This rainwater harvesting calculator estimates how much water your roof sheds — and how much of it you can actually capture — from the roof’s footprint area, the rainfall amount and your system’s collection efficiency.
To turn this into a yearly estimate, enter your area’s annual rainfall as the rainfall figure. A year of rain on an average roof is measured in tens of thousands of gallons — storage, not supply, is usually the binding constraint.
The 0.623 Rule
One inch of rain on one square foot of roof is 0.623 gallons — a conversion worth memorizing (it comes from a square foot × inch of water = 144 cubic inches ≈ 0.623 gal). So: gallons = roof footprint (sq ft) × rainfall (in) × 0.623 × efficiency. Use the footprint — the horizontal shadow of the roof — not the sloped shingle area, because rain falls vertically: a steeply pitched roof catches no more rain than the flat outline beneath it. This calculator is part of our Ecology Calculators collection.
Worked Example (Verified)
A 1,000 sq ft roof in a 1-inch rain:
- Gross runoff: 1000 × 1 × 0.623 = 623 gallons — one modest storm delivers more water than most homes use in two days
- At a realistic 85% collection efficiency: 530 gallons harvestable — about 9.6 standard rain barrels from a single inch of rain
- Scale it up: 30 inches of annual rain on that roof is ~15,900 captured gallons a year — real irrigation supply, if you can store it
Where the Efficiency Goes
- Roof material: smooth metal sheds 90–95%; asphalt shingles ~85%; rough tile or gravel can drop to 70% through splash and absorption.
- First-flush diverters deliberately discard the first few gallons (the dusty, bird-seasoned wash) — quality bought with quantity.
- Gutters and screens leak, clog and overflow in downpours; the heaviest storms — the biggest supply — are exactly when capture efficiency dips.
- Storage rules reality: a 55-gallon barrel under a 1,000 sq ft roof fills in the first minutes of a real storm. Serious systems use 500–5,000-gallon cisterns, or aim barrels at modest, targeted uses like the garden beds the Mulch Calculator covers.
- Check local rules: most US states allow rain barrels freely; a few western states historically restricted collection, though laws have liberalized — verify before installing large cisterns.
Related Tools
- Plant Spacing Calculator — where the harvested water goes to work
- Ecology Calculators — the full collection
Frequently Asked Questions
How much rainwater can I collect from my roof?
Roof footprint × rainfall × 0.623 gallons, times your efficiency (~85% typical). 1,000 sq ft and 1 inch of rain yields about 530 usable gallons.
Why use the roof footprint instead of the actual roof area?
Rain falls straight down, so the catchment is the roof’s horizontal projection. The sloped surface is larger than its shadow, but it intersects the same column of falling rain.
How many rain barrels do I need?
The calculator’s third card divides your harvest by 55 gallons. For the worked example’s single inch of rain: about 10 barrels — which is why most barrel setups serve one garden zone, not the whole roof’s potential.
What efficiency should I enter?
85% is a solid default for shingle roofs with maintained gutters. Use 90–95% for metal roofs, 70–80% for rough tile, and subtract a few points if you use a first-flush diverter.
Is collected rainwater safe to drink?
Not without treatment — roof runoff carries debris, droppings and roofing chemicals. Untreated harvest is for irrigation and outdoor use; potable systems need filtration, disinfection and usually a permit.
Is this rainwater harvesting calculator free?
Yes — free, instant, no sign-up.