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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 42 | Pima County | Arizona | 23794.31 | 9187.04 | 980263 | 41 | 107 | 982108 | 41 | 107 | 989569 | 42 | 108 | 7461 | 0.76 | Tucson |
| 43 | Honolulu County | Hawaii | 1555.92 | 600.74 | 953207 | 613 | 1587 | 955636 | 614 | 1591 | 963607 | 619 | 1604 | 7971 | 0.83 | Honolulu |
| 44 | Westchester County | New York | 1114.98 | 430.50 | 949113 | 851 | 2205 | 950283 | 852 | 2207 | 955899 | 857 | 2220 | 5616 | 0.59 | White Plains |
| 45 | Milwaukee County | Wisconsin | 625.23 | 241.40 | 947735 | 1516 | 3926 | 948623 | 1517 | 3930 | 952532 | 1523 | 3946 | 3909 | 0.41 | Milwaukee |
| 46 | Fulton County | Georgia | 1363.98 | 526.63 | 920581 | 675 | 1748 | 925920 | 679 | 1758 | 949599 | 696 | 1803 | 23679 | 2.56 | Atlanta |
| 47 | Mecklenburg County | North Carolina | 1356.75 | 523.84 | 919628 | 678 | 1756 | 923141 | 680 | 1762 | 944373 | 696 | 1803 | 21232 | 2.30 | Charlotte |