What the Rainwater Harvesting Calculator does
This calculator estimates how much rainwater a roof can collect each month and how big a storage tank needs to be to carry you through the drier months. It multiplies catchment area by the monthly rainfall you enter and a runoff coefficient, then runs a month-by-month water balance against your daily use to find the smallest tank that supplies as much as any tank could.
Nothing is fetched: you enter the monthly averages for your area from your national weather service, which keeps the result tied to the place that matters - your roof.
How to use it
- Enter the roof's plan area in square metres - the footprint, not the sloping surface - counting only the parts whose gutters feed the tank.
- Choose the roof surface to set a typical runoff coefficient, and lower it if you divert a first flush or have leaky gutters.
- Paste twelve monthly rainfall averages in millimetres, January to December, from the nearest weather station's climate normals.
- Enter the water you will draw from the tank per day - garden, toilets, laundry or whole-house use.
- Optionally enter the tank you have or plan to see how it performs: how much of your demand it meets, and how much overflows in wet months.
Reading the results
Annual yield is the most the roof can deliver in an average year. If it is less than your annual use, no tank can close the gap; the best-possible-supply figure says how much of the demand can be met.
The recommended tank is the smallest (rounded to your chosen step) that supplies as much as an unlimited tank would. Bigger tanks beyond it only hold water you never use.
Overflow in wet months is normal. Shortfalls show when you will need a top-up supply with that tank.
Worked example: one wet season and one dry season
One millimetre of rain on one square metre is one litre. A 100 m² metal roof with a runoff coefficient of 0.95 collects 100 x 50 x 0.95 = 4,750 litres from a 50 mm month.
Now take a simplified climate: 200 mm a month from January to June and none from July to December, on 100 m² with a coefficient of 1. Each wet month brings 20,000 litres, and the household uses 10,000 litres a month. The year's harvest (120,000 L) exactly equals the year's use.
In the wet half the tank must bank 10,000 litres a month so that the six dry months, which need 6 x 10,000 = 60,000 litres, are covered. The recommended tank is 60,000 litres. A 30,000-litre tank supplies 90,000 litres (75%), overflows 30,000 in the wet season and runs short by 30,000 in the dry one.
Formulas and scoring rules
- Monthly yield
litres = roof area (m²) x rainfall (mm) x runoff coefficient1 mm on 1 m² = 1 L.- Monthly balance
used = min(demand, stored + collected); stored = min(tank, stored + collected - used); the rest overflowsRun for two years from empty and the second year reported, so the result does not depend on starting full.- Recommended size
smallest multiple of the step for which annual supply equals the supply of an unlimited tank- Runoff coefficients
metal 0.95; tile 0.90; concrete 0.85; asphalt 0.85; gravel 0.70; green roof 0.30Typical values; losses to first-flush diversion reduce them further.
Limitations: what the result does not prove
- Averages hide dry years. A tank sized on average rainfall will run short in a drought year; add margin, or repeat the calculation with a dry year's monthly figures.
- Monthly steps assume rain and use are spread evenly through each month. In reality a month's rain may fall in two storms, so small tanks overflow more than the model shows. Keep at least one to three weeks of use in storage.
- Water quality is outside the calculation. Drinking use needs filtration and disinfection and may be regulated; check local health and plumbing rules.
- Some places restrict or permit rainwater collection. Check local regulations before installing.
Privacy: where your data goes
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Standards and sources
- Texas Water Development Board - Rainwater Harvesting Manual - checked 19 Sep 2026
- US EPA - Soak up the rain: rain barrels
Frequently asked questions
How much rainwater can I collect from my roof?
Multiply the roof's plan area in square metres by the rainfall in millimetres and by a runoff coefficient of about 0.8-0.95. A 100 m² roof in a place with 800 mm a year can collect roughly 100 x 800 x 0.9 = 72,000 litres in an average year.
What size rainwater tank do I need?
It depends on how uneven the rain is. Where it rains every month, a tank holding a few weeks of use may be enough; where there is a long dry season, the tank must hold most of the dry season's demand. Enter your monthly rainfall and use to find the size for your climate.
What is a runoff coefficient?
It is the share of the rain landing on a roof that actually reaches the tank. Smooth metal roofs lose little (0.9-0.95); tiles and concrete a bit more; gravel and planted roofs much more. Diverting the first flush of each storm for cleaner water lowers it further.
Should I use the roof's sloping area or its footprint?
Use the footprint (plan area). Rain falls roughly vertically, so a steep roof catches no more rain than a flat one over the same ground; the sloping surface area would overstate the harvest.
Where do I find monthly rainfall for my area?
National weather services publish climate averages (normals), usually 30-year monthly means for each station. Choose the nearest station with a similar altitude, and consider also running a dry year's figures to see how the tank copes.
Why is there overflow even when I run short later?
Because the tank fills up in the wettest months and cannot store more. Water that overflows in July is not available in December. A bigger tank captures more of it, up to the recommended size; beyond that there is no extra water left to catch.
Last reviewed by the A2Z.Tools team against the sources listed above.
