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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 6 | Orange County | California | 2047.56 | 790.57 | 3010232 | 1470 | 3808 | 3017598 | 1474 | 3817 | 3055745 | 1492 | 3865 | 38147 | 1.26 | Santa Ana |
| 7 | Miami-Dade County | Florida | 4915.06 | 1897.72 | 2496435 | 508 | 1315 | 2503034 | 509 | 1319 | 2554766 | 520 | 1346 | 51732 | 2.07 | Miami |
| 8 | Kings County | New York | 183.41 | 70.82 | 2504700 | 13656 | 35370 | 2508515 | 13677 | 35423 | 2532645 | 13809 | 35764 | 24130 | 0.96 | Brooklyn |
| 9 | Dallas County | Texas | 2256.60 | 871.28 | 2368139 | 1049 | 2718 | 2375207 | 1053 | 2726 | 2416014 | 1071 | 2773 | 40807 | 1.72 | Dallas |
| 10 | Queens County | New York | 281.10 | 108.53 | 2230722 | 7936 | 20553 | 2233895 | 7947 | 20583 | 2247848 | 7997 | 20711 | 13953 | 0.62 | Jamaica |
| 11 | Riverside County | California | 18664.70 | 7206.48 | 2189641 | 117 | 304 | 2202978 | 118 | 306 | 2239620 | 120 | 311 | 36642 | 1.66 | Riverside |