Seattle, WA water hardness

Tap water hardness
31mg/L
1.8gpg
SoftReports range 31–53

Reported by Seattle Public Utilities · October 2025

WA typical
36 mg/L
EPA health violations since 2016
0
Data
Utility report

Tap water in Seattle has about 31 mg/L of hardness, which is 1.8 grains per gallon. On the USGS scale that counts as soft. The number comes from Seattle Public Utilities, dated October 2025. Reports for this system disagree, with values from 31 to 53 mg/L.

Seattle on the hardness scale, with nearby cities
060120180240300360+WA typical 36Kenmore: 25 mg/LKenmore 25Lake Forest Park: 30 mg/LLake Forest Park 30White Center: 31 mg/LWhite Center 31Shoreline: 31 mg/LShoreline 31Burien: 33 mg/LBurien 33Kirkland: 34 mg/LKirkland 34Bellevue: 34 mg/LBellevue 34Bryn Mawr-Skyway: 35 mg/LBryn Mawr-Skyway 35Mercer Island: 39 mg/LMercer Island 39Bainbridge Island: 70 mg/LBainbridge Island 70Seattle 31mg/L as calcium carbonate060120180240300360+WA 36White Center: 31 mg/LWhite Cent. 31Shoreline: 31 mg/LShoreline 31Kirkland: 34 mg/LKirkland 34Bellevue: 34 mg/LBellevue 34Mercer Island: 39 mg/LMercer Isl. 39Bainbridge Island: 70 mg/LBainbridge. 7031 mg/Lmg/L as calcium carbonate
Soft 0–60 mg/LModerately hard 61–120 mg/LHard 121–180 mg/LVery hard 181+ mg/L

At 31 mg/L, Seattle is close to the typical Washington figure of 36 mg/L. Of the 10 nearest cities we track, 2 have softer water and 6 have harder. The range nearby runs from 25 mg/L in Kenmore to 70 in Bainbridge Island.

What 31 mg/L means at home

Soap and shampoo lather with very little product. Kettles and showerheads stay mostly free of white crust.

Some people notice that soap feels slow to rinse off in soft water. That slippery feel is normal and harmless.

Hardness is not a health problem, and EPA sets no limit for it. It is a matter of scale and soap.

What to do about it

Nothing, as far as hardness goes. At 31 mg/L a softener would have almost nothing to remove.

Carbon filter

If the water tastes of chlorine, a pitcher or faucet carbon filter takes care of that. It does not change hardness.

EPA record for Seattle's water system

Seattle Public Utilities · PWSID WA5377050

No health-based violations since 2016

Serves about 1,161,961 people from surface water.

Health-based
0
Monitoring or reporting
0
Open now
0

Health-based means a contaminant above its legal limit or a missed treatment step. Monitoring and reporting violations are late or missing tests and notices. Source: EPA ECHO, 2026Q3 export, violations with periods starting 2016 or later. Full EPA record.

Questions

Is Seattle water hard?

No. It measures about 31 mg/L (1.8 grains per gallon), which the USGS classes as soft.

What is the water hardness in Seattle in grains per gallon?

About 1.8 gpg. One grain per gallon equals 17.1 mg/L, so 31 mg/L divided by 17.1 gives 1.8.

Has the Seattle water system had EPA violations?

EPA records show no health-based violations for Seattle Public Utilities since 2016.

Nearby cities

All 145 cities in Washington

Hardness data: TapWaterData US Water Hardness (utility report, October 12, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.