Phoenix, AZ rainwater: 6,732 gallons off a roof
Phoenix, AZ: gallons off the roof each year
6,732gallons a year
That is a 1,500 sq ft asphalt roof under Phoenix, AZ's normal 8.5 inches of rain, about 795 gallons for every inch that falls.
Phoenix, AZ's rain arrives in a wet season and stops, so storage is what matters. It takes a cistern of 1,500 gallons to carry spring rain into the dry months.
Rainfall: NOAA 1991-2020 climate normals, the Glendale 2.5 Ene station (3.4 mi away). Phoenix, AZ ranks 30th wettest of the 44 cities covered in Arizona.
Month by month in Phoenix
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 1.13 | 898 | |
| Feb | 1.16 | 922 | |
| Mar | 1.09 | 866 | |
| Apr | 0.30 | 238 | 505 |
| May | 0.15 | 119 | 524 |
| Jun | 0.04 | 32 | 537 |
| Jul | 0.73 | 580 | 451 |
| Aug | 1.01 | 803 | 416 |
| Sep | 0.64 | 509 | 463 |
| Oct | 0.69 | 548 | 456 |
| Nov | 0.67 | 533 | |
| Dec | 0.86 | 684 |
Feb is the wettest month (1.16 in, 922 gal off the roof); Jun the driest (0.04 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 Phoenix
Best fit: a 1,500-gallon cistern, which delivers about 3,235 gallons to the garden in a normal year (96.5% of its need).
If you set one up, ask about a rebate first: cistern-sized storage costs real money, and a buried or tall tank may need a permit. Place it where a full tank (about 8.3 lb a gallon) sits on firm, level ground.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 495 | 5,831 |
| 110 gal | 983 | 5,343 |
| 165 gal | 1,423 | 4,903 |
| 275 gal | 1,988 | 4,338 |
| 550 gal | 2,357 | 3,969 |
| 1,000 gal | 2,807 | 3,519 |
| 1,500 gal | 3,235 | 3,091 |
| 2,500 gal | 3,235 | 3,091 |
| 5,000 gal | 3,235 | 1,182 |
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