Gilroy, CA rainwater: 15,713 gallons off a roof
How much rain a roof in Gilroy, CA collects
15,713gallons a year
That is a 1,500 sq ft asphalt roof under Gilroy, CA's normal 19.8 inches of rain, about 795 gallons for every inch that falls.
Gilroy, CA's rain arrives in a wet season and stops, so storage is what matters. It takes a cistern of 2,500 gallons to carry spring rain into the dry months.
Rainfall: NOAA 1991-2020 climate normals, the Gilroy station (1.4 mi away). Gilroy, CA ranks 90th wettest of the 376 cities covered in California.
Month by month in Gilroy
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 4.55 | 3,616 | |
| Feb | 4.27 | 3,394 | |
| Mar | 2.59 | 2,059 | |
| Apr | 1.30 | 1,033 | 380 |
| May | 0.56 | 445 | 473 |
| Jun | 0.15 | 119 | 524 |
| Jul | 0.00 | 0 | 542 |
| Aug | 0.03 | 24 | 539 |
| Sep | 0.04 | 32 | 537 |
| Oct | 0.85 | 676 | 436 |
| Nov | 1.74 | 1,383 | |
| Dec | 3.69 | 2,933 |
Jan is the wettest month (4.55 in, 3,616 gal off the roof); Jul the driest (0.00 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 Gilroy?
Best fit: a 2,500-gallon cistern, which delivers about 3,350 gallons to the garden in a normal year (97.6% 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 | 385 | 14,628 |
| 110 gal | 712 | 14,302 |
| 165 gal | 885 | 14,128 |
| 275 gal | 1,151 | 13,862 |
| 550 gal | 1,431 | 13,583 |
| 1,000 gal | 1,881 | 13,133 |
| 1,500 gal | 2,381 | 12,633 |
| 2,500 gal | 3,350 | 11,664 |
| 5,000 gal | 3,350 | 11,664 |
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 California?
Legal. Use of rainwater collected from rooftops does not require a water right permit (Rainwater Capture Act of 2012, Water Code 10570 et seq.). The Act does not change existing water rights; local building/plumbing codes still apply to installed systems.
Checked against the current source on 2026-11-06. Source