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Calculate water volume collected from rainfall for rain barrels and irrigation planning.
Rainfall volume is the total amount of water collected over a given area from a measured rainfall depth, useful for rain harvesting, irrigation planning, and stormwater management.
| Description | Amount | Volume |
|---|---|---|
| Light rain | 0.1 in / 2.5 mm | ~1.5 gal per 100 sq ft |
| Moderate rain | 0.5 in / 12.7 mm | ~3.1 gal per 100 sq ft |
| Heavy rain | 1.0 in / 25.4 mm | ~62.3 gal per 100 sq ft |
| Very heavy rain | 2.0 in / 50.8 mm | ~124.6 gal per 100 sq ft |
| Extreme event | 4.0 in / 101.6 mm | ~249.2 gal per 100 sq ft |
Formula
Volume (gal) = Rainfall (in) × Area (sq ft) × 144 / 231Volume = Total water collected in US gallons
Rainfall = Precipitation depth in inches
Area = Collection surface area in square feet
144 = Square inches per square foot
231 = Cubic inches per US gallon
Worked Example
1 inch of rain on 1,000 sq ft roof
Did you know? A 2,000 sq ft roof can collect over 1,200 gallons from just 1 inch of rain — enough to fill 24 standard rain barrels (source: EPA WaterSense).
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