Rainwater Harvesting Calculator
Estimate how much rainwater you can collect from a roof area and annual rainfall.
Accounts for evaporation, first-flush diversion, and other losses (typically 80–90%).
Annual Harvestable Water
Volume in Cubic Meters
Calculation Steps
Understanding Rainwater Harvesting
Rainwater harvesting is the practice of collecting and storing rain that falls on a roof or other catchment surface for later use. It reduces reliance on mains water, lowers utility bills, and helps manage stormwater runoff. The amount you can collect depends on the size of your catchment area, how much rain falls in your region, and how efficiently your system captures and stores the water.
The Harvesting Formula
Because 1 millimeter of rain falling on 1 square meter of surface equals exactly 1 liter of water, the harvestable volume is calculated as:
Liters = Area (m²) × Rainfall (mm) × (Efficiency ÷ 100)
Divide the result by 1,000 to express the volume in cubic meters (1 m³ = 1,000 liters).
Tips for Maximizing Collection
- Use a first-flush diverter to discard the dirtiest initial runoff
- Keep gutters and downpipes clear of leaves and debris
- Install mesh screens to prevent mosquitoes and contaminants
- Smooth, non-porous roof materials (metal, tile) collect more efficiently than rough surfaces
- Size your storage tank to your local rainfall pattern and demand
Note: This calculator provides estimates based on average annual rainfall and a single efficiency factor. Actual collection varies with seasonal rainfall distribution, roof material, evaporation, overflow losses, and storage capacity. Always treat harvested rainwater appropriately before any potable use.