Rain Barrel Capacity Calculator
A roof collects a surprising amount of rainwater: every inch of rain on 1,000 square feet of roof yields over 600 gallons. This calculator estimates how much rainwater your roof captures from a given rainfall, applies a collection efficiency for losses, and tells you how many rain barrels of a chosen size you would need to store it.
Rainwater collection formula
Gross gallons = roof area * rain depth * 0.623
Net gallons = gross gallons * (efficiency / 100)
Barrels = ceiling( net gallons / barrel size )
0.623 = gallons per sq ft per inch of rain
One inch of rain over one square foot is 144 cubic inches of water, which is about 0.623 US gallons (144 / 231 cubic inches per gallon). Collection efficiency accounts for splashing, first-flush diversion, evaporation, and gutter losses, commonly assumed around 75 to 90 percent.
Worked example
A 1,000 square foot roof in a 1 inch rainfall: gross = 1,000 times 1 times 0.623 = 623.00 gallons. At 85 percent efficiency, net captured = 529.55 gallons. To store it all in 55 gallon barrels you would need ceiling(529.55 / 55) = 10 barrels, which is why larger cisterns are common for full capture.
Rainwater harvesting notes
- Every inch of rain yields about 0.623 gallons per square foot of roof, or roughly 623 gallons per 1,000 square feet.
- Collection efficiency of 75 to 90 percent accounts for first-flush diversion, splash, and gutter losses.
- Roof catchment area is the footprint (plan area), not the sloped surface area.
- Check local rules: some jurisdictions regulate or restrict rainwater harvesting.
- Use captured water for irrigation; treat it before any potable use.
Rain Barrel Capacity Calculator: frequently asked questions
How much rainwater can I collect from my roof?
About 0.623 gallons per square foot of roof for every inch of rain. A 1,000 square foot roof collects roughly 623 gallons in a 1 inch rainfall before losses, or about 530 gallons at 85 percent collection efficiency.
What is the 0.623 figure?
One inch of rain falling on one square foot is 144 cubic inches of water. A US gallon is 231 cubic inches, so 144 divided by 231 is about 0.623 gallons. That constant converts roof area and rainfall depth into gallons.
Do I use the roof's flat area or sloped area?
Use the flat (plan) footprint of the roof, not the longer sloped surface. Rain falls vertically, so the catchment is the horizontal area the roof covers when viewed from above.
Is rainwater harvesting legal where I live?
It depends on your location. Many places encourage it, but some jurisdictions regulate or limit rainwater collection. Check your state and local rules before installing a system, and treat collected water before any drinking use.
Sources and methodology
- U.S. Environmental Protection Agency: Soak Up the Rain: Rain Barrels.
- The 0.623 gallons per square foot per inch factor derives from fixed unit conversions (144 cubic inches per inch-square-foot, 231 cubic inches per gallon); efficiency is a user input.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.