Direct answer: Multiply your roof's footprint in square feet by 0.6, then multiply by inches of rain. A 1,000 sq ft roof in a 30-inch rainfall area collects about 18,000 gallons a year in theory. Real yield is usually 70–85% of that after first-flush diversion, evaporation, and overflow.
The formula
Gallons = roof footprint (sq ft) × 0.6 × rainfall (inches)
Why 0.6? One cubic foot of water is 7.48 gallons. One inch of rain on one square foot is 1/12 cubic foot, which is 0.623 gallons. The 0.6 constant just makes the arithmetic easy.
Use the footprint, not the sloped surface area. A 30x40 ft house with a gable roof has a 1,200 sq ft footprint, and the rain that lands on both slopes is what the gutters catch.
Worked example
A 1,200 sq ft roof in a region with 35 inches of annual rain:
- 1,200 × 0.6 = 720 gallons per inch
- 720 × 35 = 25,200 gallons per year
- At 80% realistic capture: about 20,000 gallons
For scale, a typical US household uses 8,000–10,000 gallons a month indoors. Roof catchment is serious garden water and emergency supply, not whole-house independence on its own.
What reduces the real yield
- First-flush diversion. The first 10–20 gallons per 1,000 sq ft is dumped. In a region with many small storms this can cost 20–30% of the total.
- Gutter losses. Overshoot at corners, splash-out, and leaks.
- Storage size. If the tank is full, the rest goes down the overflow. A 275-gallon tank in a 40-inch rainfall area is full after half an inch of rain.
- Roof material. Rough surfaces, snow, and steep pitches shed less cleanly than smooth metal.
- Winter. Snow sits on the roof until melt, and frozen pipes stop collection entirely.
Sizing the tank to the storm
A practical rule: one inch of rain on 1,000 sq ft delivers 600 gallons, so a 275-gallon IBC tote fills after 0.46 inches. If your area routinely gets one-inch storms, plan for multiple tanks or a cistern, otherwise most of the rain is lost to overflow.
Frequently asked questions
Does roof pitch change the calculation?
No. Rain falls vertically onto the footprint. Pitch affects flow speed and gutter sizing, not total volume.
How many gallons is an inch of rain on 2,000 square feet?
About 1,200 gallons. Multiply 2,000 by 0.6.
Can I collect from a metal roof?
Yes — smooth metal is one of the best surfaces, provided it is unpainted or coated with a potable-water-safe finish and has no lead flashing.
For the full build sequence, see the DIY rainwater harvesting guide. The water chapter sits alongside storage and power in our review of The Self-Sufficient Backyard.
Frequently asked questions
Does roof pitch change the calculation?
No. Rain falls vertically onto the footprint. Pitch affects flow speed and gutter sizing, not total volume.
How many gallons is an inch of rain on 2,000 square feet?
About 1,200 gallons — multiply 2,000 by 0.6.
Can I collect from a metal roof?
Yes. Smooth metal is one of the best surfaces, provided it is unpainted or coated with a potable-water-safe finish and has no lead flashing.