| County | Population | Area
(sq.mi.) |
Density |
| Adams | 66,090 | 862 | 76.7 |
| Alexander | 10,626 | 229 | 46.4 |
| Bond | 14,991 | 378 | 39.7 |
| Boone | 30,806 | 283 | 108.9 |
| Brown | 5,836 | 306 | 19.1 |
| Bureau | 35,688 | 866 | 41.2 |
| Calhoun | 5,322 | 247 | 21.5 |
| Carroll | 16,805 | 456 | 36.9 |
| Cass | 13,437 | 371 | 36.2 |
| Champaign | 173,025 | 1,000 | 173.0 |
| Christian | 34,418 | 709 | 48.5 |
| Clark | 15,921 | 505 | 31.5 |
| Clay | 14,460 | 464 | 31.2 |
| Clinton | 33,944 | 434 | 78.2 |
| Coles | 51,644 | 506 | 102.1 |
| Cook | 5,105,044 | 954 | 5,351.2 |
| Crawford | 19,464 | 443 | 43.9 |
| Cumberland | 10,670 | 347 | 30.7 |
| DeKalb | 77,932 | 636 | 122.5 |
| De Witt | 16,516 | 399 | 41.4 |
| Douglas | 19,464 | 420 | 46.3 |
| Du Page | 781,689 | 337 | 2,319.6 |
| Edgar | 19,595 | 628 | 31.2 |
| Edwards | 7,440 | 225 | 33.1 |
| Effingham | 31,704 | 481 | 65.9 |
| Fayette | 20,893 | 703 | 29.7 |
| Ford | 14,275 | 488 | 29.3 |
| Franklin | 40,319 | 434 | 92.9 |
| Fulton | 38,080 | 877 | 43.4 |
| Gallatin | 6,909 | 328 | 21.1 |
| Greene | 15,317 | 543 | 28.2 |
| Grundy | 32,337 | 432 | 74.9 |
| Hamilton | 8,499 | 435 | 19.5 |
| Hancock | 21,373 | 797 | 26.8 |
| Hardin | 5,189 | 183 | 28.4 |
| Henderson | 8,096 | 376 | 21.5 |
| Henry | 51,159 | 826 | 61.9 |
| Iroquois | 30,787 | 1,122 | 27.4 |
| Jackson | 61,067 | 605 | 100.9 |
| Jasper | 10,609 | 495 | 21.4 |
| Jefferson | 37,020 | 573 | 64.6 |
| Jersey | 20,539 | 376 | 54.6 |
| Jo Daviess | 21,821 | 606 | 36.0 |
| Johnson | 11,347 | 345 | 32.9 |
| Kane | 317,471 | 520 | 610.5 |
| Kankakee | 96,255 | 678 | 142.0 |
| Kendall | 39,413 | 320 | 123.2 |
| Knox | 56,393 | 728 | 77.5 |
| Lake | 516,418 | 457 | 1130.0 |
| La Salle | 106,913 | 1,150 | 93.0 |
| Lawrence | 15,972 | 374 | 42.7 |
| Lee | 34,392 | 728 | 47.2 |
| Livingston | 39,301 | 1,043 | 37.7 |
| Logan | 30,798 | 622 | 49.5 |
| Macon | 117,206 | 578 | 202.8 |
| Macoupin | 47,679 | 872 | 54.7 |
| Madison | 249,238 | 733 | 340.0 |
| Marion | 41,561 | 579 | 71.8 |
| Marshall | 12,846 | 391 | 32.9 |
| Mason | 16,269 | 541 | 30.1 |
| Massac | 14,752 | 245 | 60.2 |
| McDonough | 35,244 | 582 | 60.6 |
| McHenry | 183,241 | 610 | 300.4 |
| McLean | 129,180 | 1,173 | 110.1 |
| Menard | 11,164 | 312 | 35.8 |
| Mercer | 17,290 | 556 | 31.1 |
| Monroe | 22,422 | 382 | 58.7 |
| Montgomery | 30,728 | 705 | 43.6 |
| Morgan | 36,397 | 561 | 64.9 |
| Moultrie | 13,930 | 326 | 42.7 |
| Ogle | 45,957 | 758 | 60.6 |
| Peoria | 182,827 | 623 | 293.5 |
| Perry | 21,412 | 439 | 48.8 |
| Piatt | 15,548 | 437 | 35.6 |
| Pike | 17,577 | 828 | 21.2 |
| Pope | 4,373 | 381 | 11.5 |
| Pulaski | 7,523 | 204 | 36.9 |
| Putnam | 5,730 | 160 | 35.8 |
| Randolph | 34,583 | 594 | 58.2 |
| Richland | 16,545 | 364 | 45.5 |
| Rock Island | 148,723 | 424 | 350.8 |
| Saint Clair | 262,852 | 673 | 390.6 |
| Saline | 26,551 | 383 | 69.3 |
| Sangamon | 178,386 | 879 | 202.9 |
| Schuyler | 7,498 | 434 | 17.3 |
| Scott | 5,644 | 251 | 22.5 |
| Shelby | 22,261 | 752 | 29.6 |
| Stark | 6,534 | 291 | 22.5 |
| Stephenson | 48,052 | 568 | 84.6 |
| Tazewell | 123,692 | 652 | 189.7 |
| Union | 17,619 | 416 | 42.4 |
| Vermillion | 88,257 | 899 | 98.2 |
| Wabash | 13,111 | 222 | 59.1 |
| Warren | 19,181 | 541 | 35.5 |
| Washington | 14,965 | 564 | 26.5 |
| Wayne | 17,241 | 715 | 24.1 |
| White | 16,522 | 502 | 32.9 |
| Whiteside | 60,186 | 687 | 87.6 |
| Will | 357,313 | 847 | 421.9 |
| Williamson | 57,733 | 429 | 134.6 |
| Winnebago | 252,913 | 519 | 487.3 |
| Woodford | 31,653 | 528 | 61.8 |
| 11,430,602 | 55,760 |