Collecting rainwater from your roof is a sustainable way to water your garden and reduce utility bills. This calculator estimates how many gallons you can collect annually based on your roof area and local rainfall.
Collection Details
sq ft
in
Scenario: You have 1,000 sq ft of roof area and your area receives 40 inches of rain per year.
Result: Annual collection: 1,000 x 40 x 0.623 = 24,920 gallons. That's about 2,077 gallons per month, or 38 fifty-five-gallon barrels spread across the year.
Multiply your roof area (sq ft) by annual rainfall (inches) by 0.623 for gallons. A 1,000 sq ft roof in an area with 30 inches of rain collects about 18,690 gallons per year. Actual yield is about 75-90% of theoretical due to evaporation and spillage.
It depends on how much water you use and how often it rains. A 55-gallon barrel fills in about 0.1 inches of rain on a 1,000 sq ft roof. Two to four barrels is a good starting point for garden watering. For serious collection, consider a larger cistern.
Laws vary by state and municipality. Most states allow residential rainwater collection with no restrictions. Colorado, Utah, and a few others have specific regulations on volume or use. Check your local water authority's rules before installing a system.
Not without treatment. Roof runoff picks up dust, bird droppings, pollen, and roofing chemicals. Use collected rainwater for gardens, lawns, car washing, and toilet flushing. If you want potable water, you'll need filtration, UV treatment, and regular testing.
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