Water hardness in Illinois
Illinois has a median ambient water hardness of 285 mg/L, which is 16.6 grains per gallon and classes as very hard. That is the middle of 77,575 samples across 102 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 Illinois
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 |
|---|---|---|---|---|---|
| Saline County | 461 | 26.9 | Very hard | 288 to 916 | 1,210 |
| Fulton County | 390 | 22.8 | Very hard | 329 to 810 | 1,639 |
| Kendall County | 389 | 22.7 | Very hard | 343 to 414 | 384 |
| Boone County | 377 | 22 | Very hard | 346 to 400 | 409 |
| Henry County | 377 | 22 | Very hard | 345 to 400 | 406 |
| Jo Daviess County | 370 | 21.6 | Very hard | 340 to 440 | 748 |
| Cass County | 368 | 21.5 | Very hard | 332 to 400 | 224 |
| Stark County | 364 | 21.3 | Very hard | 337 to 391 | 637 |
| Carroll County | 360 | 21 | Very hard | 340 to 378 | 187 |
| Lee County | 357 | 20.9 | Very hard | 317 to 417 | 75 |
| Ford County | 355 | 20.7 | Very hard | 334 to 400 | 42 |
| Stephenson County | 351 | 20.5 | Very hard | 330 to 370 | 601 |
| DeKalb County | 350 | 20.4 | Very hard | 311 to 380 | 306 |
| Logan County | 350 | 20.4 | Very hard | 310 to 380 | 955 |
| Winnebago County | 350 | 20.4 | Very hard | 321 to 377 | 1,356 |
| Whiteside County | 349 | 20.4 | Very hard | 309 to 370 | 1,030 |
| Knox County | 343 | 20 | Very hard | 300 to 380 | 409 |
| Livingston County | 342 | 20 | Very hard | 310 to 374 | 565 |
| Woodford County | 342 | 20 | Very hard | 306 to 376 | 126 |
| Tazewell County | 341 | 19.9 | Very hard | 310 to 371 | 1,004 |
| Bureau County | 340 | 19.9 | Very hard | 291 to 381 | 887 |
| McHenry County | 340 | 19.9 | Very hard | 304 to 395 | 1,622 |
| Moultrie County | 340 | 19.9 | Very hard | 300 to 370 | 817 |
| Ogle County | 340 | 19.9 | Very hard | 316 to 360 | 876 |
| Grundy County | 338 | 19.7 | Very hard | 296 to 370 | 769 |
| McLean County | 330 | 19.3 | Very hard | 274 to 370 | 330 |
| Iroquois County | 320 | 18.7 | Very hard | 282 to 350 | 911 |
| LaSalle County | 319 | 18.6 | Very hard | 279 to 353 | 1,753 |
| Henderson County | 318 | 18.6 | Very hard | 294 to 340 | 355 |
| Kankakee County | 314 | 18.3 | Very hard | 284 to 340 | 916 |
| Piatt County | 313 | 18.3 | Very hard | 287 to 340 | 486 |
| Vermilion County | 313 | 18.3 | Very hard | 279 to 340 | 2,012 |
| Kane County | 311 | 18.2 | Very hard | 272 to 344 | 1,021 |
| Rock Island County | 311 | 18.2 | Very hard | 282 to 340 | 492 |
| Mason County | 310 | 18.1 | Very hard | 271 to 334 | 940 |
| Menard County | 310 | 18.1 | Very hard | 275 to 340 | 362 |
| Peoria County | 310 | 18.1 | Very hard | 270 to 364 | 950 |
| Champaign County | 307 | 17.9 | Very hard | 262 to 340 | 1,135 |
| Coles County | 304 | 17.8 | Very hard | 252 to 340 | 548 |
| De Witt County | 303 | 17.7 | Very hard | 262 to 344 | 672 |
| Warren County | 300 | 17.5 | Very hard | 270 to 327 | 196 |
| Will County | 293 | 17.1 | Very hard | 240 to 350 | 2,745 |
| Cumberland County | 290 | 16.9 | Very hard | 251 to 320 | 455 |
| Douglas County | 290 | 16.9 | Very hard | 245 to 312 | 698 |
| DuPage County | 290 | 16.9 | Very hard | 220 to 360 | 1,619 |
| Lake County | 290 | 16.9 | Very hard | 138 to 368 | 2,758 |
| Mercer County | 290 | 16.9 | Very hard | 250 to 320 | 337 |
| St. Clair County | 290 | 16.9 | Very hard | 217 to 378 | 1,111 |
| Putnam County | 287 | 16.8 | Very hard | 257 to 320 | 277 |
| Schuyler County | 286 | 16.7 | Very hard | 245 to 325 | 468 |
