| County | Rank | Change (2020) ↓ | 2026 Pop. | 2020 Pop. |
|---|---|---|---|---|
| Kendall County | 1 | 11.67% | 147,815 | 132,364 |
| Grundy County | 2 | 3.51% | 54,383 | 52,538 |
| McHenry County | 3 | 3.16% | 319,772 | 309,977 |
| Bond County | 4 | 2.72% | 17,318 | 16,859 |
| DeKalb County | 5 | 2.67% | 103,049 | 100,366 |
| Will County | 6 | 2.67% | 715,526 | 696,901 |
| Kane County | 7 | 2.27% | 528,084 | 516,364 |
| Champaign County | 8 | 2.2% | 210,766 | 206,222 |
| Brown County | 9 | 2.12% | 6,364 | 6,232 |
| Piatt County | 10 | 1.51% | 16,940 | 16,688 |
| Lake County | 11 | 0.95% | 720,188 | 713,377 |
| Douglas County | 12 | 0.7% | 19,912 | 19,774 |
| Clinton County | 13 | 0.69% | 37,168 | 36,915 |
| Boone County | 14 | 0.5% | 53,610 | 53,344 |
| DuPage County | 15 | 0.48% | 935,945 | 931,473 |
| Johnson County | 16 | 0.47% | 13,379 | 13,317 |
| Williamson County | 17 | 0.37% | 67,376 | 67,127 |
| Montgomery County | 18 | 0.25% | 28,315 | 28,244 |
| Crawford County | 19 | 0.24% | 18,675 | 18,631 |
| McLean County | 20 | 0.07% | 171,326 | 171,210 |
| Putnam County | 21 | -0.05% | 5,621 | 5,624 |
| Winnebago County | 22 | -0.2% | 284,383 | 284,939 |
| Stark County | 23 | -0.24% | 5,369 | 5,382 |
| Randolph County | 24 | -0.4% | 29,972 | 30,091 |
| Ogle County | 25 | -0.47% | 51,524 | 51,765 |
| Effingham County | 26 | -0.57% | 34,497 | 34,695 |
| Tazewell County | 27 | -0.9% | 130,116 | 131,300 |
| Coles County | 28 | -0.95% | 46,641 | 47,087 |
| Sangamon County | 29 | -0.97% | 194,304 | 196,202 |
| Woodford County | 30 | -0.99% | 38,024 | 38,405 |
| Madison County | 31 | -1.07% | 262,903 | 265,746 |
| Monroe County | 32 | -1.1% | 34,575 | 34,961 |
| Cook County | 33 | -1.2% | 5,200,459 | 5,263,651 |
| Moultrie County | 34 | -1.28% | 14,300 | 14,485 |
| Washington County | 35 | -1.31% | 13,549 | 13,729 |
| Christian County | 36 | -1.52% | 33,444 | 33,960 |
| Jersey County | 37 | -1.53% | 21,165 | 21,493 |
| Rock Island County | 38 | -1.53% | 142,147 | 144,357 |
| Shelby County | 39 | -1.54% | 20,598 | 20,921 |
| LaSalle County | 40 | -1.59% | 107,671 | 109,411 |
| Peoria County | 41 | -1.62% | 178,565 | 181,505 |
| Wayne County | 42 | -1.62% | 15,881 | 16,143 |
| Fulton County | 43 | -1.69% | 32,995 | 33,562 |
| Hamilton County | 44 | -1.74% | 7,857 | 7,996 |
| Jackson County | 45 | -1.82% | 52,320 | 53,289 |
| Adams County | 46 | -2.01% | 64,318 | 65,637 |
| Carroll County | 47 | -2.01% | 15,370 | 15,686 |
| Kankakee County | 48 | -2.02% | 105,236 | 107,402 |
| Bureau County | 49 | -2.15% | 32,501 | 33,215 |
| Hardin County | 50 | -2.16% | 3,575 | 3,654 |
| Franklin County | 51 | -2.16% | 36,878 | 37,694 |
| Jefferson County | 52 | -2.21% | 36,182 | 36,998 |
| Clark County | 53 | -2.39% | 15,087 | 15,456 |
| Henry County | 54 | -2.39% | 47,997 | 49,174 |
| Livingston County | 55 | -2.45% | 34,930 | 35,808 |
