Olympia, WA rainwater: 46,901 gallons off a roof
Rainwater from a roof in Olympia, WA
46,901gallons a year
That is a 1,500 sq ft asphalt roof under Olympia, WA's normal 59.0 inches of rain, about 795 gallons for every inch that falls.
Olympia, WA has gaps between storms long enough to drain a single barrel. Around 165 gallons of storage catches most of what is worth catching.
Rainfall: NOAA 1991-2020 climate normals, the Olympia 1.3 S station (1.2 mi away). Olympia, WA ranks 5th wettest of the 91 cities covered in Washington.
Month by month in Olympia
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 8.87 | 7,050 | |
| Feb | 5.74 | 4,562 | |
| Mar | 6.39 | 5,079 | |
| Apr | 4.24 | 3,370 | 14 |
| May | 2.52 | 2,003 | 228 |
| Jun | 1.85 | 1,470 | 312 |
| Jul | 0.66 | 525 | 460 |
| Aug | 1.10 | 874 | 405 |
| Sep | 2.16 | 1,717 | 273 |
| Oct | 6.19 | 4,920 | |
| Nov | 9.67 | 7,686 | |
| Dec | 9.62 | 7,646 |
Nov is the wettest month (9.67 in, 7,686 gal off the roof); Jul the driest (0.66 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 Olympia
Best fit: 3 linked 55-gallon barrels, which delivers about 1,398 gallons to the garden in a normal year (82.6% of its need).
If you buy one, 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 | 955 | 44,237 |
| 110 gal | 1,258 | 43,933 |
| 165 gal | 1,398 | 43,793 |
| 275 gal | 1,460 | 43,732 |
| 550 gal | 1,460 | 43,732 |
| 1,000 gal | 1,460 | 43,732 |
| 1,500 gal | 1,460 | 43,732 |
| 2,500 gal | 1,460 | 43,732 |
| 5,000 gal | 1,460 | 43,732 |
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 Washington?
Collecting and storing rooftop rainwater for use on site needs no water right permit, provided the roof is part of a fixed above-ground structure whose primary purpose is not rain collection. Ecology may set local restrictions on new systems if they harm instream values or existing water rights. No state storage cap in the policy.
Checked against the current source on 2026-11-06. Source