Denver, CO rainwater: 12,208 gallons off a roof
Rainwater from a roof in Denver, CO
12,208gallons a year
That is a 1,500 sq ft asphalt roof under Denver, CO's normal 15.4 inches of rain, about 795 gallons for every inch that falls.
In Denver, CO a single barrel does most of the work. Storms come often enough that a small tank refills before it runs dry, and a bigger one would mostly sit full.
Rainfall: NOAA 1991-2020 climate normals, the Denver-Stapleton station (0.6 mi away). Denver, CO ranks 34th wettest of the 53 cities covered in Colorado.
Month by month in Denver
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 0.46 | 366 | |
| Feb | 0.53 | 421 | |
| Mar | 1.12 | 890 | |
| Apr | 1.67 | 1,327 | 334 |
| May | 2.29 | 1,820 | 257 |
| Jun | 1.68 | 1,335 | 333 |
| Jul | 2.12 | 1,685 | 278 |
| Aug | 1.83 | 1,454 | 314 |
| Sep | 1.51 | 1,200 | 354 |
| Oct | 0.98 | 779 | 420 |
| Nov | 0.70 | 556 | |
| Dec | 0.47 | 374 |
May is the wettest month (2.29 in, 1,820 gal off the roof); Jan the driest (0.46 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
What size rain barrel for Denver
Best fit: 2 linked 55-gallon barrels, which delivers about 1,979 gallons to the garden in a normal year (86.4% of its need).
If you buy one, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Denver's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 1,464 | 10,174 |
| 110 gal | 1,979 | 9,660 |
| 165 gal (over cap) | 2,007 | 9,631 |
| 275 gal (over cap) | 2,007 | 9,631 |
| 550 gal (over cap) | 2,007 | 9,631 |
| 1,000 gal (over cap) | 2,007 | 9,631 |
| 1,500 gal (over cap) | 2,007 | 9,631 |
| 2,500 gal (over cap) | 2,007 | 9,631 |
| 5,000 gal (over cap) | 2,007 | 9,631 |
Simulated storm by storm through a normal year using the station's count of days with 0.1 in or more of rain each month.
Is rainwater collection legal in Colorado?
Most households may use rain barrels: a maximum of two rain barrels with a combined storage of 110 gallons or less per household, rooftop runoff only, used outdoors on the same property (HB16-1005). Larger rooftop collection only for qualifying well-permit holders (SB09-080) or pilot projects (HB09-1129).
Checked against the current source on 2026-11-06. Source