Rainwater Collection Calculator

Estimate roof runoff, rain-barrel equivalents and storage overflow for a rainfall event.

Your roof and rainfall

Use the horizontal area draining to your collection system. For a rectangular catchment, length × width: 40 ft × 25 ft = 1,000 sq ft. Do not multiply by roof slope.

Enter the total for one event. Zero is allowed; this is not an annual forecast.

80% is an example estimate you can change. It allows for first flush, splash, gutter losses, evaporation and imperfect capture.

Enter space available before the rain, not total capacity if partly full. 55 gallons is one empty example barrel; enter 0 for no storage.

How it works

Theoretical US gallons = horizontal roof catchment area × rainfall in inches × 0.623. Estimated runoff = theoretical volume × collection efficiency ÷ 100. Even a modest roof covers many square feet, so a single inch of rain can produce hundreds of gallons.

One square foot covered by one inch of water is 144 cubic inches. Dividing by 231 cubic inches per US gallon gives about 0.623 gallons. This rounded conversion and an estimated efficiency do not predict an exact measured yield.

Retained water is the smaller of available runoff and empty storage. Overflow is the remainder. The 55-gallon barrel equivalent rounds up to hold the calculated event. This assumes no storage drawdown during rain.

Assumptions and useful information

Use the roof’s footprint as seen from above, including only the parts whose gutters drain to your collection system. Include contributing overhangs; do not use the larger sloped roof surface. Roof pitch does not increase the horizontal rainfall catchment. Add separate contributing rectangles if needed.

Efficiency is a rough assumption. Actual yield varies with roof material, first-flush diversion, rainfall intensity, gutter design and maintenance. This event calculator does not size gutters, overflow plumbing or determine annual supply reliability.

For non-potable uses, consider watering ornamental plants. Collected roof runoff should not be assumed safe to drink without appropriate treatment and testing. Keep it separate from drinking-water plumbing. Check local authorities for collection, storage and use rules; requirements vary.

Plan a safe overflow route away from foundations, and maintain covers/screens and the collection system. Available storage means empty volume before rainfall; actual usable barrel capacity may be below its nominal rating.

References: Texas Water Development Board collection-volume FAQ, rainwater manual and catchment-area explanation, and CDC rainwater health guidance.

Estimate only. Check your catchment, storage, drainage and local requirements before installing a collection system.