Rain barrel calculator

Enter the size of the roof area that drains to your downspout and how much rain falls. You'll get the gallons you can collect and how many barrels that fills.

How the math works

One inch of rain on one square foot of roof is about 0.62 gallons. Multiply that by the roof area and the rainfall, then reduce it for water lost to splashing, evaporation and gutter overflow. A 1,000 square foot roof in a 1 inch storm sheds about 620 gallons, or about 500 gallons at 80% efficiency.

Which part of the roof counts?

Only the section that drains to the downspout feeding your barrel. If your house has four downspouts, each one typically drains about a quarter of the roof. Use the roof's footprint, the flat area it covers, rather than the length of the slope.

Common questions

Why does one storm fill so many barrels?

Roofs collect a lot of water fast. A single 55 gallon barrel overflows in a moderate rain on most roofs, which is why many people link two or three barrels together or add an overflow hose that leads away from the foundation.

Is collecting rainwater legal?

Rain barrels are allowed in every US state, but a few states limit how much you can store, and some offer rebates for buying barrels. Check your state and city rules.

Can I drink rain barrel water?

No. Roof runoff picks up dirt, droppings and material from the roof. Use it for gardens, lawns and washing, and cover barrels to keep mosquitoes out.

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