event.

Do You Need a First-Flush Diverter?

For non-potable uses — garden irrigation, toilet flushing, laundry (where permitted), and livestock troughs — a first-flush diverter is not strictly necessary but will extend tank cleanliness and reduce the frequency of tank cleaning. Sediment and organic matter from unfiltered roof runoff accumulate at the tank base over months, creating conditions for bacterial growth and odour.

For any potable or near-potable application — drinking, cooking, bathing — a first-flush diverter is essential, not optional. It is the first line of defence in a harvesting system that should also include a pre-tank filter, tank screening, and post-tank treatment (UV, chlorination, or filtration). No single device makes roof-harvested water safe to drink on its own.

In areas with long dry spells between rains — common in much of Australia, southern Africa, and inland India — contamination accumulates on the roof surface over weeks or months. After a six-week dry spell, the first flush is highly polluted. Diverting only 4 litres from a 100 m² roof may not be enough; some practitioners double the diverter volume in such climates.

Common Mistakes

Mistake 1: Undersizing the diverter based on roof area alone, ignoring local conditions. A house near a chicken farm, under a large tree, or next to a busy road carries a much higher contamination load than one in a clean rural setting. The 1 L per 25 m² rule is a baseline for average conditions. In high-contamination environments, undersizing means contaminated water enters the tank on every event.

Mistake 2: Fitting a diverter and assuming the tank water is safe without further treatment. A diverter removes the worst of the surface contamination but does not filter bacteria, protozoa, or dissolved chemical contaminants. E. coli and other pathogens can be present in roof runoff even after a diverter. Any system supplying water for drinking requires downstream treatment.

Mistake 3: Not maintaining the slow-drain orifice. The small drainage hole at the base of the diverter chamber blocks easily with leaf particles, insect nests, and mineral scale. A blocked orifice means the chamber never resets — all subsequent rainfall bypasses the diverter and goes directly to the tank. Check and clear the orifice every 3 to 6 months.

Mistake 4: Fitting a diverter but not screening the tank inlet. Insects, frogs, and small animals will enter an unscreened tank inlet. A diverter stops the first flush; it does not stop creatures entering through open pipes. Fit a fine mesh screen (maximum 1 mm) at the tank inlet to exclude vectors.

Related Calculators You Might Need

Sizing a first-flush diverter is one step in designing a complete rainwater harvesting system. The roof catchment area calculator converts your roof dimensions to an effective catchment area accounting for pitch and material. From there, the annual rainwater collection calculator estimates how much water your roof actually yields over a year given local rainfall data. Once you know the yield, the rainwater harvesting calculator sizes the storage tank to balance yield against demand. And if you are evaluating the financial case for the system, the rainwater harvesting ROI calculator models payback against your current water costs.

Frequently Asked Questions

How much water does a first-flush diverter waste?

A standard diverter for a 100 m² suburban roof discards approximately 4 litres per rain event. In a location with 80 rain events per year, that is 320 litres annually — a negligible loss against the thousands of litres collected. In drought-prone areas where every litre counts, some users fit diverters with a collection point for the flush water and use it for non-contact irrigation rather than discarding it to drain.

Can I make a DIY first-flush diverter?

Yes. A common DIY design uses a vertical PVC standpipe connected inline with the downpipe, sized to the required divert volume (roughly 20 mm pipe holds about 0.3 L per metre length). A ball float valve at the connection point closes when the standpipe is full, diverting subsequent flow to the tank. A 2 to 3 mm orifice drilled in the standpipe cap allows slow drainage. Commercial units offer the same function with better weathering and easier maintenance access.

Does a first-flush diverter help with water quality testing results?

It improves results but does not guarantee them. Studies show first-flush diversion reduces E. coli counts and turbidity significantly compared to undiverted systems, but detectable contamination often remains. For potable use, treat diverted roof water with chlorination or UV regardless. The water tank disinfection calculator helps you determine correct disinfection doses.

How often should I clean a first-flush diverter?

