Rain Barrel Calculator
Enter the roof area feeding your downspout and how many 55-gallon barrels you run. The calculator shows the gallons one inch of rain harvests - hundreds on an ordinary roof - how fast they overflow, and the season total.
The shock is the point: a modest rain moves hundreds of gallons off your roof. The two rules that keep the barrel a blessing: a screened self-closing lid so mosquitoes cannot breed, and an overflow plan that routes water to garden beds instead of the foundation.
How to use
- Roof area is footprint, not slope - pitch does not change harvest.
- One inch of rain yields about 0.62 gallons per square foot.
- Raise the barrel on blocks for gravity pressure at the spigot.
Frequently asked questions
How much water comes off a roof in rain?
About 0.62 gallons per square foot per inch of rain - one inch on a 1,200 square foot roof is roughly 750 gallons. Even one 55-gallon barrel fills several times over in a modest shower.
Is rainwater good for the garden?
Better than most tap water - soft, neutral pH, no chlorine. Plants visibly prefer it, which is why the barrel pays for itself in plant health before the water bill notices.
Do rain barrels attract mosquitoes?
Only if the water sits open. A screened, self-closing lid makes breeding impossible - the one rule that is non-negotiable, and the overflow hose should leave the barrel area too.
Are rain barrels legal?
Most places yes; a few states carried historic restrictions that have mostly been repealed with incentives instead. Check your local rules once - the calculator handles the math, the statute needs one search.