| Calhoun County | 56 | -2.48% | 4,290 | 4,399 |
| Jo Daviess County | 57 | -2.53% | 21,532 | 22,090 |
| Jasper County | 58 | -2.57% | 9,015 | 9,253 |
| Logan County | 59 | -2.68% | 27,145 | 27,892 |
| St. Clair County | 60 | -2.73% | 250,041 | 257,057 |
| Cumberland County | 61 | -2.82% | 10,138 | 10,432 |
| Marshall County | 62 | -2.89% | 11,375 | 11,713 |
| De Witt County | 63 | -2.96% | 14,992 | 15,450 |
| Macoupin County | 64 | -3% | 43,579 | 44,929 |
| Mercer County | 65 | -3.01% | 15,166 | 15,637 |
| Warren County | 66 | -3.02% | 16,328 | 16,837 |
| Ford County | 67 | -3.05% | 13,106 | 13,519 |
| Whiteside County | 68 | -3.12% | 53,819 | 55,555 |
| Pike County | 69 | -3.14% | 14,176 | 14,635 |
| Lee County | 70 | -3.19% | 32,977 | 34,063 |
| Edgar County | 71 | -3.23% | 16,264 | 16,806 |
| Morgan County | 72 | -3.28% | 32,391 | 33,490 |
| Iroquois County | 73 | -3.3% | 26,078 | 26,967 |
| Pope County | 74 | -3.41% | 3,627 | 3,755 |
| Stephenson County | 75 | -3.48% | 43,018 | 44,570 |
| Marion County | 76 | -3.7% | 36,263 | 37,656 |
| Menard County | 77 | -3.84% | 11,802 | 12,273 |
| Vermilion County | 78 | -3.98% | 71,062 | 74,008 |
| Fayette County | 79 | -3.99% | 20,626 | 21,483 |
| Schuyler County | 80 | -3.99% | 6,591 | 6,865 |
| Clay County | 81 | -3.99% | 12,741 | 13,271 |
| Saline County | 82 | -4.33% | 22,714 | 23,741 |
| Scott County | 83 | -4.48% | 4,843 | 5,070 |
| Cass County | 84 | -4.48% | 12,396 | 12,978 |
| Knox County | 85 | -4.5% | 47,570 | 49,812 |
| Perry County | 86 | -4.5% | 19,944 | 20,884 |
| Richland County | 87 | -4.67% | 15,070 | 15,808 |
| Wabash County | 88 | -4.69% | 10,788 | 11,319 |
| Union County | 89 | -4.73% | 16,480 | 17,299 |
| Macon County | 90 | -4.92% | 98,651 | 103,761 |
| Hancock County | 91 | -5.12% | 16,670 | 17,570 |
| McDonough County | 92 | -5.2% | 25,717 | 27,128 |
| Massac County | 93 | -5.32% | 13,388 | 14,141 |
| White County | 94 | -5.48% | 13,051 | 13,807 |
| Mason County | 95 | -5.49% | 12,333 | 13,050 |
| Greene County | 96 | -5.8% | 11,233 | 11,924 |
| Henderson County | 97 | -6.36% | 5,937 | 6,340 |
| Edwards County | 98 | -6.41% | 5,809 | 6,207 |
| Gallatin County | 99 | -7.78% | 4,549 | 4,933 |
| Pulaski County | 100 | -8.1% | 4,757 | 5,176 |
| Lawrence County | 101 | -9.58% | 13,813 | 15,277 |
| Alexander County | 102 | -12.43% | 4,529 | 5,172 |
Illinois's counties show varied growth patterns from 2020 to 2026. Kendall County leads with 1.86% annualized growth, followed by Grundy County (0.58%) and McHenry County (0.52%). The largest counties are Cook County (5,200,459), DuPage County (935,945), and Lake County (720,188). Of the 102 counties in Illinois, 20 are growing while 82 have experienced population decline. Overall, counties averaged -0.37% annualized growth, with total population growing by -0.08% annually.
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