Rainwater harvesting calculator: yield, monthly table and tank size
This rainwater harvesting calculator estimates how much rain your roof can collect in a year — in litres or US gallons — from the roof area, the rainfall and a runoff coefficient for the roof material. Pick a city preset (Mumbai, Delhi, Bengaluru, Chennai, London, Houston, Sydney and more) to load a month-by-month rainfall pattern, or type your own figures, and the tool shows the harvest for each month, how many days of household demand it covers and a sensible tank size.
The core formula is the one used in Indian municipal RWH guidelines and by rainwater associations elsewhere: one millimetre of rain on one square metre of roof is one litre, before losses. A 100 m² RCC roof in a 1,000 mm climate therefore yields about 80,000 litres a year after the 0.8 runoff coefficient — enough to cover a family of four for roughly five months at the 135 L per person per day norm.
Tank sizing balances two limits: the water your household uses during a dry spell, and what a single month can actually deliver. The calculator suggests the smaller of the two.
How to use the Rainwater Harvesting Calculator
- Choose metric or imperial and enter the roof plan length and width (or the plan area directly). Use the footprint, not the sloped area — rain falls on the plan area.
- Select the roof type to set the runoff coefficient, or type a custom value between 0 and 1.
- Pick a city preset for the monthly rainfall, or type the annual rainfall (the months scale to match). Open “Monthly rainfall” to fine-tune individual months.
- Set the first-flush loss if you divert the first rain of each event, then enter the people in the house, water use per person and how many dry days the tank should bridge.
- Add the water price per 1,000 L or 1,000 gallons to see the yearly saving, read the results and monthly table, and copy or share the summary.
Rainwater Harvesting Calculator features
City rainfall presets
Twelve Indian cities plus London, New York, Houston, Seattle, Los Angeles, Sydney, Dubai, Nairobi, Singapore and Lagos, with monthly distributions you can edit.
Runoff by roof type
Metal 0.90, tiles 0.85, RCC 0.80, clay/asbestos 0.75, gravel 0.60, green roof 0.40, or custom.
Monthly yield table
Litres or gallons per month with a bar for each, and how many days of your household demand that month covers.
Tank sizing
Demand × dry days, capped at the best month’s harvest, in kilolitres or gallons.
First-flush and demand
Allow for a first-flush diverter, set people and litres per person (135 L Indian norm, 300 L US).
Savings estimate
Water-bill saving per year from your tariff per 1,000 L or 1,000 gallons, in ₹, $, £ or €.
How the harvest is calculated
- Harvest (litres) = roof plan area (m²) × rainfall (mm) × runoff coefficient × (1 − first-flush loss). 100 m² × 1,000 mm × 0.8 = 80,000 L.
- Imperial: 1 ft² × 1 in of rain = 0.623 US gallons, so 1,000 ft² × 40 in × 0.8 ≈ 19,950 gallons.
- Monthly harvest uses the same formula with each month’s rainfall, which is why a Mumbai roof collects nearly all its water between June and September.
- Days covered = harvest ÷ (people × litres per person per day). 80,000 L ÷ 540 L/day = 148 days for four people at 135 L.
- Suggested tank = the smaller of (daily demand × dry days) and the best month’s harvest. A 10,800 L sump covers 20 dry days for that family.
Runoff coefficients and losses
Evaporation from a hot roof, splash-out of gutters and the first flush that washes dust and bird droppings off the roof all reduce the yield further, which is why Indian guidelines often quote an overall collection efficiency of 0.7–0.8 for RCC roofs.
| Roof surface | Coefficient | Why |
|---|---|---|
| Galvanised / colour-coated metal | 0.85–0.95 | Smooth, sheds almost everything |
| Concrete or glazed tiles | 0.80–0.90 | Small joints hold a little water |
| RCC flat roof (India) | 0.70–0.80 | Ponding and absorption on the slab |
| Clay (Mangalore) tiles, asbestos | 0.70–0.80 | Porous surface, overlaps |
| Gravel / built-up roof | 0.50–0.70 | Gravel traps water |
| Green roof | 0.30–0.50 | Soil and plants absorb most rain |
Planning the system
- A tank bigger than the best month’s harvest will never fill. Spend the extra on a recharge pit instead so surplus monsoon water tops up the groundwater.
- Size gutters and downpipes for the peak storm, not the annual total: a 100 m² roof in a 50 mm/hour downpour delivers 1.4 L per second.
- Fit a leaf screen and a first-flush diverter before the tank, and keep the tank dark and sealed against algae and mosquitoes.
- The roof area and pitch are covered by the roofing calculator; an underground sump is a job for the concrete calculator; a solar pump pairs well with the solar panel calculator.
Rainwater Harvesting Calculator FAQ
How much rainwater can I collect from my roof?
Roof plan area × annual rainfall × runoff coefficient. For a 1,200 ft² (111 m²) roof in Bengaluru (about 920 mm) with an RCC roof: 111 × 920 × 0.8 ≈ 82,000 litres a year. The rainwater harvesting calculator also breaks that down by month.
How many litres is 1 mm of rain on 1 m²?
Exactly one litre before losses. Multiply by the runoff coefficient (0.8 for a concrete roof) to get the amount that reaches the tank: about 0.8 litres.
What size rainwater tank do I need?
Enough for the dry spell you want to bridge — daily use × dry days — but no larger than the most a single month can deliver. For four people at 135 L/day and 20 dry days that is 10,800 L; a 10 kL sump is a common choice in Indian homes.
Is rainwater harvesting mandatory in India?
Many states and cities (Tamil Nadu, Karnataka, Delhi, Maharashtra among others) require RWH for new buildings above a certain plot size, usually with a minimum storage or recharge volume. Check your local building bye-laws; the calculator gives the numbers to size the system.
Can I use the calculator for a rain barrel in the US?
Yes — switch to imperial, enter the area of the roof section feeding the barrel and the local rainfall in inches. A 500 ft² section with 1 in of rain yields about 250 gallons, so a 55-gallon barrel fills in a light shower.
Does the harvested water need treatment?
For gardens, flushing and washing, a leaf screen and first flush are enough. For drinking it needs filtration and disinfection; many Indian households use harvested water to recharge borewells or as a non-potable supply.