Inspect every 3 months minimum. Clean the slow-drain orifice with a pin or thin wire. Remove the standpipe or chamber cap and flush out any sediment annually. In areas with high leaf fall or heavy bird activity, inspect monthly. A blocked diverter is worse than no diverter — it gives a false sense of protection while passing unfiltered runoff into the tank.

Do I need a first-flush diverter if I already have a pre-tank filter?

Both serve different purposes and work best together. The diverter removes the most contaminated initial water before it reaches the filter. Without a diverter, the filter processes the full contamination load of the first flush on every event — shortening filter life and increasing maintenance frequency. A diverter upstream of a filter reduces clogging, extends filter media life, and produces cleaner water at the filter outlet.

A gravity-feed water system delivers water using elevation difference alone — no pump, no pressure vessel, no electricity. The tank sits above the point of use, and the weight of the water column above the outlet creates usable pressure. Every 1 metre of vertical height between the tank outlet and the tap produces approximately 0.098 bar (1.42 psi) of static pressure. That number determines everything: what you can and cannot run off a gravity-fed supply.

Before sizing a tank for gravity feed, use the gravity feed flow rate calculator to confirm your height gives you enough flow for the fixtures you plan to run.

How the Pressure Equation Works

The physics is straightforward. Hydrostatic pressure at the base of a water column equals the fluid density multiplied by gravitational acceleration multiplied by height: P = ρgh. For water, this simplifies to roughly 9.81 kPa per metre of head, or 0.098 bar/m. A tank mounted 3 m above a shower outlet generates around 0.29 bar of static pressure before any friction losses in the pipework are accounted for.

Static pressure is not the same as dynamic pressure. Once water flows, pressure drops due to pipe friction, fittings, and elevation changes along the route. A 25 mm pipe carrying 10 L/min loses significantly less pressure per metre than a 15 mm pipe carrying the same flow. If your pipes are undersized, the working pressure at the fixture will be well below what the height calculation suggests. Use the pipe size and flow rate calculator to account for friction losses.

Pressure by Tank Height: What You Can Actually Run

Most shower heads require a minimum of 0.1 bar to function, but a comfortable shower requires at least 0.3 bar dynamic pressure at the head — meaning static pressure at the tank needs to be higher to cover friction losses. The minimum tank height for shower pressure calculator works this out for your specific setup.

Hard Limits of Gravity-Feed Systems

Gravity feed fails predictably in three scenarios. First, when height is insufficient: most modern appliances — washing machines, dishwashers, combination boilers — require a minimum inlet pressure of 0.5 to 1.5 bar. At 5 m head you get roughly 0.49 bar static, which leaves almost no margin once pipe losses are accounted for. These appliances will either refuse to operate or underperform.

Second, when flow demand exceeds what the pipe and head can deliver simultaneously. A 3 m head through a 15 mm supply pipe might sustain one shower adequately. Open a second outlet and dynamic pressure at both drops sharply. This is not fixable by adding a bigger tank — the constraint is head height and pipe diameter, not storage volume.

Third, when the tank position is constrained. Rooftop installations in single-storey buildings rarely achieve more than 2 to 3 m of usable head above upper-floor outlets. A second-storey bathroom fed from a rooftop tank at the same level has effectively zero pressure. The solution is either a pumped header tank or a pressurised system.

Common Mistakes

Mistake 1: Measuring height from the tank base rather than the tank outlet. The pressure equation uses the vertical distance from the water surface in the tank to the outlet — not the floor the tank sits on. A 1,000 L rooftop tank sitting 3 m above the roofline but with only 0.5 m of water remaining inside gives you only 3.5 m of effective head, not 4 m. As the tank empties, pressure drops.

Mistake 2: Ignoring pipe friction over long horizontal runs. A tank 6 m above a tap sounds like plenty of head. But if the supply pipe runs 40 m horizontally in 15 mm copper with multiple bends, friction losses can consume 0.3 bar or more — cutting effective pressure at the outlet nearly in half.

Mistake 3: Sizing the tank for storage without checking structural load. A 2,000 L tank weighs over 2 tonnes when full. Rooftop installations require a structural assessment before installation. The rooftop load bearing calculator gives a baseline figure, but a structural engineer sign-off is mandatory for anything over 500 L on a residential roof.

