List of the most populous counties in the United States List of the most populous US counties sorted according to the mid-2011 official estimate by the USCB List of the most populous US counties sorted according to the mid-2011 official estimate by the USCB

| Rank_ | County | State_ | Land_area_km² | Land_area_mi² | April_1_2010_census | April_1_2010_density_km² | April_1_2010_density_mi² | July_1_2010_official_estimate | July_1_2010_density_km² | July_1_2010_density_mi² | July_1_2011_official_estimate | July_1_2011_density_km² | July_1_2011_density_mi² | Mid_2010_to_mid_2011_change_Absolute | Mid_2010_to_mid_2011_change_ | County_seat_or_courthouse |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 84 | Lake County | Illinois | 1149.10 | 443.67 | 703462 | 612 | 1586 | 704492 | 613 | 1588 | 706222 | 615 | 1592 | 1730 | 0.25 | Waukegan |
| 85 | DeKalb County | Georgia | 693.03 | 267.58 | 691893 | 998 | 2586 | 692858 | 1000 | 2589 | 699893 | 1010 | 2616 | 7035 | 1.02 | Decatur |
| 86 | Cobb County | Georgia | 879.43 | 339.55 | 688078 | 782 | 2026 | 689749 | 784 | 2031 | 697553 | 793 | 2054 | 7804 | 1.13 | Marietta |
| 87 | San Joaquin County | California | 3603.51 | 1391.32 | 685306 | 190 | 493 | 687661 | 191 | 494 | 696214 | 193 | 500 | 8553 | 1.24 | Stockton |
| 88 | Denton County | Texas | 2275.13 | 878.43 | 662614 | 291 | 754 | 667138 | 293 | 759 | 686406 | 302 | 781 | 19268 | 2.89 | Denton |
| 89 | Will County | Illinois | 2167.58 | 836.91 | 677560 | 313 | 810 | 678873 | 313 | 811 | 681545 | 314 | 814 | 2672 | 0.39 | Joliet |