🌧️ Rainwater Harvesting Calculator
Enter your roof’s catchment area, average rainfall, and runoff coefficient to estimate how many liters and gallons of rainwater you could collect over a year.
What Is Rainwater Harvesting?
Rainwater harvesting is the practice of collecting and storing rain, usually from a roof, for later use in irrigation, cleaning, or (with proper treatment) household needs. It reduces demand on municipal or well water supplies and can lessen stormwater runoff.
The Formula
This calculator uses the standard catchment formula: Volume (liters) = Roof Area (m²) × Rainfall (mm) × Runoff Coefficient. This works because one millimeter of rain falling on one square meter of surface equals one liter of water. The runoff coefficient accounts for losses from evaporation, splash, first-flush diversion, and roof material — a typical hard roof (metal, tile) is around 0.8-0.9, while a more absorbent or textured surface will be lower.
How to Use the Result
The annual harvest estimate assumes rainfall is spread evenly enough across the year to be captured effectively and that your storage capacity can hold what falls; in practice, tank size, seasonal rainfall patterns, and overflow during heavy storms will affect how much you actually use. Use this as a general planning estimate before sizing a rainwater collection system.
- For general planning and educational purposes only.
- Check local regulations and consult a professional before installing a rainwater harvesting system.