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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 30 | Hillsborough County | Florida | 2642.34 | 1020.21 | 1229226 | 465 | 1205 | 1233373 | 467 | 1209 | 1267775 | 480 | 1243 | 34402 | 2.79 | Tampa |
| 31 | Allegheny County | Pennsylvania | 1890.88 | 730.07 | 1223348 | 647 | 1676 | 1224833 | 648 | 1678 | 1227066 | 649 | 1681 | 2233 | 0.18 | Pittsburgh |
| 32 | Oakland County | Michigan | 2247.24 | 867.66 | 1202362 | 535 | 1386 | 1202682 | 535 | 1386 | 1210145 | 539 | 1395 | 7463 | 0.62 | Pontiac |
| 33 | Franklin County | Ohio | 1378.36 | 532.19 | 1163414 | 844 | 2186 | 1165789 | 846 | 2191 | 1178799 | 855 | 2215 | 13010 | 1.12 | Columbus |
| 34 | Orange County | Florida | 2339.87 | 903.43 | 1145956 | 490 | 1268 | 1148787 | 491 | 1272 | 1169107 | 500 | 1294 | 20320 | 1.77 | Orlando |
| 35 | Hennepin County | Minnesota | 1433.79 | 553.59 | 1152425 | 804 | 2082 | 1154067 | 805 | 2085 | 1168431 | 815 | 2111 | 14364 | 1.24 | Minneapolis |