How hard is the water in St. Louis County, MO?
15.2 grains per gallon
A softener will pay its way here.
The median hardness measured in St. Louis County, MO is 261 mg/L as calcium carbonate, which is 15.2 gpg. That is very hard on the USGS scale. It comes from 1,519 samples at 72 sites between 1990 and 2026, drawn from every sampling site in the county, wells and surface water together.
How much it varies here
Half the sites came in between 13.2 and 19.4 gpg. The whole middle of that range gives the same answer, so a test of your own tap is unlikely to change it. Confidence in the county figure: high (how it is measured).
What to do next
At 15.2 gpg a softener pays its way here. Before you buy, check the quoted unit's grain capacity against the sizing table below, and ask whether it regenerates on demand, which uses less salt than a timer. Call your utility for its own report if you are on city water.
More on sizing a softener for your household.
From 10.2 to 31.3 gpg
The softest single sample on record in St. Louis County was 174 mg/L and the hardest 536 mg/L, a 3-fold gap. The top quarter of sites read above 19.4 gpg and the bottom quarter below 13.2 gpg.
Wells 24.3 gpg, rivers and lakes 15.1 gpg
3 groundwater samples from 3 wells put the median at 416 mg/L. 1,516 surface samples from 69 sites put it at 259 mg/L. In St. Louis County the wells come out 9.2 gpg harder. Groundwater sits in contact with rock for years and picks up calcium and magnesium as it goes, so wells usually read harder. A utility here that draws from the river can deliver noticeably different water from a private well next door.
What a softener would cost to run
For a household of 3, a 24,000-grain unit covers a week between regenerations. Expect about 332 lb of salt a year (9 bags of 40 lb, about $60), 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 $160 a year.
Sizing at 15.2 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 912 | 24,000 | 111 lb |
| 2 | 1,824 | 24,000 | 222 lb |
| 4 | 3,648 | 32,000 | 444 lb |
| 6 | 5,472 | 40,000 | 666 lb |
St. Louis County against the rest of Missouri
Among the 110 Missouri counties with data, St. Louis County is number 17 of 110 counting from the hardest. Across the whole country it is harder than 70% of counties.
Softest in Missouri: Livingston County at 4.1 gpg. Hardest: Reynolds County at 96.7 gpg. Closest to St. Louis County: Morgan County (15.2), Camden County (15.1), Jefferson County (15).