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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 60 | New Haven County | Connecticut | 1565.66 | 604.51 | 862477 | 551 | 1427 | 862438 | 551 | 1427 | 861113 | 550 | 1424 | -1325 | -0.15 | New Haven and Waterbury |
| 61 | Kern County | California | 21061.57 | 8131.92 | 839631 | 40 | 103 | 842813 | 40 | 104 | 851710 | 40 | 105 | 8897 | 1.06 | Bakersfield |
| 62 | Macomb County | Michigan | 1241.18 | 479.22 | 840978 | 678 | 1755 | 840944 | 678 | 1755 | 842145 | 679 | 1757 | 1201 | 0.14 | Mount Clemens |
| 63 | Ventura County | California | 4773.69 | 1843.13 | 823318 | 172 | 447 | 825445 | 173 | 448 | 831771 | 174 | 451 | 6326 | 0.77 | Ventura |
| 64 | Gwinnett County | Georgia | 1114.69 | 430.38 | 805321 | 722 | 1871 | 808291 | 725 | 1878 | 824941 | 740 | 1917 | 16650 | 2.06 | Lawrenceville |
| 65 | El Paso County | Texas | 2622.86 | 1012.69 | 800647 | 305 | 791 | 803995 | 307 | 794 | 820790 | 313 | 811 | 16795 | 2.09 | El Paso |