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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 90 | Jackson County | Missouri | 1565.55 | 604.46 | 674158 | 431 | 1115 | 674953 | 431 | 1117 | 676360 | 432 | 1119 | 1407 | 0.21 | Independence, Missouri and Kansas City |
| 91 | Norfolk County | Massachusetts | 1025.91 | 396.11 | 670850 | 654 | 1694 | 672107 | 655 | 1697 | 675436 | 658 | 1705 | 3329 | 0.50 | Dedham |
| 92 | Bernalillo County | New Mexico | 3006.53 | 1160.83 | 662564 | 220 | 571 | 664425 | 221 | 572 | 670968 | 223 | 578 | 6543 | 0.98 | Albuquerque |
| 93 | Jefferson County | Alabama | 2878.19 | 1111.28 | 658466 | 229 | 593 | 658386 | 229 | 592 | 658931 | 229 | 593 | 545 | 0.08 | Birmingham |
| 94 | Hudson County | New Jersey | 119.63 | 46.19 | 634266 | 5302 | 13732 | 634979 | 5308 | 13747 | 641224 | 5360 | 13882 | 6245 | 0.98 | Jersey City |
| 95 | El Paso County | Colorado | 5508.39 | 2126.80 | 622263 | 113 | 293 | 627159 | 114 | 295 | 636963 | 116 | 299 | 9804 | 1.56 | Colorado Springs |