Rainwater Harvesting Calculator

Rainwater harvesting collects roof runoff for garden irrigation, toilet flushing, and laundry. UK average rainfall of 600-1,200mm annually can supply 30-50% of non-potable water needs

The calculator uses the standard published formula for rainwater harvest. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.

This calculator works with inches, litres, percentages. You can enter values in the units shown — the calculator handles all conversions internally.

The core formula is: Collection potential = Roof area (m2) x Annual rainfall (mm) x Efficiency factor (0.85). Each step in the calculation is shown so you can verify the result manually.