Calculate how much rainwater you can collect from your roof. Get annual gallons, monthly averages, barrel or cistern sizing, and estimated water bill savings.
Gallons collected per year at common roof sizes and rainfall levels.
| Roof (sqft) | 15"/yr (dry) | 37"/yr (moderate) | 50"/yr (wet) |
|---|---|---|---|
| 1,000 | 7,476 | 18,441 | 24,920 |
| 1,500 | 11,214 | 27,662 | 37,380 |
| 2,000 | 14,952 | 36,882 | 49,840 |
| 2,500 | 18,690 | 46,103 | 62,300 |
A 1,500-square-foot roof in Seattle (37 inches of rain) at 80% collection efficiency produces about 27,662 gallons per year — that's 2,305 gallons per month. At $4 per 1,000 gallons, that's $111 in annual water value. Each inch of rainfall produces 747 gallons from your roof.
Every 1 inch of rain on 1 square foot of roof produces 0.623 gallons. A 1,500-square-foot roof in an area with 37 inches of annual rainfall can collect about 27,662 gallons per year at 80% efficiency. Even a small 500-square-foot roof section can produce over 9,000 gallons annually in moderate rainfall areas.
A standard 55-gallon rain barrel fills up quickly — a 1,000-square-foot roof produces about 498 gallons from just 1 inch of rain (at 80% efficiency). For serious collection, consider a 275-gallon IBC tote or a larger cistern. Multiple connected barrels work well for moderate use like garden watering.
Rainwater collection is legal in most US states, though some states have restrictions on volume or require permits. Colorado, for example, limits collection to two 55-gallon barrels per household. Check your state and local regulations before installing a large collection system.
At the US average water rate of about $4 per 1,000 gallons, collecting 20,000 gallons per year saves roughly $80. In areas with higher water rates ($8–$12 per 1,000 gallons), annual savings can exceed $200. The biggest savings come from reducing irrigation water use during summer months.
Collection efficiency accounts for water lost to evaporation, splash, first-flush diversion, and gutter overflow. A well-designed system with proper gutters achieves 80–90% efficiency. Systems with first-flush diverters (which discard the first few gallons to remove roof debris) typically run 75–85% efficient.
Rainwater can be made potable with proper filtration and treatment (sediment filter, carbon filter, UV sterilization), but most residential systems are used for irrigation, toilet flushing, and laundry. Using rainwater for gardens and landscaping requires no treatment. Check local health codes for indoor non-potable use requirements.
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