Category: Concepts & Explainers

The runoff coefficient is the fraction of rainfall that actually reaches your tank after accounting for losses from absorption, evaporation, splashing, and wetting of the catchment surface. It is expressed as a decimal between 0 and 1: a coefficient of 0.85 means 85% of the rain that falls on your roof ends up as collectible water. The remaining 15% is lost.

Every rainwater harvesting calculation depends on this number. Apply it using the annual rainwater collection calculator, which incorporates roof area, local rainfall, and runoff coefficient to give you 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: if 50 mm of rain falls on a 100 m² roof with a coefficient of 0.85, the collectible volume is 0.050 m × 100 m² × 0.85 = 4.25 m³ (4,250 litres). Without the coefficient, you would overestimate yield by 15%, leading to a tank that runs out more often than modelled.

The coefficient is not a fixed property of the roof material alone — it varies with rainfall intensity, roof condition, antecedent dryness, 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. Long-run average coefficients, used for annual yield calculations, account for this variation across all 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 reflect long-run average performance. For a conservative tank sizing calculation — where you want to avoid running out — use the lower bound of the coefficient range for your roof material. For estimating maximum potential yield, use the upper bound.

Other Factors That Adjust the Coefficient

Roof slope: Steeper roofs shed water faster, reducing evaporation time during and after rain. A roof pitched at 30° or more performs closer to the upper end of its material coefficient range. Flat roofs (under 5° slope) retain water longer and experience more evaporation, shifting 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. Lichen and moss on any roof surface can reduce effective coefficient by 0.05 to 0.15.

First-flush losses: If your system includes a first-flush diverter, the volume diverted reduces effective yield and is sometimes incorporated into an adjusted runoff coefficient. For a system diverting 4 litres per event across 80 annual events, that is 320 litres of annual loss — subtract this from gross yield when sizing storage.

Gutter and downpipe losses: Water remaining in gutters after rain, overflow from gutters during high-intensity events, and evaporation from hot metal gutters between events all reduce effective yield. A system efficiency factor of 0.85 to 0.90 is applied on top of the roof coefficient in some Australian standards to account for these distribution losses.

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 gives approximately 42,300 litres of usable annual yield.

This figure feeds directly into tank sizing — the rainwater harvesting calculator matches this yield against household demand and seasonal rainfall distribution to give a minimum tank size that avoids running dry.

Common Mistakes

Mistake 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. Assuming 100% collection consistently overestimates yield, leading to a tank that runs dry more often than planned. Use 0.90 to 0.95 as the realistic range for metal roofing.

Mistake 2: Using the same coefficient year-round in climates with marked wet and dry seasons. In a dry season, the roof surface is hotter and drier — initial wetting losses and evaporation rates are higher for every event. An annual average coefficient of 0.80 may mask a dry-season effective coefficient of 0.65 and a wet-season coefficient of 0.88. If you are sizing storage for dry-season survival, use dry-season figures for that calculation.

Mistake 3: Applying the coefficient to the building footprint rather than the effective catchment area. The catchment area is the horizontal projection of the roof, not its surface area. A steeply pitched roof covers more surface than footprint, but rainfall is measured vertically. The roof catchment area calculator converts roof dimensions to correct horizontal catchment area.

Mistake 4: Applying a textbook coefficient to a roof that has not been inspected. A roof listed as concrete tiles gets the concrete tile coefficient in most calculations, but if it is covered in moss and has cracked or missing tiles, actual performance may be 20 points below the standard figure. Inspect the roof surface before using any coefficient 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 then 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 are unsure of your roof’s condition, use the lower end of the range for that material — it produces a conservative (safer) tank size estimate.

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 is 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 annual rainfall implicitly blend these events, making the long-run average coefficient the appropriate value for sizing.

How do I measure my actual runoff coefficient?

The simplest method is empirical: know your roof area precisely, measure rainfall with a gauge, measure how much actually enters the tank, and divide tank inflow by (rainfall × roof area). Do this over 10 or more events across different rainfall intensities and average the results. This gives a site-specific coefficient that accounts for your specific roof, gutters, and downpipes — far more accurate than any table value.

