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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 54 | Pinellas County | Florida | 709.14 | 273.80 | 916542 | 1292 | 3347 | 916220 | 1292 | 3346 | 917398 | 1294 | 3351 | 1178 | 0.13 | Clearwater |
| 55 | Marion County | Indiana | 1026.41 | 396.30 | 903393 | 880 | 2280 | 904789 | 882 | 2283 | 911296 | 888 | 2300 | 6507 | 0.72 | Indianapolis |
| 56 | Bergen County | New Jersey | 603.49 | 233.01 | 905116 | 1500 | 3884 | 906184 | 1502 | 3889 | 911004 | 1510 | 3910 | 4820 | 0.53 | Hackensack |
| 57 | Hartford County | Connecticut | 1903.89 | 735.10 | 894014 | 470 | 1216 | 894127 | 470 | 1216 | 894705 | 470 | 1217 | 578 | 0.06 | Hartford |
| 58 | Prince George`s County | Maryland | 1250.16 | 482.69 | 863420 | 691 | 1789 | 865219 | 692 | 1792 | 871233 | 697 | 1805 | 6014 | 0.70 | Upper Marlboro |
| 59 | Duval County | Florida | 1974.07 | 762.19 | 864263 | 438 | 1134 | 865420 | 438 | 1135 | 870709 | 441 | 1142 | 5289 | 0.61 | Jacksonville |