Rainwater Harvesting Calculator
Estimate how much rainwater a roof or catchment can collect.
Please note: An estimate based on the figures you entered. Real-world results vary with conditions this calculation cannot see, so treat it as a starting point rather than a final answer.
What the Rainwater Harvesting Calculator does
Rainwater collection is area multiplied by rainfall depth, reduced by what the surface loses. The conversion is exact and simple — a millimetre on a square metre is a litre — so the only real variables are the runoff coefficient and how much you divert as first flush.
Formula
Litres = Area (m²) × Rainfall (mm) × Runoff coefficientNet = Gross × (1 − First flush share)1 mm on 1 m² = 1 litre
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Catchment area | m² | Yes | Accepts more than 0. |
| Surface | one of 5 options | Yes | — |
| Rainfall | mm | Yes | Accepts more than 0. |
| Rainfall period | one of 3 options | Yes | — |
| Daily water demand | litres | Optional | Accepts 0 or more. |
| First flush diverted | % | Optional | Water discarded to wash dirt off the roof. |
How to use it
- Choose Surface and Rainfall period.
- Enter Catchment area and Rainfall.
- Optionally add Daily water demand and First flush diverted.
- Select Calculate.
Worked example
A 200 m² metal roof with 800 mm of annual rainfall and 5% first flush diverted.
- Area
- 200 m²
- Surface
- Corrugated metal
- Rainfall
- 800 mm/year
- First flush
- 5%
200 × 800 × 0.9 = 144,000 litres gross, and 136,800 after first flush — about 375 litres a day averaged over the year.
Frequently asked questions
How much water can a roof collect?
Roughly 0.9 litres per square metre per millimetre of rain on metal. A 200 m² roof in an 800 mm climate yields around 144,000 litres a year.
What size tank do I need?
Size it against the longest dry spell rather than the annual total. Around three weeks of demand is a common starting point.