How hard is the water in St. Louis, MO?

Tap water hardness
109mg/L
6.4gpg
Moderately hardReports range 109–148

Reported by ST Louis City PWS · October 2025

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

The water in St. Louis carries roughly 109 mg/L of dissolved calcium and magnesium, or 6.4 grains per gallon. That is moderately hard water. The number comes from ST Louis City PWS, dated October 2025. Reports for this system disagree, with values from 109 to 148 mg/L.

St. Louis on the hardness scale, with nearby cities
060120180240300360+MO typical 121Murphy: 104 mg/LMurphy 104Maryland Heights: 121 mg/LMaryland Heights 121Chesterfield: 121 mg/LChesterfield 121Kirkwood: 130 mg/LKirkwood 130Ladue: 155 mg/LLadue 155St. Peters: 168 mg/LSt. Peters 168Arnold: 212 mg/LArnold 212Town and Country: 234 mg/LTown and Country 234Ballwin: 242 mg/LBallwin 242Des Peres: 263 mg/LDes Peres 263St. Louis 109mg/L as calcium carbonate060120180240300360+MO 121Maryland Heights: 121 mg/LMaryland H. 121Kirkwood: 130 mg/LKirkwood 130Ladue: 155 mg/LLadue 155Arnold: 212 mg/LArnold 212Town and Country: 234 mg/LTown and C. 234Des Peres: 263 mg/LDes Peres 263109 mg/Lmg/L as calcium carbonate
Soft 0–60 mg/LModerately hard 61–120 mg/LHard 121–180 mg/LVery hard 181+ mg/L

St. Louis sits 12 mg/L below the typical Missouri figure of 121. Of the 10 nearest cities we track, 1 has softer water and 9 have harder. The range nearby runs from 104 mg/L in Murphy to 263 in Des Peres.

What 109 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 St. Louis

3 people × 75 gallons a day × 6.4 gpg = 1,433 grains a day. A 24,000-grain unit regenerates about every 16 days.

Size it for your household

EPA record for St. Louis's water system

ST Louis City PWS · PWSID MO6010715

No health-based violations since 2016

Serves about 304,709 people from surface water.

Health-based
0
Monitoring or reporting
8
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 St. Louis water hard?

Somewhat. It measures about 109 mg/L (6.4 grains per gallon), which the USGS classes as moderately hard.

What is the water hardness in St. Louis in grains per gallon?

About 6.4 gpg. One grain per gallon equals 17.1 mg/L, so 109 mg/L divided by 17.1 gives 6.4.

What size water softener do I need in St. Louis?

For 3 people using 75 gallons each per day, 6.4 gpg works out to about 1,433 grains a day. A 24,000-grain softener covers that with a regeneration about every 16 days.

Has the St. Louis water system had EPA violations?

EPA records show no health-based violations for ST Louis City PWS since 2016. There were also 8 monitoring or reporting violations, which are about late or missing tests and notices.

Nearby cities

All 90 cities in Missouri

Hardness data: TapWaterData US Water Hardness (utility report, October 31, 2025), CC BY 4.0. Population: US Census Bureau. Violations: EPA ECHO. How we use them.