How it works
The formula
gallons = roof area (sq ft) × rain (in.) × 0.623 × share collected
rain to fill the barrels = barrel gallons ÷ (roof area × 0.623 × share collected)
rain to fill the barrels = barrel gallons ÷ (roof area × 0.623 × share collected)
- 0.623 gallons fall on each square foot of roof per inch of rain (1/12 cu ft × 7.48 gal). The Texas Water Development Board's Rainwater Harvesting Manual works its examples at 85% collected, after splash-over, first flush and evaporation.
- Rainfall is NOAA's 1991-2020 normals (rain plus melted snow) from the station nearest your county's center of population. The days with heavy rain come from the same normals.
- Runoff is what reaches the downspout, not what you keep. Once the barrels are full the rest overflows, so how much you save depends on their size and how often you use the water between rains. The "days that can fill them" line counts days with at least that much rain, from NOAA's day counts at 0.10, 0.25, 0.50 and 1.00 in.; a few lighter days can fill them too.
- Measure the roof by its footprint, not its sloped surface: rain falls straight down, so a steep roof catches no more than a flat one over the same ground.
Worked example
1,000 sq ft of roof in ZIP 10001, New York County, NY
- 1,000 × 0.623 × 85% = 530 gallons per inch of rain
- 49.5 in. a year × 530 = 26,239 gallons
- 2 barrels of 55 gal fill with 0.21 in. of rain
Roof runoff from 1,000 sq ft, by city
| City | Rain a year | Runoff a year |
|---|---|---|
| New York | 49.5 in. | 26,239 |
| Los Angeles | 14.3 in. | 7,551 |
| Chicago | 34.4 in. | 18,238 |
| Houston | 60.5 in. | 32,052 |
| Phoenix | 7.2 in. | 3,826 |
| Philadelphia | 50.7 in. | 26,859 |
| San Antonio | 34.0 in. | 18,000 |
| San Diego | 11.5 in. | 6,083 |
| Dallas | 38.3 in. | 20,305 |
| Austin | 36.3 in. | 19,208 |
| Jacksonville | 47.6 in. | 25,222 |
| Columbus | 44.7 in. | 23,664 |
| Charlotte | 43.6 in. | 23,102 |
| Seattle | 36.5 in. | 19,335 |
| Denver | 17.9 in. | 9,479 |
| Washington, DC | 45.6 in. | 24,157 |
| Boston | 43.6 in. | 23,097 |
| Nashville | 50.5 in. | 26,764 |
| Portland, OR | 44.4 in. | 23,537 |
| Las Vegas | 4.2 in. | 2,215 |
| Atlanta | 54.1 in. | 28,671 |
| Minneapolis | 34.7 in. | 18,376 |
| Miami | 67.4 in. | 35,719 |
| Salt Lake City | 20.6 in. | 10,899 |
| Albuquerque | 10.3 in. | 5,458 |
At 85% collected.
Questions people ask
How much water does 1 inch of rain give from a roof?
About 0.62 gallons per square foot, so 623 gallons from 1,000 sq ft before losses, or about 530 gallons with 85% collected. That fills 9 55 gallon barrels and most of another.
How many rain barrels do I need?
One barrel on a downspout fills in a light shower. Size by how you'll use it: enough to water between rains, which the monthly table shows. Most homes link two to four barrels per downspout; past that, a larger tank is cheaper per gallon.
Can I drink rainwater from a barrel?
Not without treatment. Roof runoff picks up debris, bird droppings and roofing material; use barrel water for the garden, or follow your health department's guidance for a filtered, disinfected system.