Free planning tool
Rainwater potential calculator
Estimate collection potential for a rainfall period. Use the horizontal footprint draining into the system, not the sloping roof surface. The result is collected volume before tank overflow and demand.
How the result is worked out
Metric: litres = catchment area (m²) × rainfall (mm) × efficiency. Imperial inputs are converted to metric using 1 ft = 0.3048 m and 1 in = 25.4 mm. We also show US gallons, using 3.785411784 litres per US gallon.
What to keep in mind
A potential-volume calculation is not a tank-size design or a guarantee of supply. Seasonal rainfall, first-flush losses, overflow, treatment and local rules matter. Collected rainwater is not automatically safe to drink.
Read the related guide: Estimate rainwater collection from an A-frame roof →