Mistake 4: Assuming gravity feed works for all fixtures once it works for one. A system tested on a single ground-floor tap may fail completely when a first-floor shower is added. Every metre of vertical rise above the outlet subtracts from available head. Test with all intended fixtures active simultaneously.

Related Calculators You Might Need

Once you have confirmed your gravity head is adequate, the next check is whether your tank can sustain supply between refills. The how long will my tank last calculator tells you how many days of consumption your stored volume covers. If you are sizing a new tank from scratch, the water tank size for home calculator combines daily demand with backup duration to give a minimum capacity figure. For rooftop installations, check the safe rooftop tank load calculator before committing to a tank size — a full 2,000 L tank exerts over 2 tonnes of force on the mounting surface. And if you need to understand how long a gravity-fed tank takes to refill from a low-pressure supply, the tank refill time calculator handles that calculation.

Frequently Asked Questions

What is the minimum height for a gravity-fed shower?

A shower needs at least 0.1 bar at the head to function, which equates to roughly 1 m of head. In practice, that produces a trickle. A usable gravity-fed shower requires at least 3 m of vertical distance between the tank water surface and the shower head — and more if the pipe run is long or has multiple bends. Premium shower heads with multiple jets need 0.5 bar or more, requiring at least 5 m of head before pipe losses are considered.

Can I run a washing machine off a gravity-feed tank?

Most washing machines require a minimum inlet pressure of 0.5 to 1.0 bar. That means your tank surface needs to sit at least 5 to 10 m above the machine’s inlet valve. In a standard residential setup, this is rarely achievable without a pump. Check your machine’s specification plate — the minimum operating pressure is listed there. If gravity head is insufficient, a small pressure-boosting pump in line is the practical solution.

Why does my gravity-fed tap run fine but the shower is weak?

The tap is almost certainly at a lower elevation than the shower head. Every metre of height you lose going up to a first-floor bathroom subtracts from your available head. A ground-floor tap 2 m below the tank might get 0.20 bar, while the shower head 1.5 m above that tap is only 0.5 m below the tank — giving 0.05 bar and barely any flow. Gravity systems are acutely sensitive to the elevation of the end fixture, not just the distance from the tank.

How does tank volume affect pressure in a gravity system?

Volume does not directly affect static pressure — only the height of the water surface matters. However, as the tank empties, the water level drops, reducing effective head and therefore pressure. A larger tank maintains a higher water level for longer, giving more consistent pressure throughout the day. Use the water pressure calculator to see how pressure changes at different fill levels.

What pipe size should I use for a gravity-feed system?

For a single household with moderate demand, 25 mm internal diameter pipe from tank to distribution point is the minimum worth installing. Smaller pipe generates higher friction losses per metre, which eats into your available head. For longer runs (over 20 m) or multi-fixture systems, 32 mm or 40 mm main supply pipe is preferable, stepping down at branch points. Avoid reducing pipe diameter at the tank outlet — this creates a bottleneck that limits the entire system regardless of how large the tank is.

Does tank shape affect pressure in a gravity system?

Shape does not affect pressure — only the vertical height of the water surface above the outlet matters. A tall, narrow tank and a wide, shallow tank at the same base elevation will produce the same pressure when both are full. The difference appears as they empty: the narrow tank maintains a higher water level and therefore more consistent pressure for longer, while the wide tank’s level drops more rapidly per litre consumed.

Rainwater harvesting captures precipitation from a roof, filters it of debris and first-flush contaminants, stores it in a tank, and delivers it for domestic or agricultural use. A typical residential system in a 600 mm/year rainfall area with a 100 m² roof can collect 48,000–54,000 litres per year — enough to cover toilet flushing, garden irrigation, and laundry for a family of four, or to provide full household supply in regions with reliable seasonal rain. This article explains each stage of the system, how collection volume is calculated, and what determines real-world yield.

The quick answer

Collection volume depends on three inputs: catchment area, annual rainfall, and runoff coefficient. The formula is:

Annual yield (litres) = Roof area (m²) × Annual rainfall (mm) × Runoff coefficient

Runoff coefficients by roof material (FAO Technical Paper 267, 1994):

Use the rainwater harvesting calculator to model your system, input local rainfall data, and get an annual yield estimate.

How the collection system works

The path from rainfall to storage involves four functional components: the catchment surface, the conveyance system, the first flush diverter, and the storage tank.

Catchment surface. The roof acts as an impermeable collection surface. Effective catchment area is the horizontal projected area — not the total roof surface area if the roof is pitched. A 10 m × 12 m footprint building has a 120 m² catchment area regardless of roof pitch. Use the roof catchment area calculator if your roof has multiple sections at different angles.

Gutters and downpipes. Gutters collect water from the roof edge and channel it to downpipes. Undersized gutters overflow during heavy rain, causing significant collection losses. The standard rule is 1 cm² of gutter cross-section per 1 m² of roof in moderate rainfall areas. In high-intensity tropical rainfall, double this. Downpipes should be sized to handle the peak gutter flow without backing up.

First flush diverter. The first 2–3 mm of rain on a roof carries the highest concentration of bird droppings, dust, atmospheric fallout, and debris. A first flush diverter captures and discards this initial volume before directing clean water to the tank. The standard sizing rule is 1 litre of diverter capacity per 10 m² of roof — so a 100 m² roof needs a 10-litre first flush chamber. Use the first flush diverter size calculator to size yours correctly.

Storage tank. The tank size determines how much of the collected rainfall you can actually store versus what overflows. Tank sizing depends on whether you are trying to smooth daily demand, bridge a dry season, or maximise annual collection. A tank that is too small overflows and wastes rain. A tank that is too large costs more and may allow water to sit long enough to degrade.

Key variables that change the yield

Seasonal distribution of rainfall. Annual rainfall figures can be misleading. Two locations with 800 mm/year may have completely different harvesting potential if one receives rain year-round and the other has a 5-month dry season. In the latter, you need a tank large enough to bridge the dry period — potentially 60,000–80,000 litres for a family-scale system. Local monthly rainfall data is essential for accurate tank sizing.

Roof material and cleanliness. Moss, lichen, and accumulated debris reduce effective runoff coefficient and contribute contaminants. A metal roof in good condition with regular cleaning achieves the top end of its coefficient range. An old asphalt roof with moss may perform at 0.60 or below. Inspect the roof annually and factor in the actual condition, not theoretical specifications.

Household demand versus yield. Harvesting only makes financial sense if annual yield exceeds a meaningful fraction of demand. For toilet flushing alone in a 4-person household (approximately 30 litres per person per day), annual demand is about 43,800 litres. In areas with 600 mm/year rainfall and a 100 m² roof, annual yield of 42,000–54,000 litres means rainwater can cover the full toilet demand in most years.

First flush volume and its impact. If the first flush diverter is undersized, contaminated water enters the tank, increasing treatment load. If it is oversized, clean water is wasted. Proper sizing can recover an additional 2–5% of annual yield that would otherwise be discarded.

Common mistakes

Using total roof surface area instead of horizontal projection. A 45-degree pitched roof with 160 m² of actual surface has a horizontal catchment of only 113 m². Using the larger number overstates yield by 40%. Always calculate from plan (footprint) dimensions, not slope dimensions.

Sizing the tank to peak collection without considering demand cycles. Selecting a tank that holds your full annual yield sounds safe but is rarely efficient. If you have moderate year-round demand, a smaller tank that turns over frequently holds fresher water and costs less. Tank sizing should be based on the longest expected dry period plus a buffer — not total annual yield. Model the demand-supply balance month by month.

Ignoring the first flush diverter entirely. Systems without a first flush diverter accumulate bird droppings, animal contamination, and atmospheric pollutants in the tank over time. This leads to high bacterial counts, turbidity, and biofilm growth. Many harvesting system failures that are attributed to storage are actually caused by inadequate pre-filtration. This is especially critical in urban areas with higher atmospheric pollution.

