🌧️ Rainwater Harvest Calculator for Gardening
Estimate roof runoff captured from rainfall.
This free tool helps you turn project measurements into a practical planning estimate. Enter values in Vaelden’s interactive calculator, review the result, and confirm important assumptions against supplier data, measurements, codes or qualified advice before committing materials or money.
Open the Rainwater Harvest calculator →
How to calculate rainwater harvest
Method: Captured litres = roof area × rainfall millimetres × collection efficiency
- Use horizontal roof catchment area draining to the tank.
- Use rainfall depth for the event or period assessed.
- Reduce theoretical yield for first flush, leaks, splash, overflow and dirty catchments.
Worked example
Using these planning inputs:
- Roof catchment: 100
- Rainfall: 25 mm
- Collection efficiency: 85 %
Potential capture: 2125.00 L
Reality check: Tank sizing should use local rainfall sequences and demand, not one average event. Follow health and plumbing rules for intended water use.
Before you calculate
Use consistent units and the best measured inputs available. Real materials, equipment, site conditions and tolerances can change the result. Vaelden shows planning estimates; regulated, structural, electrical, plumbing, pressure and other safety-critical work still requires appropriate local compliance and professional advice.