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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 36 | Fairfax County | Virginia | 1012.60 | 390.97 | 1081726 | 1068 | 2767 | 1085934 | 1072 | 2778 | 1100692 | 1087 | 2815 | 14758 | 1.36 | Fairfax |
| 37 | Contra Costa County | California | 1854.27 | 715.94 | 1049025 | 566 | 1465 | 1052605 | 568 | 1470 | 1066096 | 575 | 1489 | 13491 | 1.28 | Martinez |
| 38 | Travis County | Texas | 2564.61 | 990.20 | 1024266 | 399 | 1034 | 1030588 | 402 | 1041 | 1063130 | 415 | 1074 | 32542 | 3.16 | Austin |
| 39 | Salt Lake County | Utah | 1922.50 | 742.28 | 1029655 | 536 | 1387 | 1033223 | 537 | 1392 | 1048985 | 546 | 1413 | 15762 | 1.53 | Salt Lake City |
| 40 | St. Louis County | Missouri | 1315.20 | 507.80 | 998954 | 760 | 1967 | 998772 | 759 | 1967 | 998692 | 759 | 1967 | -80 | -0.01 | Clayton |
| 41 | Montgomery County | Maryland | 1272.34 | 491.25 | 971777 | 764 | 1978 | 975439 | 767 | 1986 | 989794 | 778 | 2015 | 14355 | 1.47 | Rockville |