Rainwater Harvest Calculator
Estimate how much rainwater you can collect from your roof. Find the right tank size and see potential water savings for your homestead.
Collection Calculator
This is the building's footprint (length × width), not the total sloped roof area. For a rectangular house, multiply length by width. For example, a 30×50 ft house = 1,500 sq ft.
US national average is approximately 38 inches. See sidebar for major city rainfall data.
85%
Good
Efficiency is determined by roof type. Smoother surfaces collect more water; porous or rough surfaces lose more.
Average US household uses about 300 gallons/day. For garden-only use, estimate 50–100 gallons/day during growing season.
If you already have tanks, enter the total capacity to see how many days of supply they provide.
Collection Estimates
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Annual Collection (gallons)
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Monthly Average (gallons)
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Recommended Tank Size
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Avg Daily Collection (gallons)
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Days of Supply
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Municipal Water Cost Saved
How It Works
Our rainwater harvest calculator estimates how much water you can collect based on your roof area, local rainfall, and roof material. One inch of rain on 1,000 square feet of roof yields about 623 gallons of water — more than most people realize.
The 0.623 Conversion Factor
The magic number 0.623 comes from basic geometry: 1 inch of rain falling on 1 square foot of surface equals 1/12 of a cubic foot of water. One cubic foot contains 7.48 gallons. So: 1/12 × 7.48 = 0.623 gallons per square foot per inch of rain.
Formula
Annual Collection (gallons) = Roof Area (sq ft) × Annual Rainfall (in) × 0.623 × Collection Efficiency
Monthly Average = Annual Collection ÷ 12
Recommended Tank = Daily Usage × 30 days (one month of supply)
Days of Supply = Existing Tank Size ÷ Daily Usage
Cost Saved = Annual Collection ÷ 748 × Municipal Water Rate ($/100 cu ft)
(assumes avg US rate of $3.50 per 100 cubic feet / 748 gallons)
Pro Tip: A 1,500 sq ft roof in an area with 40 inches of annual rain can collect over 30,000 gallons per year. Even with a 50% green roof, that's still 15,000+ gallons — enough to water a large garden all season.