Flagstaff, AZ rainwater: 17,978 gallons off a roof
Flagstaff, AZ: gallons off the roof each year
17,978gallons a year
That is a 1,500 sq ft asphalt roof under Flagstaff, AZ's normal 22.6 inches of rain, about 795 gallons for every inch that falls.
Flagstaff, AZ has gaps between storms long enough to drain a single barrel. Around 275 gallons of storage catches most of what is worth catching.
Rainfall: NOAA 1991-2020 climate normals, the Flagstaff 4.0 E station (2.4 mi away). Flagstaff, AZ ranks 1st wettest of the 44 cities covered in Arizona.
Month by month in Flagstaff
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 2.24 | 1,780 | |
| Feb | 2.11 | 1,677 | |
| Mar | 2.27 | 1,804 | |
| Apr | 1.34 | 1,065 | 375 |
| May | 0.75 | 596 | 449 |
| Jun | 0.37 | 294 | 496 |
| Jul | 2.44 | 1,939 | 238 |
| Aug | 3.73 | 2,965 | 77 |
| Sep | 1.83 | 1,454 | 314 |
| Oct | 1.67 | 1,327 | 334 |
| Nov | 1.55 | 1,232 | |
| Dec | 2.32 | 1,844 |
Aug is the wettest month (3.73 in, 2,965 gal off the roof); Jun the driest (0.37 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
Which tank size is worth it in Flagstaff
Best fit: a 275-gallon tote or tank, which delivers about 2,001 gallons to the garden in a normal year (87.6% of its need).
If you set one up, link barrels with a short hose at the base so they fill and drain together, and check the overflow points away from the house.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 982 | 16,322 |
| 110 gal | 1,448 | 15,855 |
| 165 gal | 1,723 | 15,580 |
| 275 gal | 2,001 | 15,302 |
| 550 gal | 2,076 | 15,228 |
| 1,000 gal | 2,076 | 15,228 |
| 1,500 gal | 2,076 | 15,228 |
| 2,500 gal | 2,076 | 15,228 |
| 5,000 gal | 2,076 | 15,228 |
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 Arizona?
Legal. Any rainwater that falls on your property may be collected for later use, in rain barrels or cisterns fed from rooftop runoff. No state storage cap stated; check city rules (some, e.g. Tucson/Flagstaff, have their own requirements or incentives).
Checked against the current source on 2026-11-06. Source