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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 12 | San Bernardino County | California | 51947.23 | 20056.94 | 2035210 | 39 | 101 | 2042027 | 39 | 102 | 2065377 | 40 | 103 | 23350 | 1.14 | San Bernardino |
| 13 | Clark County | Nevada | 20438.71 | 7891.43 | 1951269 | 95 | 247 | 1953927 | 96 | 248 | 1969975 | 96 | 250 | 16048 | 0.82 | Las Vegas |
| 14 | King County | Washington | 5479.29 | 2115.57 | 1931249 | 352 | 913 | 1937157 | 354 | 916 | 1969722 | 359 | 931 | 32565 | 1.68 | Seattle |
| 15 | Tarrant County | Texas | 2236.74 | 863.61 | 1809034 | 809 | 2095 | 1816850 | 812 | 2104 | 1849815 | 827 | 2142 | 32965 | 1.81 | Fort Worth |
| 16 | Santa Clara County | California | 3341.34 | 1290.10 | 1781642 | 533 | 1381 | 1786267 | 535 | 1385 | 1809378 | 542 | 1403 | 23111 | 1.29 | San Jose |
| 17 | Wayne County | Michigan | 1585.28 | 612.08 | 1820584 | 1148 | 2974 | 1815246 | 1145 | 2966 | 1802096 | 1137 | 2944 | -13150 | -0.72 | Detroit |