Connecting the overflow to a sealed drainage system. Tank overflow during heavy rain can be significant — a 100 mm event on a 100 m² roof generates 10,000 litres in a single event. Direct overflow to a soakaway, garden bed, or secondary tank rather than a sealed drain to avoid backing up the stormwater system or losing the overflow entirely.

Related calculators you might need

Once you have estimated your annual yield, compare it to your annual water costs with the rainwater savings calculator, which shows how much your water bill decreases. If you are evaluating whether a harvesting system is financially worthwhile, the rainwater harvesting ROI calculator and rainwater harvesting payback calculator model your break-even timeline against installation cost. For agricultural use, where water requirements scale with crop area, the irrigation water requirement calculator helps determine whether your harvested volume is sufficient for your crop needs.

Frequently asked questions

How much rainwater can I collect from my roof? Multiply your roof’s horizontal footprint in square metres by your local annual rainfall in millimetres, then by the runoff coefficient for your roof material (0.75–0.90 for most hard surfaces). A 120 m² metal roof in a 700 mm rainfall area yields approximately 67,200–75,600 litres per year. Use the annual rainwater collection calculator for a precise figure with monthly breakdown.

Is harvested rainwater safe to drink? Collected rainwater contains bacteria, particulates, and potential chemical contamination from the roof and atmosphere. Without treatment, it is not reliably safe for drinking. For potable use, the minimum treatment is a first flush diverter, sediment filtration, and either chlorination or UV disinfection. In many jurisdictions, rainwater used for drinking must meet the same standards as tap water (WHO Guidelines for Drinking-water Quality, 2022). For non-potable uses — toilets, laundry, irrigation — basic screening and first flush diversion is typically sufficient.

What size tank do I need for rainwater harvesting? Tank size depends on your consumption rate and the longest gap between rainfall events in your area. For supplementary use (toilet and garden only), 5,000–10,000 litres is typical for a residential system. For primary or sole supply, you need enough storage to bridge the dry season — often 30,000–60,000 litres in subtropical climates with a 4–6 month dry season. The rainwater harvesting calculator models tank sizing against local rainfall patterns.

Does roof colour or material affect water quality? Yes. Lead-based paints, copper or zinc in metal roofing, and treated timber in roof structures can leach into collected water. Unpainted galvanised iron roofs can contribute zinc at levels that may exceed drinking water guidelines in acidic rainfall areas. Painted or powder-coated colorbond steel, concrete tiles, and terracotta are generally low-risk. Avoid collecting from roofs within 6 months of repainting and after any bitumen or asphalt application.

Can I harvest rainwater in a dry climate? Yes, but tank sizing becomes critical. In arid areas with 200–300 mm/year rainfall and high evaporation, collection efficiency drops and storage requirements increase. A 200 m² roof receiving 250 mm of annual rainfall with a 0.85 coefficient yields only 42,500 litres — less than most households use for non-potable purposes alone. Supplementary storage with a well or delivered water is typically required for reliability in areas below 400 mm annual rainfall.

UV disinfection inactivates bacteria, viruses, and protozoa by exposing them to ultraviolet light at 254 nanometres — the wavelength at which UV energy most efficiently damages microbial DNA, preventing reproduction. It does not add chemicals, does not alter taste or pH, and leaves no residual in the water. For stored water that is then distributed through a pipe system, UV must be used as a point-of-entry treatment — at the outlet from the tank, not inside the tank itself. Understanding this distinction, plus the UV dose required for effective disinfection, is critical to sizing and operating a system correctly.

The quick answer

UV dose is measured in millijoules per square centimetre (mJ/cm²). The dose required depends on the target organism. EPA and NSF/ANSI Standard 55 require a minimum of 40 mJ/cm² for UV systems certified for drinking water treatment. This achieves a 4-log (99.99%) reduction in bacteria and viruses. Cryptosporidium and Giardia — protozoa resistant to chlorine — are inactivated at doses as low as 10–12 mJ/cm², making UV one of the few practical treatments for these pathogens in point-of-use systems (NSF/ANSI 55, Class A).

Use the UV disinfection tank calculator to determine the correct UV system flow rate for your tank’s output and verify whether your existing unit is appropriately sized.

