Even if most of the U.S. counties have been created during the 19th century, the most recent county in the United States is Broomfield County in Colorado, created in 2001.
| | Smallest |
Largest |
| All States |
Kalawao County, Hawaii (13) |
North Slope Borough, Alaska (94,763) |
| Alabama | Etowah County, Alabama (535) | Baldwin County, Alabama (1,596) |
| Alaska | Bristol Bay Borough, Alaska (505) | North Slope Borough, Alaska (94,763) |
| Arizona | Santa Cruz County, Arizona (1,238) | Coconino County, Arizona (18,617) |
| Arkansas | Lafayette County, Arkansas (527) | Union County, Arkansas (1,039) |
| California | San Francisco, California (47) | San Bernardino County, California (20,062) |
| Colorado | Broomfield County, Colorado (27) | Las Animas County, Colorado (4,772) |
| Connecticut | Middlesex County, Connecticut (369) | Litchfield County, Connecticut (920) |
| Delaware | New Castle County, Delaware (426) | Sussex County, Delaware (938) |
| District of Columbia | |
| Florida | Union County, Florida (240) | Collier County, Florida (2,025) |
| Georgia | Clarke County, Georgia (121) | Ware County, Georgia (903) |
| Hawaii | Kalawao County, Hawaii (13) | Hawaii County, Hawaii (4,028) |
| Idaho | Payette County, Idaho (408) | Idaho County, Idaho (8,485) |
| Illinois | Putnam County, Illinois (160) | McLean County, Illinois (1,184) |
| Indiana | Ohio County, Indiana (87) | Allen County, Indiana (657) |
| Iowa | Dickinson County, Iowa (381) | Kossuth County, Iowa (973) |
| Kansas | Wyandotte County, Kansas (151) | Butler County, Kansas (1,428) |
| Kentucky | Gallatin County, Kentucky (99) | Pike County, Kentucky (788) |
| Louisiana | Orleans Parish, Louisiana (181) | Rapides Parish, Louisiana (1,323) |
| Maine | Sagadahoc County, Maine (254) | Aroostook County, Maine (6,672) |
| Maryland | Calvert County, Maryland (215) | Frederick County, Maryland (663) |
| Massachusetts | Nantucket County, Massachusetts (48) | Worcester County, Massachusetts (1,513) |
| Michigan | Benzie County, Michigan (321) | Marquette County, Michigan (1,821) |
| Minnesota | Ramsey County, Minnesota (156) | St. Louis County, Minnesota (6,225) |
| Mississippi | Alcorn County, Mississippi (400) | Yazoo County, Mississippi (919) |
| Missouri | Worth County, Missouri (267) | Texas County, Missouri (1,179) |
| Montana | Silver Bow County, Montana (718) | Beaverhead County, Montana (5,542) |
| Nebraska | Sarpy County, Nebraska (241) | Cherry County, Nebraska (5,961) |
| Nevada | Storey County, Nevada (264) | Nye County, Nevada (18,147) |
| New Hampshire | Strafford County, New Hampshire (369) | Coos County, New Hampshire (1,801) |
| New Jersey | Hudson County, New Jersey (47) | Burlington County, New Jersey (805) |
| New Mexico | Los Alamos County, New Mexico (109) | Catron County, New Mexico (6,928) |
| New York | New York County, New York (29) | St. Lawrence County, New York (2,686) |
| North Carolina | Chowan County, North Carolina (173) | Robeson County, North Carolina (949) |
| North Dakota | Eddy County, North Dakota (630) | McKenzie County, North Dakota (2,742) |
| Ohio | Lake County, Ohio (229) | Ashtabula County, Ohio (703) |
| Oklahoma | Marshall County, Oklahoma (371) | Osage County, Oklahoma (2,251) |
| Oregon | Multnomah County, Oregon (435) | Harney County, Oregon (10,134) |
| Pennsylvania | Montour County, Pennsylvania (131) | Lycoming County, Pennsylvania (1,235) |
| Rhode Island | Bristol County, Rhode Island (25) | Providence County, Rhode Island (413) |
| South Carolina | McCormick County, South Carolina (360) | Horry County, South Carolina (1,134) |
| South Dakota | Clay County, South Dakota (412) | Meade County, South Dakota (3,471) |
| Tennessee | Trousdale County, Tennessee (114) | Shelby County, Tennessee (755) |
| Texas | Rockwall County, Texas (129) | Brewster County, Texas (6,193) |
| Utah | Davis County, Utah (305) | San Juan County, Utah (7,821) |
| Vermont | Grand Isle County, Vermont (83) | Windsor County, Vermont (972) |
| Virginia | Arlington County, Virginia (26) | Augusta County, Virginia (972) |
| Washington | San Juan County, Washington (175) | Okanogan County, Washington (5,268) |
