About the Rainwater Tank Size Calculator
Storage tank size for rainwater harvesting, balancing supply against demand.
Also known as: water butt size, rainwater storage sizing, how big a rainwater tank, tank size for rain.
How to use this calculator
- Enter roof area, annual rainfall, daily water demand, longest dry spell and the other fields.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
tank = min(dry-spell demand, one month of average supply × 1.5)
Worked example
Using the values pre-filled in the calculator above:
- Roof area = 100 m²
- Annual rainfall = 800 mm
- Daily water demand = 150 litres
- Longest dry spell = 21
- Runoff coefficient = 0.8
the calculator returns:
- Recommended tank size: 5,000 litres
- Calculated requirement: 3,150 litres
- Annual supply: 64 m³
- Annual demand: 54.75 m³