How the UV disinfection mechanism works

UV-C light at 254 nm is absorbed by the nucleic acids in microbial DNA and RNA. This energy causes adjacent thymine bases to bond to each other (thymine dimers), creating physical damage that prevents the DNA strand from being replicated during cell division. The organism is effectively sterilised — it cannot reproduce even if it remains present in the water. Unlike chlorine, which chemically destroys the cell, UV does not physically destroy the microorganism’s body, which is why turbidity is the critical limiting factor for UV systems.

If suspended particles are present — sediment, algae, organic matter — microorganisms can shelter inside or behind particles and receive insufficient UV dose. This is called shadowing. For this reason, UV systems are always installed after filtration. Most manufacturers require water turbidity below 1 NTU (Nephelometric Turbidity Unit) and UVT (UV transmittance) above 75% for reliable performance at rated dose.

Key variables that change UV system performance

Flow rate. UV dose = lamp output (mW/cm²) × exposure time (seconds). Exposure time depends on how long the water spends in the UV chamber, which is determined by flow rate and chamber volume. If flow rate doubles, exposure time halves, and UV dose halves. This is why UV systems are rated to a maximum flow rate — never exceed it. Undersized UV units used above their rated flow deliver insufficient dose regardless of lamp wattage. Use the water flow rate calculator to determine peak demand flow before selecting a UV system.

Lamp intensity and age. UV lamps degrade over time. Most manufacturers specify lamp replacement at 9,000–12,000 hours of operation (~1 year of continuous use). At end of life, lamp output can drop to 60–70% of initial intensity — below the threshold for reliable 40 mJ/cm² delivery. UV intensity sensors and lamp-hour counters are standard on quality systems; never operate a UV system beyond its rated lamp life without replacing the lamp.

Water UV transmittance (UVT). Different water sources have very different UV transmittance. High iron (above 0.3 mg/L), natural organic matter, tannins from surface water, and turbidity all reduce UVT. Manufacturers rate UV systems at a specified UVT — typically 75–95%. If your water has lower UVT, the effective dose is reduced proportionally. Pre-treatment with activated carbon filtration typically improves UVT significantly for surface-water-derived supply.

Temperature. Low-pressure mercury UV lamps produce optimal output at a lamp temperature of around 40°C. In cold-water installations (below 10°C), lamp output can drop 20–30% unless the unit has a temperature-compensated sleeve. High-output amalgam lamps are more temperature-stable and preferred for cold climates or high-flow installations.

UV disinfection versus chlorination: when to use each

For stored water in a tank that is refilled periodically and distributed over hours or days, chlorination with a maintained residual is the appropriate primary treatment. UV is best suited as a point-of-entry final polishing step before consumption, installed on the outlet pipe from the tank. In areas with Cryptosporidium risk — particularly surface water sources — combining both chlorination (for residual) and UV (for protozoa) is the recommended approach (WHO Guidelines for Drinking-water Quality, 2022).

Common mistakes

Installing UV inside or on the tank instead of on the outlet pipe. UV treats water that passes through the lamp chamber at the time of treatment. It does not create a disinfected environment in the tank — any water that bypasses the lamp (re-contamination, settling, biofilm growth) is untreated. The unit must be installed on the pipe delivering water to points of consumption, not inside the storage vessel.

Not replacing lamps on schedule. Running a UV lamp beyond 12,000 hours — or one year of continuous operation — does not simply reduce effectiveness gradually. Output can collapse rapidly as the quartz sleeve ages and the mercury distribution changes. Many users operate on the assumption that a functioning lamp is an effective lamp. Install a lamp-hour counter and treat replacement as non-optional maintenance.

Installing UV before filtration. UV must be the final treatment step, after sediment filtration and activated carbon (if required for UVT improvement). Installing UV upstream of a filter exposes the treated water to re-contamination from the filter. The correct order is: pre-filtration (sediment) → activated carbon (if needed) → UV lamp → point of use.

