Rain barrel calculator
Estimate how much water runs off the roof area that drains to one downspout, and how many barrels you would need to hold it.
Calculator
Results appear here.
How to use it
- Estimate the roof area that drains to the downspout (measure the footprint plus overhangs; split complex roofs into sections).
- Enter a design rainfall — UNL uses 1 inch because about 90% of Nebraska rain events are 1 inch or less.
- Compare the result with your barrel size and plan where overflow goes.
The formula
Safety: UNL does not recommend rain-barrel water for watering fruit and vegetable gardens or any use with direct food or body contact, because roof runoff can carry bacteria and chemicals. Use it for ornamentals, and direct overflow away from foundations. Local rules on rainwater collection vary — check yours.
Gear for this job
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- Rain barrel with screen and overflow — [affiliate link placeholder]
- Barrel linking kit — [affiliate link placeholder]
- Downspout diverter — [affiliate link placeholder]
- Low-pressure soaker hose — [affiliate link placeholder]
Frequently asked questions
How much water comes off a roof in 1 inch of rain?
About 0.623 gallons per square foot. UNL's example: a 1,200 sq ft roof yields roughly 750 gallons from a one-inch rain.
Will one 55-gallon barrel be enough?
Often not. UNL shows 300 sq ft of roof producing about 185 gallons per inch of rain, so a single 55-gallon barrel overflows. Link barrels or route overflow to a rain garden.
Can I water vegetables with rain barrel water?
UNL Extension does not recommend it for fruit and vegetable gardens because of possible bacteria and roof chemicals. Use it on ornamental plants.
Sources
Last reviewed 2026-10-22. Found an error? Tell us.