A roof collects far more water than most people expect. The Texas Water Development Board's Texas Manual on Rainwater Harvesting puts the theoretical yield at about 0.62 gallons per square foot of collection surface per inch of rain. One inch on a 2,500 ft² roof is about 1,560 gallons before losses. In practice some water is lost to first-flush diversion, evaporation, splash-out and gutter overshoot in hard rain, and the manual notes that most installers assume an efficiency of 75–90%. This rainwater harvesting calculator applies the 0.62 rule and your efficiency to a single storm, a month or a year. It also shows how many barrels or what size tank would hold the water, and how much to divert as first flush.
Planning where the water goes? The raised bed soil calculator sizes garden beds. Before you build, check local rules: rainwater collection is encouraged in many places but regulated in some.
How to use the rainwater harvesting calculator
- Enter the roof footprint, the area seen from directly above including overhangs. Pitch doesn’t matter, because rain falls vertically onto the footprint.
- Only count roof sections whose gutters drain to your collection point.
- Enter rainfall for the period you care about: one storm, an average month or a year. Use local climate normals from NOAA or your weather service.
- Keep the efficiency at 75–90%. Use the lower end for rough roofs such as wood shakes or gravel.
- Enter your container size to see how many you’d need to capture it all.
Formula
One inch of water over one square foot is 144 in³, and a US gallon is 231 in³, so 144 ÷ 231 = 0.623 gal. TWDB rounds this to 0.62. In metric it is even simpler: 1 mm of rain on 1 m² is exactly 1 liter. The first-flush figure uses TWDB's rule of thumb of diverting a minimum of 10 gallons per 1,000 ft² of collection surface.
Worked example
A 2,500 ft² roof footprint, 1 inch of rain, 85% efficiency, 55-gallon barrels.
- Theory: 2,500 × 1 × 0.623 = 1,558 gal.
- At 85%: 1,325 gallons (5,014 L).
- Barrels: 1,325 ÷ 55 = 24.1, so 25 barrels to hold it all, which shows why serious systems use tanks.
- First flush: at least 25 gallons per storm.
TWDB's manual uses the same method with its 0.62 figure: 1.97 inches of rain × 2,500 ft² × 0.62 × 0.85 gives about 2,595 gallons for that month.
Gallons per inch of rain at 85% efficiency
| Roof footprint | Gallons per inch |
|---|---|
| 500 ft² | 265 |
| 1,000 ft² | 530 |
| 1,500 ft² | 795 |
| 2,000 ft² | 1,060 |
| 2,500 ft² | 1,325 |
| 3,000 ft² | 1,590 |
Sizing storage
You rarely need to store a whole year's rain. Storage is sized to bridge the gap between wet and dry months, and TWDB's manual walks through a month-by-month water balance: collected supply minus demand, carried forward in the tank. For garden use, a few barrels at each downspout can capture the first part of most storms. For household supply, the manual describes larger tanks, filtration and disinfection. Keep tanks covered and screened against mosquitoes, opaque to stop algae, and fitted with an overflow directed away from foundations.
Roofing material matters for water quality. TWDB notes that smoother roofs shed more water, that metal roofing such as Galvalume is commonly used for rainwater harvesting, and that lead must not be used as gutter solder in potable systems. Follow TWDB guidance or local health rules before drinking harvested rainwater.
Caveats and assumptions
- Results are estimates; real collection depends on storm intensity, gutters, filters and tank overflow.
- Use footprint area, not roof surface area.
- Water rights and plumbing rules on rainwater collection vary by state and country; check local regulations.
- Harvested rainwater is not safe to drink without proper treatment.
Frequently asked questions
How many gallons of water come off a roof per inch of rain?
About 0.62 gallons per square foot of footprint in theory, so 620 gallons per 1,000 ft². Expect 75–90% of that in practice.
How much rain will fill a 55-gallon barrel?
From a 100 ft² section of roof at 85% efficiency, about 1.04 inches of rain. A 500 ft² section fills it with about 0.21 inch.
Do I use the roof’s sloped area?
No. Use the footprint, the area seen from above. The amount of rain a roof intercepts depends on its horizontal area, not its slope.
How much should I divert as first flush?
TWDB cites a rule of thumb of at least 10 gallons per 1,000 ft² of roof, more where roofs are dusty or under trees.
How do I convert in metric?
One millimeter of rain on one square meter is one liter. Multiply footprint m² × rainfall mm × efficiency.
Sources
- Texas Water Development Board — The Texas Manual on Rainwater Harvesting (3rd ed.) — 0.62 gal/ft²/in, 75–90% efficiency, first-flush rule of thumb, footprint method
- NOAA National Centers for Environmental Information — U.S. Climate Normals — average monthly and annual rainfall
- NIST Handbook 44, Appendix C — 1 US gallon = 231 in³
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.