Ignoring quartz sleeve fouling. The quartz sleeve surrounds the UV lamp and must be transparent to UV-C. Iron deposits, calcium carbonate scaling, and biofilm on the sleeve block UV transmission and reduce dose delivered to the water. Clean the sleeve with citric acid solution every 3–6 months depending on water quality. A fouled sleeve can reduce effective dose by 50% or more while the lamp appears to be functioning normally.

Related calculators you might need

Before installing a UV system, verify that your water’s turbidity and quality are suitable using the TDS water calculator as a starting point for water quality assessment. If you are using UV in combination with chlorination — which is recommended for surface water sources — calculate the chemical dose with the chlorine dosage calculator. For systems where the UV unit is installed in a filter housing or after a pressure filter, the water filter flow rate calculator ensures the pre-filter does not restrict flow below the UV system’s minimum operating threshold. And if your tank is sized for emergency storage rather than daily use, the safe water storage duration calculator helps determine when to re-treat or rotate stock.

Frequently asked questions

Does UV light disinfect the water tank itself? No. UV disinfection treats water as it flows through a lamp chamber at the point of delivery — it does not affect water sitting in the tank, biofilm on tank walls, or sediment at the bottom. For tank disinfection, use chemical treatment (chlorination) followed by physical scrubbing and rinsing. UV is a point-of-use or point-of-entry technology, not a tank treatment technology.

How long does UV disinfection take? The treatment is instantaneous — as water passes through the UV chamber, it is exposed to the UV dose. There is no contact time requirement as with chlorination. The critical variable is ensuring that all water passes through the chamber without bypassing, and that flow rate does not exceed the system’s rated maximum. Use the UV disinfection tank calculator to confirm your system handles your peak flow rate.

Can UV kill Cryptosporidium in water? Yes. Cryptosporidium oocysts are highly resistant to chlorination but are effectively inactivated by UV at doses as low as 5–10 mJ/cm² for 2-log (99%) reduction. This is well below the 40 mJ/cm² minimum for NSF 55 Class A certification, meaning any certified residential UV system will achieve adequate Cryptosporidium inactivation. This is the primary reason UV is the preferred treatment for surface water and rainwater sources in areas with known protozoan contamination.

What maintenance does a UV water purifier need? Three maintenance tasks: (1) replace the UV lamp every 9,000–12,000 hours (approximately annually for continuous operation); (2) clean the quartz sleeve every 3–6 months with citric acid solution or as indicated by the manufacturer; (3) inspect and replace pre-filters as recommended. Systems with UV intensity monitors should be checked monthly — a drop in output below threshold triggers immediate lamp replacement regardless of hours elapsed.

Is UV treatment enough on its own for drinking water? UV effectively inactivates biological contaminants but does not remove chemicals, heavy metals, nitrates, or dissolved solids. In areas with agricultural runoff, industrial contamination, or naturally high arsenic or fluoride, UV alone is not sufficient for drinking water safety. A complete treatment train for surface water or rainwater typically includes: coarse filtration → sediment filter → activated carbon → UV. For chemical contamination, reverse osmosis before UV may be required.

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 based on flow rate alone — without checking head — is the most common cause of pump failure to perform in water tank installations. 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. Understanding this relationship is essential for correct 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

Use the pump head pressure calculator to calculate 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.

Static head: 10 m

Friction head from pipe: Using Darcy-Weisbach, 25mm pipe at 15 L/min flow generates approximately 0.8 m friction per 10 m of pipe. For 25 m: 2.0 m. Each 90° elbow adds ~0.6 m equivalent. Four elbows: 2.4 m. Total friction head: 4.4 m

Pressure head: 0.5 bar × 10.2 = 5.1 m

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 cannot overcome the total resistance. The pump curve (head vs. flow rate graph provided by manufacturers) 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 that shows how head and flow interact. At zero flow, the pump achieves its maximum head (shut-off head). As flow increases, achievable head decreases. The intersection of the pump curve with the system curve — which represents TDH at various flow rates — is the operating point.

