Rainwater harvesting calculator
How many gallons of rain your roof collects in a storm or a year, and how many rain barrels that fills.
- Barrels filled
How to use it
Enter the roof area that drains to your downspout or barrel, and the rainfall in inches: one storm, a month, or your area's yearly total. Some water is always lost to splashing, evaporation and overflowing gutters, so set the share actually collected; 75 to 90% is typical. The calculator shows the gallons collected and how many barrels that fills.
Use the roof's footprint (the area it covers seen from above), not its sloped surface area. Rain falls straight down, so a steep roof catches no more than a flat one of the same footprint.
How it's calculated
1 inch of rain on 1 square foot is 144 cubic inches of water. A US gallon is 231 cubic inches, so:
Gallons = roof footprint (sq ft) × rainfall (in) × 0.623 × share collected
| Roof footprint | 1 in of rain (100%) | 1 in of rain (80%) |
|---|---|---|
| 500 sq ft | 312 gal | 249 gal |
| 1,000 sq ft | 623 gal | 499 gal |
| 1,500 sq ft | 935 gal | 748 gal |
| 2,000 sq ft | 1,247 gal | 997 gal |
A single 55-gallon barrel fills from less than a tenth of an inch of rain on a 1,000 sq ft roof, which is why many people link several barrels or add an overflow.
Tips
- Check local rules. Most US states allow rain barrels, but a few limit how much you can collect or how you use it.
- Not for drinking without proper treatment. Barrel water is fine for gardens and lawns.
- Fit a screen and a lid to keep out debris and mosquitoes, and add an overflow pointed away from your foundation.