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Every inch of rain on a 1,000 sq ft roof yields about 623 gallons — far more than most people expect, and enough to overflow a single rain barrel in one storm. Enter the roof area draining to your downspout and the rainfall to see what you can collect and how much storage you’d need.
The formula
Gallons = roof area (sq ft) × rainfall (in) × 0.623 × runoff coefficient. In metric: liters = roof area (m²) × rainfall (mm) × runoff coefficient. The 0.623 comes from one inch of water over one square foot (144 cubic inches) divided by 231 cubic inches per gallon.
| Roof material | Runoff coefficient |
|---|---|
| Metal | 0.95 |
| Asphalt shingle, clay or concrete tile | 0.90 |
| Flat membrane / concrete | 0.85 |
| Tar and gravel | 0.70 |
How much can you collect?
| Roof area | 1 in of rain | 30 in per year |
|---|---|---|
| 250 sq ft (one downspout) | 140 gal | 4,200 gal |
| 500 sq ft | 280 gal | 8,400 gal |
| 1,000 sq ft | 561 gal | 16,800 gal |
| 1,500 sq ft | 841 gal | 25,200 gal |
Figures use a 0.9 coefficient (asphalt shingle).
FAQ
How do I measure roof area?
Use the footprint — length × width as seen from above, including overhangs — not the sloped surface. For a single downspout, estimate the share of the roof that drains to it.
Is it legal to collect rainwater?
In most places, yes, and many cities offer rebates for rain barrels. A few US states limit volume or require permits, so check your state’s rules before installing large tanks.
Can I drink collected rainwater?
Not without proper filtration and disinfection. Untreated roof water is fine for gardens, lawns and washing tools.
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