Water hardness in Idaho
Idaho has a median ambient water hardness of 130 mg/L, which is 7.6 grains per gallon and classes as hard. That is the middle of 33,816 samples across 44 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 Idaho
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 |
|---|---|---|---|---|---|
| Oneida County | 320 | 18.7 | Very hard | 250 to 480 | 278 |
| Franklin County | 290 | 16.9 | Very hard | 214 to 327 | 1,026 |
| Bannock County | 280 | 16.4 | Very hard | 222 to 330 | 628 |
| Twin Falls County | 270 | 15.8 | Very hard | 190 to 330 | 1,557 |
| Bear Lake County | 260 | 15.2 | Very hard | 240 to 291 | 720 |
| Minidoka County | 240 | 14 | Very hard | 190 to 297 | 494 |
| Jerome County | 230 | 13.4 | Very hard | 200 to 250 | 569 |
| Power County | 227 | 13.3 | Very hard | 187 to 315 | 485 |
| Cassia County | 222 | 13 | Very hard | 139 to 304 | 690 |
| Bingham County | 202 | 11.8 | Very hard | 151 to 241 | 881 |
| Bonneville County | 200 | 11.7 | Very hard | 154 to 231 | 898 |
| Caribou County | 199 | 11.6 | Very hard | 174 to 322 | 809 |
| Elmore County | 190 | 11.1 | Very hard | 100 to 202 | 1,686 |
| Butte County | 188 | 11 | Very hard | 159 to 247 | 1,361 |
| Teton County | 187 | 10.9 | Very hard | 147 to 216 | 112 |
| Gooding County | 170 | 9.9 | Hard | 150 to 210 | 809 |
| Jefferson County | 161 | 9.4 | Hard | 118 to 230 | 462 |
| Blaine County | 160 | 9.3 | Hard | 122 to 190 | 429 |
| Clark County | 151 | 8.8 | Hard | 101 to 208 | 196 |
| Canyon County | 150 | 8.8 | Hard | 80 to 200 | 2,017 |
| Payette County | 150 | 8.8 | Hard | 60 to 226 | 253 |
| Lincoln County | 144 | 8.4 | Hard | 123 to 180 | 214 |
| Lemhi County | 140 | 8.2 | Hard | 73 to 190 | 399 |
| Lewis County | 127 | 7.4 | Hard | 115 to 173 | 64 |
| Ada County | 112 | 6.5 | Moderately hard | 61 to 189 | 1,193 |
| Boundary County | 110 | 6.4 | Moderately hard | 80 to 130 | 385 |
| Fremont County | 104 | 6.1 | Moderately hard | 61 to 166 | 382 |
| Latah County | 103 | 6 | Moderately hard | 37 to 120 | 265 |
| Washington County | 100 | 5.8 | Moderately hard | 48 to 172 | 555 |
| Bonner County | 86 | 5 | Moderately hard | 66 to 98 | 870 |
| Gem County | 79 | 4.6 | Moderately hard | 35 to 130 | 256 |
| Madison County | 74 | 4.3 | Moderately hard | 66 to 139 | 424 |
| Idaho County | 60 | 3.5 | Soft | 27 to 77 | 503 |
| Adams County | 55 | 3.2 | Soft | 42 to 76 | 158 |
| Custer County | 54 | 3.2 | Soft | 27 to 110 | 1,097 |
| Camas County | 50 | 2.9 | Soft | 35 to 67 | 134 |
| Owyhee County | 46 | 2.7 | Soft | 18 to 197 | 840 |
| Nez Perce County | 39 | 2.3 | Soft | 14 to 120 | 342 |
| Shoshone County | 38 | 2.2 | Soft | 19 to 71 | 5,881 |
| Boise County | 29 | 1.7 | Soft | 15 to 36 | 256 |
| Valley County | 26 | 1.5 | Soft | 13 to 39 | 577 |
| Benewah County | 23 | 1.3 | Soft | 17 to 63 | 479 |
| Kootenai County | 22 | 1.3 | Soft | 20 to 37 | 1,995 |
| Clearwater County | 20 | 1.2 | Soft | 12 to 71 | 187 |
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 Idaho
At 7.6 grains per gallon, a family of four would need a softener of about 16,000 grains to regenerate roughly weekly, using around 312 lb of salt a year. Size it for your own household.
44 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.