Rain Barrel Calculator
Rainwater you can capture from a roof.
Rain you can capture per year
37,380
gallons (assuming you can store it all)
Per inch of rain
935
gallons
For a 1″ storm
935
gallons
Barrels needed
17
at 55 gal each
Roof catchment formula: roof sq ft × rainfall inches × 0.623 = gallons (assumes all the runoff reaches the barrel). Real-world capture is closer to 75 to 90 percent. Some states regulate or ban rain barrels - check your state water laws.
About
Enter your roof area, annual rainfall, and barrel size. The tool returns gallons captured per inch of rain, annual capture, and how many barrels you need to catch a single storm.
How to use
- Enter roof area in sq ft.
- Enter annual rainfall.
- Pick a barrel size.
FAQ
Are rain barrels legal?+
Mostly yes, but a few western US states (notably parts of Colorado historically) have restrictions because of water rights law. Check your state water board.