How to use this calculator
- Enter or choose roof catchment area (plan).
- Enter or choose average annual rainfall.
- Enter or choose runoff coefficient.
- Enter or choose collection efficiency.
- Enter or choose daily water demand.
- The tank size for the dry period updates as you type. Use Copy result or Share to keep it.
Method
Yield (L) = Area (m²) × Rainfall (mm) × Runoff × Efficiency; Tank = Daily demand × Dry days
- Area
- roof plan area
- Runoff
- runoff coefficient of the roof surface
- Efficiency
- collection and filter efficiency
Worked example
- Yield = 100 × 800 × 0.8 × 0.9 = 57,600 L a year.
- Demand = 150 × 365 = 54,750 L, so rain can meet all of it on average.
- Tank = 150 L × 30 days = 4,500 L.
A 100 m² roof in an 800 mm rainfall area yields about 57,600 L a year; a 4,500 L tank covers 30 dry days at 150 L a day.
What the result means
If the yearly yield is below demand, plan for a mains top-up. A monthly water balance with local rainfall records gives a better tank size than the dry-period rule.
Assumptions and limits
- Rainfall is an annual average; yields vary year to year.
- The tank is full at the start of the dry period.
- Seasonal patterns and overflow are not modelled.
- Water quality and treatment for drinking use are outside the scope of this calculator.
Questions
Do I use the roof’s sloped area?
No. Use the plan (footprint) area: rain falls vertically, so the horizontal projection is what catches it.
Why is my tank smaller than the yearly yield?
The tank only needs to hold water between refills, not a year’s supply.
Please note: Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials.