Water hardness in California
California has a median ambient water hardness of 156 mg/L, which is 9.1 grains per gallon and classes as hard. That is the middle of 107,626 samples across 58 counties.
This is ambient water, not tap water. These figures come from samples taken from wells, streams and springs by USGS, EPA and state agencies. Municipal utilities treat, blend and sometimes soften water before it reaches a tap, so a city supply can be considerably softer than the ground beneath it. Treat this as a good guide for a private well and an approximate one for city water. Your utility's annual water quality report gives the real number for a municipal tap, and a test strip gives it for yours.
Every monitored county in California
Ranked hardest first. The middle 50% column shows how much the samples within a county vary, which matters more than the median where a county spans two geologies.
| County | mg/L | gpg | Class | Middle 50% | Samples |
|---|---|---|---|---|---|
| San Luis Obispo County | 617 | 36 | Very hard | 425 to 916 | 1,060 |
| Santa Barbara County | 519 | 30.3 | Very hard | 350 to 800 | 8,267 |
| San Benito County | 500 | 29.2 | Very hard | 129 to 644 | 245 |
| Ventura County | 474 | 27.7 | Very hard | 358 to 710 | 1,432 |
| Imperial County | 370 | 21.6 | Very hard | 293 to 580 | 4,357 |
| San Diego County | 305 | 17.8 | Very hard | 210 to 530 | 4,102 |
| Solano County | 260 | 15.2 | Very hard | 180 to 380 | 274 |
| San Francisco County | 251 | 14.7 | Very hard | 225 to 276 | 315 |
| Santa Cruz County | 246 | 14.4 | Very hard | 150 to 380 | 1,195 |
| Alameda County | 240 | 14 | Very hard | 190 to 330 | 6,470 |
| Stanislaus County | 236 | 13.8 | Very hard | 130 to 411 | 3,305 |
| Monterey County | 226 | 13.2 | Very hard | 160 to 340 | 915 |
| Orange County | 220 | 12.9 | Very hard | 140 to 360 | 1,966 |
| Riverside County | 212 | 12.4 | Very hard | 130 to 340 | 6,566 |
| Los Angeles County | 180 | 10.5 | Hard | 100 to 283 | 4,170 |
| Colusa County | 180 | 10.5 | Hard | 110 to 300 | 756 |
| Yolo County | 170 | 9.9 | Hard | 130 to 250 | 1,200 |
| Santa Clara County | 170 | 9.9 | Hard | 137 to 230 | 1,810 |
| Contra Costa County | 170 | 9.9 | Hard | 90 to 350 | 670 |
| Inyo County | 170 | 9.9 | Hard | 70 to 693 | 551 |
| Merced County | 160 | 9.3 | Hard | 73 to 340 | 2,713 |
| San Bernardino County | 151 | 8.8 | Hard | 80 to 270 | 11,441 |
| Glenn County | 140 | 8.2 | Hard | 55 to 220 | 897 |
| San Joaquin County | 130 | 7.6 | Hard | 76 to 200 | 3,693 |
| Lake County | 130 | 7.6 | Hard | 110 to 160 | 911 |
| Kern County | 128 | 7.5 | Hard | 54 to 245 | 3,405 |
| Kings County | 128 | 7.5 | Hard | 52 to 560 | 355 |
| Fresno County | 120 | 7 | Moderately hard | 21 to 440 | 3,974 |
| Sonoma County | 117 | 6.8 | Moderately hard | 100 to 130 | 1,371 |
| Napa County | 109 | 6.4 | Moderately hard | 68 to 160 | 783 |
| Amador County | 107 | 6.3 | Moderately hard | 72 to 182 | 24 |
| Calaveras County | 98 | 5.7 | Moderately hard | 62 to 1,760 | 251 |
| Madera County | 93 | 5.4 | Moderately hard | 55 to 175 | 437 |
| Marin County | 86 | 5 | Moderately hard | 66 to 122 | 212 |
| Tehama County | 85 | 5 | Moderately hard | 50 to 140 | 2,057 |
| Mendocino County | 78 | 4.6 | Moderately hard | 57 to 100 | 1,430 |
| Siskiyou County | 77 | 4.5 | Moderately hard | 54 to 140 | 1,116 |
| Humboldt County | 74 | 4.3 | Moderately hard | 55 to 110 | 2,516 |
| Sutter County | 74 | 4.3 | Moderately hard | 56 to 150 | 1,301 |
| Lassen County | 67 | 3.9 | Moderately hard | 40 to 81 | 425 |
| Plumas County | 65 | 3.8 | Moderately hard | 49 to 93 | 183 |
| San Mateo County | 65 | 3.8 | Moderately hard | 58 to 70 | 655 |
| Yuba County | 63 | 3.7 | Moderately hard | 38 to 120 | 432 |
| Del Norte County | 61 | 3.6 | Moderately hard | 46 to 72 | 855 |
| Mono County | 60 | 3.5 | Soft | 40 to 83 | 1,527 |
| Modoc County | 59 | 3.4 | Soft | 44 to 82 | 445 |
| Sacramento County | 53 | 3.1 | Soft | 32 to 71 | 2,858 |
| Butte County | 50 | 2.9 | Soft | 38 to 75 | 1,326 |
| Trinity County | 46 | 2.7 | Soft | 43 to 66 | 233 |
| Shasta County | 44 | 2.6 | Soft | 35 to 56 | 3,085 |
| Nevada County | 36 | 2.1 | Soft | 31 to 46 | 1,499 |
| Placer County | 35 | 2 | Soft | 33 to 69 | 1,030 |
| Sierra County | 34 | 2 | Soft | 23 to 52 | 21 |
| Tulare County | 30 | 1.8 | Soft | 5 to 76 | 1,789 |
| Alpine County | 30 | 1.8 | Soft | 23 to 37 | 239 |
| El Dorado County | 14 | 0.8 | Soft | 8 to 32 | 919 |
| Mariposa County | 7 | 0.4 | Soft | 4 to 12 | 1,182 |
| Tuolumne County | 3 | 0.2 | Soft | 2 to 10 | 410 |
Source: Water Quality Portal (USGS, EPA and state agencies), waterqualitydata.us, retrieved 2026-09-19. 2,904,346 samples across 3,110 counties.
What this means for a household in California
At 9.1 grains per gallon, a family of four would need a softener of about 24,000 grains to regenerate roughly weekly, using around 374 lb of salt a year. Size it for your own household.
57 of these counties include well samples, so the figures are a reasonable guide for private well water there. Municipal supplies in the same counties may be softer, because utilities often blend or treat.
Hardness follows geology rather than borders. Where a county sits on limestone or dolomite the water is hard; on granite, basalt or sandstone it is soft. That is why neighbouring counties in this table can differ by a factor of several.