| Gallatin County | 284 | 16.6 | Very hard | 200 to 420 | 527 |
| Marshall County | 284 | 16.6 | Very hard | 256 to 320 | 281 |
| Sangamon County | 283 | 16.5 | Very hard | 229 to 330 | 1,414 |
| Scott County | 283 | 16.5 | Very hard | 252 to 318 | 968 |
| Calhoun County | 280 | 16.4 | Very hard | 250 to 310 | 317 |
| Edgar County | 278 | 16.2 | Very hard | 250 to 300 | 254 |
| Madison County | 273 | 15.9 | Very hard | 219 to 353 | 2,110 |
| Greene County | 272 | 15.9 | Very hard | 213 to 310 | 582 |
| Clark County | 270 | 15.8 | Very hard | 202 to 310 | 51 |
| Crawford County | 270 | 15.8 | Very hard | 224 to 320 | 1,120 |
| Macon County | 270 | 15.8 | Very hard | 238 to 304 | 869 |
| Perry County | 261 | 15.2 | Very hard | 150 to 573 | 162 |
| Brown County | 260 | 15.2 | Very hard | 226 to 281 | 160 |
| Cook County | 260 | 15.2 | Very hard | 140 to 338 | 4,718 |
| Christian County | 258 | 15.1 | Very hard | 209 to 290 | 956 |
| Jasper County | 258 | 15.1 | Very hard | 205 to 300 | 429 |
| Wabash County | 256 | 15 | Very hard | 161 to 313 | 14 |
| McDonough County | 248 | 14.5 | Very hard | 206 to 271 | 605 |
| Fayette County | 246 | 14.4 | Very hard | 200 to 290 | 1,272 |
| Effingham County | 239 | 14 | Very hard | 182 to 280 | 430 |
| Monroe County | 236 | 13.8 | Very hard | 215 to 266 | 138 |
| Pike County | 234 | 13.7 | Very hard | 200 to 294 | 780 |
| Lawrence County | 231 | 13.5 | Very hard | 180 to 270 | 357 |
| Shelby County | 228 | 13.3 | Very hard | 194 to 259 | 942 |
| Adams County | 223 | 13 | Very hard | 183 to 252 | 473 |
| Hancock County | 223 | 13 | Very hard | 187 to 255 | 174 |
| Hamilton County | 221 | 12.9 | Very hard | 179 to 300 | 50 |
| Randolph County | 217 | 12.7 | Very hard | 160 to 374 | 1,149 |
| Jersey County | 215 | 12.6 | Very hard | 196 to 239 | 447 |
| Alexander County | 209 | 12.2 | Very hard | 180 to 230 | 931 |
| Morgan County | 206 | 12 | Very hard | 160 to 266 | 79 |
| Clay County | 200 | 11.7 | Very hard | 150 to 250 | 424 |
| Montgomery County | 200 | 11.7 | Very hard | 151 to 240 | 421 |
| Williamson County | 190 | 11.1 | Very hard | 115 to 390 | 1,059 |
| Macoupin County | 189 | 11 | Very hard | 124 to 280 | 382 |
| Jefferson County | 186 | 10.9 | Very hard | 146 to 242 | 1,003 |
| Edwards County | 184 | 10.7 | Very hard | 126 to 235 | 511 |
| Clinton County | 176 | 10.3 | Hard | 146 to 210 | 1,150 |
| Washington County | 170 | 9.9 | Hard | 130 to 202 | 649 |
| Jackson County | 167 | 9.8 | Hard | 97 to 334 | 1,867 |
| Bond County | 166 | 9.7 | Hard | 110 to 211 | 73 |
| Union County | 166 | 9.7 | Hard | 97 to 210 | 187 |
| White County | 166 | 9.7 | Hard | 113 to 222 | 703 |
| Marion County | 160 | 9.3 | Hard | 110 to 229 | 985 |
| Hardin County | 159 | 9.3 | Hard | 121 to 190 | 42 |
| Wayne County | 157 | 9.2 | Hard | 114 to 205 | 749 |
| Richland County | 146 | 8.5 | Hard | 104 to 199 | 456 |
| Pulaski County | 126 | 7.4 | Hard | 110 to 144 | 1,244 |
| Massac County | 123 | 7.2 | Hard | 65 to 185 | 55 |
| Franklin County | 120 | 7 | Moderately hard | 98 to 180 | 1,044 |
| Johnson County | 90 | 5.3 | Moderately hard | 67 to 114 | 437 |
| Pope County | 51 | 3 | Soft | 39 to 103 | 544 |
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 Illinois
At 16.6 grains per gallon, a family of four would need a softener of about 40,000 grains to regenerate roughly weekly, using around 684 lb of salt a year. Size it for your own household.
82 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.