Rainwater Harvesting Calculator
Rainwater you can collect from a roof in a year, and the average per day.
Results
Volume
64,260
L/a
Volume
64.26
m³/a
Volume
176.1
L/d
Volume
5.36
m³
What this tool does
One millimetre of rain on one square metre is exactly one litre, which makes the arithmetic simple — the losses are what need judgement. Tiles and slate shed around 0.8 to 0.9 of what falls, flat gravel roofs much less, and the first flush diverter and filter take a further share.
Formula
volume = catchment area × rainfall × runoff coefficient
Variables
| Symbol | Meaning | Unit |
|---|---|---|
a | Roof catchment area | m² |
r | Annual rainfall | mm/a |
rc | Runoff coefficient | — |
fe | Filter efficiency | % |
V | Volume | L/a |
M3 | Volume | m³/a |
MD | Volume | L/d |
MM | Volume | m³ |
Worked example
- Roof catchment area120 m²
- Annual rainfall700 mm/a
- Runoff coefficient0.85
- Filter efficiency90 %
- Volume64,260 L/a
- Volume64.26 m³/a
- Volume176.1 L/d
- Volume5.36 m³
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.