How hard is the water in Yellowstone County, MT?
27.7 grains per gallon
A softener will pay its way here.
The median hardness measured in Yellowstone County, MT is 475 mg/L as calcium carbonate, which is 27.7 gpg. That is very hard on the USGS scale. It comes from 17 samples at 14 sites between 1990 and 2026, drawn from groundwater wells, which is what most taps and private wells in the county draw on.
How much it varies here
Half the sites came in between 21.1 and 36 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: medium (how it is measured).
What to do next
At 27.7 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 4.5 to 154.2 gpg
The softest single sample on record in Yellowstone County was 77.2 mg/L and the hardest 2,640 mg/L, a 34-fold gap. The top quarter of sites read above 36 gpg and the bottom quarter below 21.1 gpg.
Wells 27.7 gpg, rivers and lakes 10.8 gpg
17 groundwater samples from 14 wells put the median at 475 mg/L. 426 surface samples from 49 sites put it at 185 mg/L. In Yellowstone County the wells come out 16.9 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 40,000-grain unit covers a week between regenerations. Expect about 606 lb of salt a year (16 bags of 40 lb, about $109), 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 $209 a year.
Sizing at 27.7 gpg
| People | Grains a day | Unit | Salt a year |
|---|---|---|---|
| 1 | 1,662 | 24,000 | 202 lb |
| 2 | 3,324 | 24,000 | 404 lb |
| 4 | 6,648 | 48,000 | 809 lb |
| 6 | 9,972 | 80,000 | 1,213 lb |
Yellowstone County against the rest of Montana
Among the 56 Montana counties with data, Yellowstone County is number 16 of 56 counting from the hardest. Across the whole country it is harder than 93% of counties.
Softest in Montana: Lincoln County at 3.4 gpg. Hardest: Rosebud County at 138.9 gpg. Closest to Yellowstone County: Fergus County (28.9), Richland County (29.1), Fallon County (25.8).