What this calculator does
Rainwater harvesting starts with one number: how much a roof actually yields in a year. This calculator applies the rainfall, the roof runoff coefficient and the collection efficiency, then converts that yield into a monthly average and, if you state a daily demand, a cistern size.
How to use it
- Enter the roof plan area and the annual rainfall for your location in millimetres.
- Set the runoff coefficient — 0.85 to 0.9 for a clean tile or metal roof.
- Set the collection efficiency, 85–95 % after filter and gutter losses.
- Add your daily demand and the days of autonomy you want, and the cistern size follows.
Governing formula
V_year = A · (P/1000) · C · η Where:
-
A= Roof plan area [m²] -
P= Annual rainfall [mm] -
C= Runoff coefficient [–] -
η= Collection efficiency [–]
Derived Equations:
V_tank = q_daily · n_days / 1000 Worked example — 180 m² roof with 750 mm annual rainfall
The figures below were produced by running this page's own engine with the example inputs (Roof plan area (m²), Annual rainfall (mm), Roof runoff coefficient (–), Collection efficiency (%), Daily demand (L/day), Storage autonomy (days)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Annual yield | 103.28 m³/yr |
|---|---|
| Monthly average | 8.61 m³/month |
| Daily average | 282.95 L/day |
| Cistern size for the stated demand | 1.4 m³ |
| Assessment | ≈ 103 m³/yr (9 m³/month average); a 1.4 m³ cistern covers 200 L/day for 7 days. |
- Yield = 180 × 0.75 × 0.85 × 0.90 ≈ 103 m³ per year, averaging 8.6 m³ a month.
- Against a demand of 200 L/day, the roof supplies about 283 L a day on average.
- A week of autonomy at 200 L/day needs a 1.4 m³ cistern.
Assumptions & limitations
Engineering calculation. Water-balance estimate of the annual yield from a roof area, with a cistern size derived from a stated daily demand and autonomy period.
- Uses total annual rainfall — a wet-season bias means the cistern will be full when demand is low and empty in a dry spell.
- No first-flush diversion, no seasonal rainfall distribution and no water-quality modelling.
- Potable use requires disinfection, and local rules govern whether harvested water may be used for drinking.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- WHO — Guidelines for safe use of rainwater harvesting.