How hard is the water in San Luis Obispo County, CA?
30.7 grains per gallon
A softener will pay its way here.
The median hardness measured in San Luis Obispo County, CA is 525 mg/L as calcium carbonate, which is 30.7 gpg. That is very hard on the USGS scale. It comes from 347 samples at 65 sites between 1990 and 2023, 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 20.7 and 51.3 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 30.7 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 4.1 to 88.2 gpg
The softest single sample on record in San Luis Obispo County was 70.4 mg/L and the hardest 1,510 mg/L, a 21-fold gap. The top quarter of sites read above 51.3 gpg and the bottom quarter below 20.7 gpg.
Wells 30.7 gpg, rivers and lakes 23.2 gpg
347 groundwater samples from 65 wells put the median at 525 mg/L. 63 surface samples from 13 sites put it at 396.5 mg/L. In San Luis Obispo County the wells come out 7.5 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 40,000-grain unit covers a week between regenerations. Expect about 670 lb of salt a year (17 bags of 40 lb, about $121), 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 $221 a year.
Sizing at 30.7 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 1,842 | 24,000 | 224 lb |
| 2 | 3,684 | 32,000 | 448 lb |
| 4 | 7,368 | 64,000 | 896 lb |
| 6 | 11,052 | 80,000 | 1,345 lb |
San Luis Obispo County against the rest of California
Among the 58 California counties with data, San Luis Obispo County is number 3 of 58 counting from the hardest. Across the whole country it is harder than 95% of counties.
Softest in California: Sierra County at 1.7 gpg. Hardest: Calaveras County at 98.3 gpg. Closest to San Luis Obispo County: Ventura County (31.5), Santa Barbara County (28.1), Contra Costa County (21.7).