Rain Barrel Water Collection Calculator

About this calculator

Rainwater harvesting potential from a roof follows a straightforward relationship: Collected volume = Roof area × Rainfall depth × Runoff coefficient, where the runoff coefficient (typically 0.8–0.9 for standard roofing) accounts for the small fraction of rain lost to evaporation, absorption, and splash-off rather than reaching the gutter.

This is the same basic calculation used in professional rainwater harvesting system design and stormwater management, where a roof's catchment area and local rainfall data determine tank sizing, and municipal stormwater engineers use nearly the same math in reverse to estimate runoff volumes that drainage systems need to handle.

Homeowners sizing a rain barrel or cistern, gardeners planning irrigation from harvested water, and sustainability-minded builders estimating a property's water-collection potential all use this formula to translate roof size and local weather into a concrete number of liters or gallons.