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 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Los Angeles County | California | 10509.87 | 4057.88 | 9818605 | 934 | 2420 | 9826773 | 935 | 2422 | 9889056 | 941 | 2437 | 62283 | 0.63 | Los Angeles |
| 2 | Cook County | Illinois | 2448.38 | 945.33 | 5194675 | 2122 | 5495 | 5199971 | 2124 | 5501 | 5217080 | 2131 | 5519 | 17109 | 0.33 | Chicago |
| 3 | Harris County | Texas | 4411.99 | 1703.48 | 4092459 | 928 | 2402 | 4109362 | 931 | 2412 | 4180894 | 948 | 2454 | 71532 | 1.74 | Houston |
| 4 | Maricopa County | Arizona | 23828.26 | 9200.14 | 3817117 | 160 | 415 | 3824834 | 161 | 416 | 3880244 | 163 | 422 | 55410 | 1.45 | Phoenix |
| 5 | San Diego County | California | 10895.12 | 4206.63 | 3095313 | 284 | 736 | 3105115 | 285 | 738 | 3140069 | 288 | 746 | 34954 | 1.13 | San Diego |