How much rain your roof can collect, and the storage to hold it
Updated October 2026
Enter the roof footprint that drains to the gutters feeding your barrel or tank: the length times width of that roof section as seen from above. If only one downspout feeds the barrel, use only the area that drains to it. Rainfall can be one storm (from a rain gauge or a local weather report) or your annual average for a yearly total. The roof surface sets the runoff coefficient. Add the storage you have and your weekly watering use to see overflow, how much rain fills your tank, and how many weeks of watering it holds.
| Roof footprint | Shingle (0.85) | Metal (0.95) |
|---|---|---|
| 1,000 sq ft | 530 gal | 592 gal |
| 1,500 sq ft | 794 gal | 888 gal |
| 2,000 sq ft | 1,059 gal | 1,184 gal |
| 2,500 sq ft | 1,324 gal | 1,480 gal |
| 3,000 sq ft | 1,589 gal | 1,776 gal |
An inch of rain on a square foot is 144 cubic inches of water, and a U.S. gallon is 231 cubic inches, so each square foot yields 144 ÷ 231 = 0.623 gallons per inch. The runoff coefficient accounts for water lost to splash, evaporation and absorption: smooth metal roofs shed nearly all of it, rougher or flat roofs less. Pitch does not catch more rain, which is why you measure the footprint and not the sloped surface.
The Texas Water Development Board example: 3.5 inches × 0.623 × 2,500 square feet × 0.85 = 4,634 gallons, or 1,324 gallons for each inch. A 55-gallon barrel under that roof fills from 55 ÷ 1,324 = 0.04 inch of rain, so almost all of a real storm overflows.
| Annual rainfall | Gallons per year | 55-gallon barrels to hold it all |
|---|---|---|
| 10 in | 7,943 | 145 |
| 20 in | 15,886 | 289 |
| 30 in | 23,830 | 434 |
| 40 in | 31,773 | 578 |
| 50 in | 39,716 | 723 |
Even dry climates produce thousands of gallons a year from an ordinary roof. Because rain arrives in bursts, what you capture depends far more on storage size and how quickly you use it than on the yearly total.
Size storage to how you will use the water and route overflow away from the foundation. A first-flush diverter that discards the first runoff improves water quality but lowers the yield. Rainwater collection is legal in most states, but a few regulate it. Colorado, for example, allows a household at most two rain barrels with a combined 110 gallons, used outdoors on the property where it was collected (House Bill 16-1005, as summarized by Colorado State University Extension, 2025). Check your state’s rules before installing large cisterns, and do not drink untreated rainwater.
Measuring the sloped roof surface instead of the footprint overstates the yield. Counting the whole roof when only one downspout feeds the barrel. Letting overflow pool at the foundation. Leaving barrels open, which invites mosquitoes and debris; use screened, covered inlets. Leaving barrels full in a hard freeze, which can split them. Assuming roof water is clean: debris and bird droppings wash into it, which is one reason untreated rainwater is for irrigation, not drinking.