Rain and storageTool
Rain catch calculator for your roof
A 1,000 square foot roof catches about 15,700 gallons in an average Santa Rosa year. Enter your ZIP and roof size to see yours.
What the numbers mean
Santa Rosa averages 31.5 inches of rain a year (1991 to 2020). A 1,000 square foot roof at 80% catch: 1,000 x 31.5 x 0.623 x 0.8 is about 15,700 gallons.
The ZIP box picks the nearest of six weather stations. Rain changes a lot over short distances in the hills, so treat the number as a starting point and edit it if you know better.
Wet years and dry years swing a lot. Over 1889 to 2000, the driest Santa Rosa year was 12.8 inches and the wettest was 56.3.
Use the roof footprint
Measure the flat area the roof covers on the ground, not the slope. A steep roof catches the same rain as the footprint under it.
Barrels fill fast
One inch of rain on 1,000 square feet is about 623 gallons. A 55 gallon barrel fills in the first few minutes. Plan for overflow.
Where will you keep it?
Work out how big a tank you need with the water storage tank size calculator.
The 80% catch figure is a rule of thumb, not a measured number. It covers splash, first flush and gutter loss.
Sources
- Station averages, 1991 to 2020 normals (NOAA, as shown in each city's Wikipedia climate table). See the rainfall page for each link.
- Extremes: KRIS Russian River database, Santa Rosa rain gage 1889 to 2000
- Unit math: 1 inch over 1 sq ft = 144 cubic inches; 231 cubic inches per US gallon
Checked 2026-10-11. Rules and programs change. If something here is out of date, the source above wins. Spot an error? See our corrections policy.
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