Size storage from the roof

Rainwater capture calculator

Translate roof area and a design storm into collectable gallons or liters with a user-set efficiency factor.

Live worksheet

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Each field explains what to measure, where to find it, and how to read the result. Change any value to update the plan instantly.

Roof footprint reaching this tanksq ftThe horizontal footprint that drains to this downspout. Use only the connected roof section, not the sloped surface area.Example: Half of a 1,000 sq ft simple roof may contribute about 500 sq ft.Rainfall in one eventinThe depth reported by a rain gauge or local weather station for the storm you are planning around.Example: Enter 1 for a one-inch rain event.Collection efficiency%The share likely to reach storage after splash, gutter, first-flush, and overflow losses.Example: 85% is a useful planning estimate when no measured system value is available.

Collectable water estimate

265US gallons

Liters1,003

55-gal barrels5

200-L tanks6

Plan screened inlets, a stable full-tank base, and overflow away from foundations. Check local rules before installation or edible-garden use.

Sources & further reading

Harvesting rainwater for use in the gardenOregon State University Extension
Soak Up the Rain: Rain BarrelsU.S. Environmental Protection Agency

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Why one inch of rain overwhelms a small barrel

Sources & further reading