Rain Barrel Calculator: how much water your roof sheds
Free rain barrel and roof runoff calculator. Enter the roof area that drains to one downspout and the rainfall, and get gallons (square feet times inches times 0.623), barrels filled, overflow, and what a full barrel weighs. With the setup cautions from Nebraska, Virginia Tech, and Clemson Extension.
The formula
Roof area in square feet times rainfall in inches times 0.623 equals gallons (Nebraska Extension, Clemson HGIC). A 1,200 square foot roof yields about 750 gallons from a 1 inch rain, and the quarter of it that drains to one downspout yields about 185 gallons, so a 55 gallon barrel overflows fast.
Cautions
- A full 55 gallon barrel weighs over 450 pounds. Set it on a level, sturdy base (Nebraska, Clemson, Virginia Tech).
- Route overflow to a planted area or another barrel, away from the foundation (Nebraska).
- Screen openings and use the water within about a week. Mosquitoes need 7 to 10 days of standing water (Nebraska, Virginia Tech).
- Nebraska Extension does not recommend rain barrel water for fruit and vegetable gardens.