How hard is the water in SeaTac, WA?

Tap water hardness
64mg/L
3.8gpg
Moderately hard

Reported by Highline Water District · June 2025

WA typical
36 mg/L
EPA health violations since 2016
0
Softener for 3 people
24k grain
Data
Utility report

The water in SeaTac carries roughly 64 mg/L of dissolved calcium and magnesium, or 3.8 grains per gallon. That is moderately hard water. The number comes from Highline Water District, dated June 2025.

SeaTac on the hardness scale, with nearby cities
060120180240300360+WA typical 36White Center: 31 mg/LWhite Center 31Burien: 33 mg/LBurien 33Bryn Mawr-Skyway: 35 mg/LBryn Mawr-Skyway 35Tukwila: 37 mg/LTukwila 37Des Moines: 48 mg/LDes Moines 48Lakeland North: 48 mg/LLakeland North 48Renton: 49 mg/LRenton 49Kent: 52 mg/LKent 52Fairwood: 53 mg/LFairwood 53Normandy Park: 64 mg/LNormandy Park 64SeaTac 64mg/L as calcium carbonate060120180240300360+WA 36White Center: 31 mg/LWhite Cent. 31Burien: 33 mg/LBurien 33Bryn Mawr-Skyway: 35 mg/LBryn Mawr-. 35Tukwila: 37 mg/LTukwila 37Des Moines: 48 mg/LDes Moines 48Normandy Park: 64 mg/LNormandy P. 6464 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 64 mg/L, SeaTac is 28 mg/L above the typical Washington figure of 36. None of the 10 nearest cities we track has harder water. The range nearby runs from 31 mg/L in White Center to 64 in Normandy Park.

What 64 mg/L means at home

You may see faint white spots on glasses and shower doors after the water dries.

Scale builds up slowly. A kettle or coffee maker needs descaling now and then, maybe every few months.

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

Optional. Plenty of households at this level live with it and descale the kettle now and then.

Water softener

Worth a look if spots on glass bother you, or if your tankless water heater manual asks for softer water.

Carbon filter

Improves taste and removes chlorine. It leaves calcium and magnesium in place.

Softener size for SeaTac

3 people × 75 gallons a day × 3.8 gpg = 844 grains a day. A 24,000-grain unit regenerates about every 28 days.

Size it for your household

EPA record for SeaTac's water system

Highline Water District · PWSID WA5340650

No health-based violations since 2016

Serves about 100,170 people from surface water.

Health-based
0
Monitoring or reporting
0
Open now
0

TapWaterData ties SeaTac to this system. EPA's service-area records do not name SeaTac, so check the utility printed on your water bill.

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 SeaTac water hard?

Somewhat. It measures about 64 mg/L (3.8 grains per gallon), which the USGS classes as moderately hard.

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

About 3.8 gpg. One grain per gallon equals 17.1 mg/L, so 64 mg/L divided by 17.1 gives 3.8.

What size water softener do I need in SeaTac?

For 3 people using 75 gallons each per day, 3.8 gpg works out to about 844 grains a day. A 24,000-grain softener covers that with a regeneration about every 28 days.

Has the SeaTac water system had EPA violations?

EPA records show no health-based violations for Highline Water District since 2016.

Nearby cities

All 145 cities in Washington

Hardness data: TapWaterData US Water Hardness (utility report, June 9, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.