A pump operating to the right of its best efficiency point (BEP) on the curve is working harder than designed, heating up, and wearing faster. A pump operating far to the left is deadheading or near deadheading, generating heat without useful work. Correctly calculating TDH ensures the operating point falls within 80–110% of the BEP flow rate

For installations that vary in demand — such as a farm tank that fills at night during low demand and delivers during high-demand irrigation — the system curve shifts. Using the pump horsepower and flow rate calculator helps verify that the motor is appropriately sized for both conditions.

Key variables that change total dynamic head

Pipe diameter. Friction head scales approximately with the square of velocity in the pipe. Doubling pipe diameter from 20mm to 40mm reduces velocity by a factor of 4, cutting friction head by approximately 16 times. For long pipe runs, upsizing the pipe is almost always cheaper than buying a higher-head pump. As a rule: for runs over 30 metres, increase pipe diameter by one standard size above the minimum.

Number of fittings. Elbows, tees, gate valves, and check valves all add equivalent pipe length. A check valve (non-return valve) required for most pump installations adds 5–10 metres of equivalent pipe length depending on the valve type. Ball valves are low-resistance (0.3–0.5 m equivalent); globe valves and angle valves are high-resistance (10–20 m equivalent). Account for every fitting in the calculation.

Suction lift. Centrifugal pumps have a maximum practical suction lift of around 7–8 metres under ideal conditions (atmospheric pressure minus vapour pressure of water). In practice, due to leaks, turbulence, and elevation of the installation site, 5–6 metres is the reliable limit. Exceeding this causes cavitation — a rapid implosion of vapour bubbles that erodes impellers and casings. Always install submersible pumps for borehole depths greater than 6 metres.

Elevation above sea level. Atmospheric pressure decreases at altitude, reducing the net positive suction head available (NPSHA). At 1,500 m above sea level, effective suction lift drops to approximately 4.5 metres. At 3,000 m, it falls to around 3 metres. This matters for installations in highland agricultural regions and mountain communities.

Common mistakes

Selecting a pump based on flow rate alone. A pump delivering 30 L/min at 5 m head cannot deliver 30 L/min at 20 m head — it delivers less, sometimes drastically less depending on the pump curve. Always cross-reference the flow requirement with the TDH on the manufacturer’s pump curve. Buy the combination, not either variable in isolation.

Ignoring friction in short pipe runs. Installers routinely assume friction head is negligible for short runs. A 10-metre run of 20mm pipe at 20 L/min generates approximately 4 metres of friction head — equivalent to lifting water an extra 4 metres. At 30 L/min in the same pipe, friction head exceeds 8 metres. In tight-budget pump selections, this unaccounted loss causes the pump to underperform from day one.

Using flow rate at max head as the selection criterion. Manufacturer specifications often show maximum head and maximum flow separately. These are not simultaneously achievable — they are the two endpoints of the performance curve. The maximum flow occurs at zero head; maximum head occurs at zero flow. Select based on the specific combination of head and flow your system demands.

Not accounting for future expansion. A pump selected to exactly meet current TDH and flow leaves no margin for expansion — adding a second building, extending the pipe run, or adding more fixtures. Size the pump for 120–130% of current TDH to allow for system growth and age-related efficiency decline.

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 are designing a gravity-fed system and comparing it to a pumped system, 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 the friction loss per metre for any pipe diameter and flow rate 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 — and the number to match against the pump’s performance curve.

How do I calculate total dynamic head for my pump? Add three components: (1) static head — the vertical height from the pump inlet to the delivery point; (2) friction head — calculated from pipe diameter, length, and fittings using a friction loss table or formula; (3) pressure head — the required outlet pressure converted to metres (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, the pump will deliver less flow than needed — or none at all if TDH exceeds shut-off head. The pump will run continuously, heat up, and eventually fail. Common symptoms include the pump running without water reaching the tank, the tank filling slowly and only partially, or pressure cuts out during high-demand periods.

Is more pump head always better? Not necessarily. Over-specifying head means the pump operates far to the left of its best efficiency point — delivering low flow at unnecessarily high energy consumption. This also puts mechanical stress on the pump and may cause pipe pressure to exceed 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 requires a pump rated to at least 30 m static head, plus friction and pressure head.