Mercer Island, WA water hardness

Tap water hardness
39mg/L
2.3gpg
Soft

Reported by Mercer Island City of · June 2025

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

The water in Mercer Island carries roughly 39 mg/L of dissolved calcium and magnesium, or 2.3 grains per gallon. That is soft water. The number comes from Mercer Island City of, dated June 2025.

Mercer Island on the hardness scale, with nearby cities
060120180240300360+WA typical 36Newcastle: 27 mg/LNewcastle 27White Center: 31 mg/LWhite Center 31Seattle: 31 mg/LSeattle 31Burien: 33 mg/LBurien 33Bellevue: 34 mg/LBellevue 34Bryn Mawr-Skyway: 35 mg/LBryn Mawr-Skyway 35Tukwila: 37 mg/LTukwila 37Issaquah: 41 mg/LIssaquah 41Renton: 49 mg/LRenton 49SeaTac: 64 mg/LSeaTac 64Mercer Island 39mg/L as calcium carbonate060120180240300360+WA 36Newcastle: 27 mg/LNewcastle 27White Center: 31 mg/LWhite Cent. 31Bellevue: 34 mg/LBellevue 34Bryn Mawr-Skyway: 35 mg/LBryn Mawr-. 35Tukwila: 37 mg/LTukwila 37Renton: 49 mg/LRenton 4939 mg/Lmg/L as calcium carbonate
Soft 0–60 mg/LModerately hard 61–120 mg/LHard 121–180 mg/LVery hard 181+ mg/L

Mercer Island sits close to the typical Washington figure of 36 mg/L. Of the 10 nearest cities we track, 7 have softer water and 3 have harder. The range nearby runs from 27 mg/L in Newcastle to 64 in SeaTac.

What 39 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 39 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 Mercer Island's water system

Mercer Island City of · PWSID WA5353640

No health-based violations since 2016

Serves about 31,057 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 Mercer Island water hard?

No. It measures about 39 mg/L (2.3 grains per gallon), which the USGS classes as soft.

What is the water hardness in Mercer Island in grains per gallon?

About 2.3 gpg. One grain per gallon equals 17.1 mg/L, so 39 mg/L divided by 17.1 gives 2.3.

Has the Mercer Island water system had EPA violations?

EPA records show no health-based violations for Mercer Island City of since 2016.

Nearby cities

All 145 cities in Washington

Hardness data: TapWaterData US Water Hardness (utility report, June 23, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.