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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 18 | Broward County | Florida | 3133.33 | 1209.79 | 1748066 | 558 | 1445 | 1752122 | 559 | 1448 | 1780172 | 568 | 1471 | 28050 | 1.60 | Fort Lauderdale |
| 19 | Bexar County | Texas | 3211.12 | 1239.82 | 1714773 | 534 | 1383 | 1723561 | 537 | 1390 | 1756153 | 547 | 1416 | 32592 | 1.89 | San Antonio |
| 20 | New York County | New York | 59.13 | 22.83 | 1585873 | 26820 | 69464 | 1587481 | 26847 | 69534 | 1601948 | 27092 | 70168 | 14467 | 0.91 | New York |
| 21 | Philadelphia County | Pennsylvania | 347.32 | 134.10 | 1526006 | 4394 | 11380 | 1528074 | 4400 | 11395 | 1536471 | 4424 | 11458 | 8397 | 0.55 | Philadelphia |
| 22 | Alameda County | California | 1914.05 | 739.02 | 1510271 | 789 | 2044 | 1513043 | 790 | 2047 | 1529875 | 799 | 2070 | 16832 | 1.11 | Oakland |
| 23 | Middlesex County | Massachusetts | 2118.14 | 817.82 | 1503085 | 710 | 1838 | 1505720 | 711 | 1841 | 1518171 | 717 | 1856 | 12451 | 0.83 | Cambridge |