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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 66 | Middlesex County | New Jersey | 800.08 | 308.91 | 809858 | 1012 | 2622 | 810747 | 1013 | 2625 | 814217 | 1018 | 2636 | 3470 | 0.43 | New Brunswick |
| 67 | San Francisco County | California | 121.40 | 46.87 | 805235 | 6633 | 17179 | 805340 | 6634 | 17181 | 812826 | 6695 | 17341 | 7486 | 0.93 | San Francisco |
| 68 | Collin County | Texas | 2178.76 | 841.22 | 782341 | 359 | 930 | 788511 | 362 | 937 | 812226 | 373 | 966 | 23715 | 3.01 | McKinney |
| 69 | Baltimore County | Maryland | 1549.59 | 598.30 | 805029 | 520 | 1346 | 805964 | 520 | 1347 | 809941 | 523 | 1354 | 3977 | 0.49 | Towson |
| 70 | Pierce County | Washington | 4324.01 | 1669.51 | 795225 | 184 | 476 | 795371 | 184 | 476 | 807904 | 187 | 484 | 12533 | 1.58 | Tacoma |
| 71 | Montgomery County | Pennsylvania | 1251.07 | 483.04 | 799874 | 639 | 1656 | 800482 | 640 | 1657 | 804210 | 643 | 1665 | 3728 | 0.47 | Norristown |