About the Rainwater Harvesting Calculator
Every millimetre of rain on a square metre of roof is one litre of water, less what the roof surface and filter lose. Enter your own annual rainfall figure — this calculator never guesses at your location — and it gives the harvestable volume and what it is worth against mains water.
Also known as: roof water collection, rainwater collection calculator, how much rain can i collect, rain harvesting.
How to use this calculator
- Enter roof plan area, annual rainfall, roof surface, filter efficiency and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Open “Step-by-step” to see how the answer was worked out.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
litres = roof area (m²) × rainfall (mm) × runoff coefficient × filter efficiency
Worked example
Using the values pre-filled in the calculator above:
- Roof plan area = 100 m²
- Annual rainfall = 800 mm
- Roof surface: Tiles or metal (0.9)
- Filter efficiency = 90 %
- Mains water cost per m³ = 1.8
the calculator returns:
- Annual harvest: 64.8 m³
- In litres: 64,800
- Monthly average: 5,400 litres
- Daily average: 177.5 litres
Working
100 m² × 800 mm = 80,000 litres of rain× 0.9 runoff × 90% filter = 64,800 litres harvested