Longmont, CO rainwater: 12,780 gallons off a roof
How much rain a roof in Longmont, CO collects
12,780gallons a year
That is a 1,500 sq ft asphalt roof under Longmont, CO's normal 16.1 inches of rain, about 795 gallons for every inch that falls.
In Longmont, 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 Longmont 1.8 E station (1.2 mi away). Longmont, CO ranks 30th wettest of the 53 cities covered in Colorado.
Month by month in Longmont
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 0.56 | 445 | |
| Feb | 0.58 | 461 | |
| Mar | 1.30 | 1,033 | |
| Apr | 2.05 | 1,629 | 287 |
| May | 2.53 | 2,011 | 227 |
| Jun | 1.75 | 1,391 | 324 |
| Jul | 1.66 | 1,319 | 335 |
| Aug | 1.82 | 1,447 | 315 |
| Sep | 1.29 | 1,025 | 382 |
| Oct | 1.21 | 962 | 391 |
| Nov | 0.75 | 596 | |
| Dec | 0.58 | 461 |
May is the wettest month (2.53 in, 2,011 gal off the roof); Jan the driest (0.56 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
Barrel or cistern in Longmont?
Best fit: 2 linked 55-gallon barrels, which delivers about 1,957 gallons to the garden in a normal year (86.5% of its need).
If you set one up, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Longmont's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 1,440 | 10,740 |
| 110 gal | 1,957 | 10,224 |
| 165 gal (over cap) | 1,964 | 10,216 |
| 275 gal (over cap) | 1,964 | 10,216 |
| 550 gal (over cap) | 1,964 | 10,216 |
| 1,000 gal (over cap) | 1,964 | 10,216 |
| 1,500 gal (over cap) | 1,964 | 10,216 |
| 2,500 gal (over cap) | 1,964 | 10,216 |
| 5,000 gal (over cap) | 1,964 | 10,216 |
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