Water hardness in Washington
Washington has a median ambient water hardness of 32 mg/L, which is 1.9 grains per gallon and classes as soft. That is the middle of 84,140 samples across 39 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 Washington
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 |
|---|---|---|---|---|---|
| Island County | 190 | 11.1 | Very hard | 117 to 390 | 150 |
| San Juan County | 167 | 9.8 | Hard | 78 to 280 | 92 |
| Grant County | 160 | 9.3 | Hard | 78 to 220 | 1,635 |
| Lincoln County | 127 | 7.4 | Hard | 85 to 150 | 338 |
| Adams County | 110 | 6.4 | Moderately hard | 51 to 230 | 319 |
| Franklin County | 110 | 6.4 | Moderately hard | 58 to 220 | 946 |
| Benton County | 94 | 5.5 | Moderately hard | 68 to 120 | 1,539 |
| Ferry County | 93 | 5.4 | Moderately hard | 62 to 140 | 366 |
| Spokane County | 88 | 5.1 | Moderately hard | 32 to 140 | 1,512 |
| Okanogan County | 87 | 5.1 | Moderately hard | 40 to 130 | 1,450 |
| Whitman County | 81 | 4.7 | Moderately hard | 28 to 120 | 1,373 |
| Garfield County | 80 | 4.7 | Moderately hard | 54 to 100 | 60 |
| Douglas County | 79 | 4.6 | Moderately hard | 65 to 170 | 116 |
| Pend Oreille County | 79 | 4.6 | Moderately hard | 73 to 85 | 193 |
| Yakima County | 78 | 4.6 | Moderately hard | 39 to 137 | 2,683 |
| Stevens County | 75 | 4.4 | Moderately hard | 60 to 110 | 1,336 |
| Kitsap County | 67 | 3.9 | Moderately hard | 43 to 91 | 566 |
| Asotin County | 58 | 3.4 | Soft | 52 to 106 | 56 |
| Clark County | 57 | 3.3 | Soft | 19 to 73 | 398 |
| Walla Walla County | 54 | 3.2 | Soft | 34 to 90 | 865 |
| Columbia County | 44 | 2.6 | Soft | 35 to 60 | 92 |
| Thurston County | 44 | 2.6 | Soft | 30 to 58 | 861 |
| Pierce County | 41 | 2.4 | Soft | 24 to 56 | 1,824 |
| King County | 40 | 2.3 | Soft | 25 to 63 | 2,276 |
| Kittitas County | 39 | 2.3 | Soft | 25 to 67 | 709 |
| Clallam County | 38 | 2.2 | Soft | 30 to 43 | 376 |
| Mason County | 35 | 2 | Soft | 28 to 43 | 489 |
| Klickitat County | 34 | 2 | Soft | 27 to 76 | 434 |
| Whatcom County | 33 | 1.9 | Soft | 20 to 69 | 2,554 |
| Jefferson County | 31 | 1.8 | Soft | 25 to 38 | 529 |
| Snohomish County | 26 | 1.5 | Soft | 16 to 46 | 53,212 |
| Grays Harbor County | 24 | 1.4 | Soft | 19 to 30 | 1,072 |
| Skagit County | 24 | 1.4 | Soft | 19 to 30 | 955 |
| Skamania County | 23 | 1.3 | Soft | 19 to 56 | 331 |
| Chelan County | 22 | 1.3 | Soft | 18 to 35 | 322 |
| Lewis County | 21 | 1.2 | Soft | 15 to 31 | 774 |
| Pacific County | 21 | 1.2 | Soft | 16 to 43 | 218 |
| Wahkiakum County | 19 | 1.1 | Soft | 15 to 23 | 72 |
| Cowlitz County | 17 | 1 | Soft | 13 to 30 | 1,047 |
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 Washington
At 1.9 grains per gallon, a family of four would need a softener of about 16,000 grains to regenerate roughly weekly, using around 77 lb of salt a year. Size it for your own household.
39 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.