Rain Barrel Calculator
Enter your roof area and local rainfall to see how many gallons you could harvest, then check that against your barrel capacity to see what you’d capture and what would overflow.
Harvest estimate
How the estimate works
One inch of rain falling on one square foot of roof is a fixed volume of water — about 0.623 gallons, from the simple geometry of a cubic foot holding 7.48 gallons and a foot containing 12 inches. Multiply that constant by your roof’s footprint area and the rainfall depth, and you get the total water that landed on the roof during that rain event or period.
Not all of it reaches your barrel, which is what the runoff coefficient accounts for. Once you have a harvest estimate, comparing it against your total barrel capacity shows how much you’d actually store and how much would spill over — useful for deciding whether one barrel is enough or whether a second, linked barrel or a larger cistern makes more sense for your roof and climate.
Frequently Asked Questions
Where does the 0.623 gallons-per-square-foot figure come from?
It's a unit conversion, not an assumption: one cubic foot holds 7.48 gallons, and one inch is 1/12 of a foot. So one inch of rain falling on one square foot of roof is 7.48 ÷ 12 ≈ 0.623 gallons. Multiply that by your roof area and the rainfall depth to get the raw volume of water that fell on the roof.
What roof area should I enter?
Use the roof's horizontal footprint — length times width as seen from above — not the larger sloped surface area, and not the whole house if only one downspout feeds your barrel. Most calculators, including this one, use the footprint because that's the figure gutter and downspout sizing guides use too.
What is a runoff coefficient and why isn't it 1.0?
Not every drop that falls on the roof reaches the barrel — some evaporates, some splashes past the gutter in heavy rain, and a little clings to the roof surface. A runoff coefficient between 0.75 and 0.9 is commonly used for typical asphalt-shingle or metal roofs to account for that loss; smoother metal roofs tend toward the higher end.
Why would water overflow even with several barrels?
A single inch of rain on an average roof is often several hundred gallons — far more than one 55-gallon barrel holds. Overflow simply means the harvest exceeded your total storage; a first-flush diverter and an overflow hose routed away from the foundation are standard ways to handle it safely.
Planning estimate only. Actual harvest varies with roof material, gutter condition, and storm intensity; check local rules on rainwater storage, overflow, and mosquito control before installing a system.