Understanding the Rain Barrel Calculator
A rain barrel calculator estimates how much rainwater you can collect from your roof during a rainfall, helping you size a barrel or cistern and understand the water savings for your garden. It is a simple tool for anyone interested in conserving water.
Roof area and rainfall
Collection potential depends on your roof’s catchment area and how much rain falls. A surprising amount adds up: even a modest rainfall over an average roof yields hundreds of gallons. The calculator multiplies area by rainfall (with a small efficiency loss) to show how quickly a barrel fills.
Putting collected water to use
Harvested rainwater is ideal for watering gardens and lawns, reducing both your water bill and stormwater runoff. Barrels fill faster than many expect, so a single barrel may overflow in a good storm — linking multiple barrels or adding an overflow diverter helps. Use collected water for plants rather than drinking unless properly treated.
Tips & things to know
- •Even small rainfalls collect hundreds of gallons off an average roof.
- •Link barrels or add an overflow diverter for big storms.
- •Use harvested water for plants, not drinking, unless treated.