Water hardness in Montana
Montana has a median ambient water hardness of 210 mg/L, which is 12.3 grains per gallon and classes as very hard. That is the middle of 88,640 samples across 56 counties.
This is ambient water, not tap water. These figures come from samples taken from wells, streams and springs by USGS, EPA and state agencies. Municipal utilities treat, blend and sometimes soften water before it reaches a tap, so a city supply can be considerably softer than the ground beneath it. Treat this as a good guide for a private well and an approximate one for city water. Your utility's annual water quality report gives the real number for a municipal tap, and a test strip gives it for yours.
Every monitored county in Montana
Ranked hardest first. The middle 50% column shows how much the samples within a county vary, which matters more than the median where a county spans two geologies.
| County | mg/L | gpg | Class | Middle 50% | Samples |
|---|---|---|---|---|---|
| Rosebud County | 2,090 | 122.1 | Very hard | 850 to 3,330 | 16,390 |
| Petroleum County | 710 | 41.5 | Very hard | 510 to 980 | 639 |
| Powder River County | 607 | 35.5 | Very hard | 420 to 1,000 | 1,615 |
| Wibaux County | 570 | 33.3 | Very hard | 220 to 740 | 170 |
| Musselshell County | 560 | 32.7 | Very hard | 400 to 730 | 687 |
| Toole County | 519 | 30.3 | Very hard | 243 to 1,420 | 108 |
| Sheridan County | 457 | 26.7 | Very hard | 297 to 728 | 397 |
| Golden Valley County | 450 | 26.3 | Very hard | 370 to 905 | 380 |
| Teton County | 438 | 25.6 | Very hard | 270 to 1,247 | 509 |
| Dawson County | 400 | 23.4 | Very hard | 149 to 670 | 369 |
| Carter County | 350 | 20.4 | Very hard | 237 to 578 | 215 |
| Custer County | 340 | 19.9 | Very hard | 220 to 530 | 2,378 |
| Wheatland County | 335 | 19.6 | Very hard | 255 to 406 | 315 |
| Phillips County | 331 | 19.3 | Very hard | 214 to 602 | 2,806 |
| Big Horn County | 327 | 19.1 | Very hard | 248 to 570 | 3,119 |
| Treasure County | 320 | 18.7 | Very hard | 268 to 380 | 725 |
| Fallon County | 300 | 17.5 | Very hard | 190 to 576 | 379 |
| Fergus County | 299 | 17.5 | Very hard | 198 to 500 | 680 |
| Cascade County | 290 | 16.9 | Very hard | 170 to 500 | 2,618 |
| Daniels County | 290 | 16.9 | Very hard | 236 to 340 | 594 |
| Blaine County | 279 | 16.3 | Very hard | 219 to 397 | 1,758 |
| Prairie County | 270 | 15.8 | Very hard | 170 to 474 | 306 |
| McCone County | 260 | 15.2 | Very hard | 220 to 585 | 812 |
| Roosevelt County | 260 | 15.2 | Very hard | 180 to 540 | 929 |
| Liberty County | 240 | 14 | Very hard | 220 to 270 | 320 |
| Richland County | 240 | 14 | Very hard | 203 to 305 | 1,540 |
| Valley County | 240 | 14 | Very hard | 165 to 340 | 1,119 |
| Pondera County | 239 | 14 | Very hard | 206 to 280 | 254 |
| Meagher County | 195 | 11.4 | Very hard | 145 to 237 | 880 |
| Garfield County | 192 | 11.2 | Very hard | 94 to 283 | 176 |
| Powell County | 192 | 11.2 | Very hard | 129 to 234 | 2,474 |
| Chouteau County | 190 | 11.1 | Very hard | 170 to 324 | 569 |
| Judith Basin County | 180 | 10.5 | Hard | 135 to 310 | 329 |
| Yellowstone County | 178 | 10.4 | Hard | 125 to 340 | 1,760 |
| Carbon County | 170 | 9.9 | Hard | 105 to 440 | 1,746 |
| Silver Bow County | 167 | 9.8 | Hard | 126 to 248 | 2,618 |
| Deer Lodge County | 153 | 8.9 | Hard | 104 to 225 | 4,144 |
| Madison County | 150 | 8.8 | Hard | 73 to 260 | 1,723 |
| Broadwater County | 147 | 8.6 | Hard | 104 to 193 | 641 |
| Gallatin County | 140 | 8.2 | Hard | 86 to 181 | 1,888 |
| Hill County | 140 | 8.2 | Hard | 98 to 187 | 553 |
| Beaverhead County | 135 | 7.9 | Hard | 30 to 242 | 1,768 |
| Granite County | 130 | 7.6 | Hard | 42 to 190 | 1,107 |
| Lewis and Clark County | 122 | 7.1 | Hard | 54 to 160 | 4,664 |
| Missoula County | 112 | 6.5 | Moderately hard | 75 to 148 | 3,870 |
| Sweet Grass County | 106 | 6.2 | Moderately hard | 53 to 148 | 303 |
| Lake County | 105 | 6.1 | Moderately hard | 72 to 141 | 465 |
| Lincoln County | 99 | 5.8 | Moderately hard | 33 to 128 | 3,158 |
| Glacier County | 96 | 5.6 | Moderately hard | 57 to 220 | 1,541 |
| Mineral County | 91 | 5.3 | Moderately hard | 60 to 121 | 380 |
| Flathead County | 87 | 5.1 | Moderately hard | 63 to 118 | 1,955 |
| Sanders County | 86 | 5 | Moderately hard | 48 to 96 | 2,030 |
| Stillwater County | 84 | 4.9 | Moderately hard | 49 to 127 | 928 |
| Park County | 71 | 4.1 | Moderately hard | 44 to 100 | 1,365 |
| Jefferson County | 46 | 2.7 | Soft | 25 to 90 | 2,885 |
| Ravalli County | 29 | 1.7 | Soft | 14 to 58 | 589 |
Source: Water Quality Portal (USGS, EPA and state agencies), waterqualitydata.us, retrieved 2026-09-19. 2,904,346 samples across 3,110 counties.
What this means for a household in Montana
At 12.3 grains per gallon, a family of four would need a softener of about 32,000 grains to regenerate roughly weekly, using around 509 lb of salt a year. Size it for your own household.
55 of these counties include well samples, so the figures are a reasonable guide for private well water there. Municipal supplies in the same counties may be softer, because utilities often blend or treat.
Hardness follows geology rather than borders. Where a county sits on limestone or dolomite the water is hard; on granite, basalt or sandstone it is soft. That is why neighbouring counties in this table can differ by a factor of several.