Hillsborough, CA rainwater: 20,434 gallons off a roof
How much rain a roof in Hillsborough, CA collects
20,434gallons a year
That is a 1,500 sq ft asphalt roof under Hillsborough, CA's normal 25.7 inches of rain, about 795 gallons for every inch that falls.
Hillsborough, 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 San Mateo 3.4 Ssw station (2.8 mi away). Hillsborough, CA ranks 42nd wettest of the 376 cities covered in California.
Month by month in Hillsborough
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 4.65 | 3,696 | |
| Feb | 4.40 | 3,497 | |
| Mar | 4.26 | 3,386 | |
| Apr | 2.26 | 1,796 | 261 |
| May | 0.74 | 588 | 450 |
| Jun | 0.18 | 143 | 520 |
| Jul | 0.01 | 8 | 541 |
| Aug | 0.06 | 48 | 535 |
| Sep | 0.13 | 103 | 526 |
| Oct | 1.02 | 811 | 415 |
| Nov | 2.51 | 1,995 | |
| Dec | 5.49 | 4,363 |
Dec is the wettest month (5.49 in, 4,363 gal off the roof); Jul the driest (0.01 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 Hillsborough
Best fit: a 2,500-gallon cistern, which delivers about 3,149 gallons to the garden in a normal year (97.0% of its need).
If you buy one, 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 | 483 | 19,192 |
| 110 gal | 844 | 18,832 |
| 165 gal | 1,053 | 18,623 |
| 275 gal | 1,252 | 18,424 |
| 550 gal | 1,527 | 18,149 |
| 1,000 gal | 1,977 | 17,699 |
| 1,500 gal | 2,477 | 17,199 |
| 2,500 gal | 3,149 | 16,527 |
| 5,000 gal | 3,149 | 16,527 |
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