Does roof colour affect the runoff coefficient?

Colour affects surface temperature, not porosity. A dark roof in full sun evaporates water faster during and immediately after light rain events — this can reduce the effective coefficient by 0.02 to 0.05 compared to a light-coloured roof of the same material. For heavy rainfall events, the effect is negligible. In hot climates where light drizzle events are common, colour is worth accounting for; in cool or high-rainfall climates, it is not a meaningful factor.

Can I increase my runoff coefficient?

The primary levers are roof material and condition. Replacing cracked concrete tiles with metal roofing increases the effective coefficient by 0.10 to 0.20. Cleaning moss, lichen, and debris from any roof improves performance noticeably. Improving gutter capacity to prevent overflow during high-intensity events also retains water that would otherwise be lost. None of these changes affect how the roof catchment area calculator computes your catchment size, but they directly increase 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

The WHO guideline for drinking water is 600 ppm, though palatability typically declines 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).

Why TDS Changes During Tank Storage

Water sitting in a tank does not stay chemically static. Several mechanisms alter TDS over time. First, evaporation from open or poorly sealed tanks removes pure water vapour but leaves dissolved solids behind — concentrating TDS in the remaining water. In hot climates with high evaporation rates, a tank losing 5% of its volume to evaporation will see TDS rise by approximately 5%.

Second, concrete and metal tanks leach minerals into stored water. Freshly cured concrete tanks add calcium hydroxide and other alkalis, which raises TDS and pH simultaneously. Corroding iron or galvanised steel tanks contribute iron and zinc ions. Plastic tanks are largely inert but can leach additives if they are low-grade or degraded by UV exposure.

Third, biological activity in improperly treated or uncovered tanks produces metabolic by-products that contribute to organic TDS. Algae and biofilm formation can raise TDS measurably in warm, light-exposed storage.

If you are treating stored water with chlorine, the residual chlorine reacts with organic matter to produce disinfection by-products — adding to TDS. 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. Combine TDS measurement with a residual chlorine check and a visual inspection for sediment, colour change, or odour.

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

Mistake 1: Treating TDS as a pass/fail safety test. A TDS reading of 250 ppm tells you the total dissolved load but nothing about what is dissolved. Arsenic contamination, nitrates, and E. coli are invisible to a TDS meter. Use TDS as a screening tool and first-line quality indicator, not as a definitive safety assessment.

Mistake 2: Ignoring TDS rise in sealed tanks during extended storage. Even sealed tanks concentrate dissolved solids slowly over weeks due to minor evaporation through 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.

Mistake 3: Interpreting very low TDS (under 50 ppm) as superior quality. RO-purified or distilled water with near-zero TDS is demineralised. Prolonged consumption of very low-TDS water leaches minerals from body tissue over time and can be mildly corrosive to metal plumbing. The WHO recommends a minimum of 100 ppm for regular drinking water.

Mistake 4: Testing TDS once and assuming it stays constant. TDS changes with season, supply source variation, tank condition, and treatment. Test at minimum quarterly for stored water, and 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 your TDS is elevated due to water hardness, the water hardness calculator will tell you whether scaling is a risk for your appliances and pipes. 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 are purchasing 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 for drinking water — enough minerals for taste and health, below the threshold where most people notice off-flavours. Water in the 300 to 600 ppm range is acceptable but may taste noticeably mineral. Above 600 ppm, WHO considers the water outside its palatability guideline. 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. The health concern depends entirely on what is causing the high 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 cannot distinguish between these — laboratory analysis is needed to identify specific ions.

How do I reduce TDS in my storage tank?

The only effective method is to treat or replace the water. Reverse osmosis reduces TDS by 90 to 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 due to tank leaching, the tank surface needs cleaning or lining.

Does chlorination affect TDS?

Yes. Adding chlorine or bleach to water 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 to TDS, which is negligible for most applications. Accurate dosing 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 in 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.

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 approximately 80 times 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).

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.

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. 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.

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.