Last updated: June 1, 2026

Rain Barrel Calculator

Calculate how much rainwater you can collect from your roof and how many barrels you need.

Roof Catchment Area (sq ft)

Monthly Rainfall (inches)

Barrel Size (gallons)

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320 × 100 Mobile Banner

336 × 280 Large Rectangle

300 × 250 Medium Rectangle

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About 0.6 gallons per square foot of roof area per inch of rain. A 1,500 sq ft roof can collect ~900 gallons from 1 inch of rain.

Standard rain barrels hold 55 gallons. Larger cisterns hold 100-500+ gallons.