Boulder, CO rainwater: 15,642 gallons off a roof
Boulder, CO: gallons off the roof each year
15,642gallons a year
That is a 1,500 sq ft asphalt roof under Boulder, CO's normal 19.7 inches of rain, about 795 gallons for every inch that falls.
In Boulder, 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 Boulder 1.3 Nw station (0.9 mi away). Boulder, CO ranks 4th wettest of the 53 cities covered in Colorado.
Month by month in Boulder
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 0.76 | 604 | |
| Feb | 0.88 | 699 | |
| Mar | 1.91 | 1,518 | |
| Apr | 2.96 | 2,353 | 173 |
| May | 3.02 | 2,400 | 166 |
| Jun | 1.79 | 1,423 | 319 |
| Jul | 1.76 | 1,399 | 323 |
| Aug | 1.42 | 1,129 | 365 |
| Sep | 2.04 | 1,621 | 288 |
| Oct | 1.55 | 1,232 | 349 |
| Nov | 0.88 | 699 | |
| Dec | 0.71 | 564 |
May is the wettest month (3.02 in, 2,400 gal off the roof); Dec the driest (0.71 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 Boulder
Best fit: 2 linked 55-gallon barrels, which delivers about 1,718 gallons to the garden in a normal year (86.6% of its need).
Before you buy, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Boulder's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 1,268 | 13,699 |
| 110 gal | 1,718 | 13,249 |
| 165 gal (over cap) | 1,718 | 13,249 |
| 275 gal (over cap) | 1,718 | 13,249 |
| 550 gal (over cap) | 1,718 | 13,249 |
| 1,000 gal (over cap) | 1,718 | 13,249 |
| 1,500 gal (over cap) | 1,718 | 13,249 |
| 2,500 gal (over cap) | 1,718 | 13,249 |
| 5,000 gal (over cap) | 1,718 | 13,249 |
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