How hard is the water in San Mateo County, CA?
20.2 grains per gallon
A softener will pay its way here.
The median hardness measured in San Mateo County, CA is 347 mg/L as calcium carbonate, which is 20.2 gpg. That is very hard on the USGS scale. It comes from 44 samples at 38 sites between 1990 and 2022, drawn from groundwater wells, which is what most taps and private wells in the county draw on.
How much it varies here
Half the sites came in between 11.4 and 27.3 gpg. The whole middle of that range gives the same answer, so a test of your own tap is unlikely to change it. Confidence in the county figure: high (how it is measured).
What to do next
At 20.2 gpg a softener pays its way here. Before you buy, check the quoted unit's grain capacity against the sizing table below, and ask whether it regenerates on demand, which uses less salt than a timer. Call your utility for its own report if you are on city water.
More on sizing a softener for your household.
From 5.5 to 206.2 gpg
The softest single sample on record in San Mateo County was 94.3 mg/L and the hardest 3,530 mg/L, a 37-fold gap. The top quarter of sites read above 27.3 gpg and the bottom quarter below 11.4 gpg.
Wells 20.2 gpg, rivers and lakes 9.9 gpg
44 groundwater samples from 38 wells put the median at 346.5 mg/L. 520 surface samples from 16 sites put it at 169 mg/L. In San Mateo County the wells come out 10.3 gpg harder. Groundwater sits in contact with rock for years and picks up calcium and magnesium as it goes, so wells usually read harder. A utility here that draws from the river can deliver noticeably different water from a private well next door.
What a softener would cost to run
For a household of 3, a 32,000-grain unit covers a week between regenerations. Expect about 442 lb of salt a year (12 bags of 40 lb, about $80), and roughly 2,346 gallons of regeneration water. Installed, most units run $1,000 to $4,500; spread over 15 years with salt, that is about $180 a year.
Sizing at 20.2 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 1,212 | 24,000 | 147 lb |
| 2 | 2,424 | 24,000 | 295 lb |
| 4 | 4,848 | 40,000 | 590 lb |
| 6 | 7,272 | 64,000 | 885 lb |
San Mateo County against the rest of California
Among the 58 California counties with data, San Mateo County is number 6 of 58 counting from the hardest. Across the whole country it is harder than 84% of counties.
Softest in California: Sierra County at 1.7 gpg. Hardest: Calaveras County at 98.3 gpg. Closest to San Mateo County: Contra Costa County (21.7), Orange County (18.4), Stanislaus County (16.5).