How hard is the water in King County, WA?
3.6 grains per gallon
Optional: a softener helps, but plenty of homes skip it.
The median hardness measured in King County, WA is 61 mg/L as calcium carbonate, which is 3.6 gpg. That is moderately hard on the USGS scale. It comes from 133 samples at 132 sites between 1991 and 2017, 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 2.6 and 4.5 gpg. That spread crosses a decision line, so some homes here land on a different answer. A $10 test strip on your own tap settles it. Confidence in the county figure: high (how it is measured).
What to do next
At 3.6 gpg this is your call. Look for white film on glasses and crust on the shower head: if you see little of either, you can live without one. If you do want one, a 24,000-grain unit is enough, so ask for that size rather than a bigger one. Because King County straddles a decision line, check your own tap with a $10 test strip before you spend anything.
More on when a softener is not worth it.
From 0.1 to 14.4 gpg
The softest single sample on record in King County was 2.2 mg/L and the hardest 246 mg/L, a 112-fold gap. The top quarter of sites read above 4.5 gpg and the bottom quarter below 2.6 gpg.
Wells 3.6 gpg, rivers and lakes 3.7 gpg
133 groundwater samples from 132 wells put the median at 60.8 mg/L. 577 surface samples from 32 sites put it at 62.9 mg/L. In King County the rivers and lakes come out 0.1 gpg harder. Here the surface water reads harder than the wells, which often points to streams fed by limestone springs or irrigation return flow. The two sources are close, so the county figure should hold whichever one your tap is on.
What a softener would cost to run
For a household of 3, a 24,000-grain unit covers a week between regenerations. Expect about 78 lb of salt a year (2 bags of 40 lb, about $14), 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 $114 a year.
Sizing at 3.6 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 216 | 24,000 | 26 lb |
| 2 | 432 | 24,000 | 53 lb |
| 4 | 864 | 24,000 | 105 lb |
| 6 | 1,296 | 24,000 | 158 lb |
King County against the rest of Washington
Among the 38 Washington counties with data, King County is number 22 of 38 counting from the hardest. Across the whole country it is harder than 25% of counties.
Softest in Washington: Chelan County at 0.6 gpg. Hardest: Island County at 20.7 gpg. Closest to King County: Klickitat County (3.7), Wahkiakum County (3.3), Benton County (3.3).