| West Virginia | Hancock County, West Virginia (83) | Randolph County, West Virginia (1,040) |
| Wisconsin | Ozaukee County, Wisconsin (232) | Marathon County, Wisconsin (1,545) |
| Wyoming | Hot Springs County, Wyoming (2,004) | Sweetwater County, Wyoming (10,426) |
| | Smallest |
Largest |
| All States |
Loving County, Texas (67) |
Los Angeles County, California (9,519,338) |
| Alabama | Greene County, Alabama (9,974) | Jefferson County, Alabama (662,047) |
| Alaska | Yakutat City and Borough, Alaska (808) | Fairbanks North Star Borough, Alaska (82,840) |
| Arizona | Greenlee County, Arizona (8,547) | Maricopa County, Arizona (3,072,149) |
| Arkansas | Calhoun County, Arkansas (5,744) | Pulaski County, Arkansas (361,474) |
| California | Alpine County, California (1,208) | Los Angeles County, California (9,519,338) |
| Colorado | San Juan County, Colorado (558) | Denver County, Colorado (554,636) |
| Connecticut | Windham County, Connecticut (109,091) | Fairfield County, Connecticut (882,567) |
| Delaware | Kent County, Delaware (126,697) | New Castle County, Delaware (500,265) |
| District of Columbia | |
| Florida | Liberty County, Florida (7,021) | Miami-Dade County, Florida (2,253,362) |
| Georgia | Taliaferro County, Georgia (2,077) | Fulton County, Georgia (816,006) |
| Hawaii | Kalawao County, Hawaii (147) | Honolulu County, Hawaii (876,156) |
| Idaho | Camas County, Idaho (991) | Ada County, Idaho (300,904) |
| Illinois | Pope County, Illinois (4,413) | Cook County, Illinois (5,376,741) |
| Indiana | Ohio County, Indiana (5,623) | Marion County, Indiana (860,454) |
| Iowa | Adams County, Iowa (4,482) | Polk County, Iowa (374,601) |
| Kansas | Greeley County, Kansas (1,534) | Johnson County, Kansas (451,086) |
| Kentucky | Robertson County, Kentucky (2,266) | Jefferson County, Kentucky (693,604) |
| Louisiana | Tensas Parish, Louisiana (6,618) | Orleans Parish, Louisiana (484,674) |
| Maine | Piscataquis County, Maine (17,235) | Cumberland County, Maine (265,612) |
| Maryland | Kent County, Maryland (19,197) | Montgomery County, Maryland (873,341) |
| Massachusetts | Nantucket County, Massachusetts (9,520) | Middlesex County, Massachusetts (1,465,396) |
| Michigan | Keweenaw County, Michigan (2,301) | Wayne County, Michigan (2,061,162) |
| Minnesota | Traverse County, Minnesota (4,134) | Hennepin County, Minnesota (1,116,200) |
| Mississippi | Issaquena County, Mississippi (2,274) | Hinds County, Mississippi (250,800) |
| Missouri | Worth County, Missouri (2,382) | St. Louis County, Missouri (1,016,315) |
| Montana | Petroleum County, Montana (493) | Yellowstone County, Montana (129,352) |
| Nebraska | Arthur County, Nebraska (444) | Douglas County, Nebraska (463,585) |
| Nevada | Esmeralda County, Nevada (971) | Clark County, Nevada (1,375,765) |
| New Hampshire | Coos County, New Hampshire (33,111) | Hillsborough County, New Hampshire (380,841) |
| New Jersey | Salem County, New Jersey (65,285) | Bergen County, New Jersey (884,118) |
| New Mexico | Harding County, New Mexico (810) | Bernalillo County, New Mexico (556,678) |
| New York | Hamilton County, New York (5,379) | Kings County, New York (2,465,326) |
| North Carolina | Tyrrell County, North Carolina (4,149) | Mecklenburg County, North Carolina (695,454) |
| North Dakota | Slope County, North Dakota (767) | Cass County, North Dakota (123,138) |
| Ohio | Vinton County, Ohio (12,806) | Cuyahoga County, Ohio (1,393,978) |
| Oklahoma | Cimarron County, Oklahoma (3,148) | Oklahoma County, Oklahoma (660,448) |
| Oregon | Wheeler County, Oregon (1,547) | Multnomah County, Oregon (660,486) |
| Pennsylvania | Forest County, Pennsylvania (4,946) | Philadelphia County, Pennsylvania (1,517,550) |
| Rhode Island | Bristol County, Rhode Island (50,648) | Providence County, Rhode Island (621,602) |
| South Carolina | McCormick County, South Carolina (9,958) | Greenville County, South Carolina (379,616) |
| South Dakota | Jones County, South Dakota (1,193) | Minnehaha County, South Dakota (148,281) |
| Tennessee | Pickett County, Tennessee (4,945) | Shelby County, Tennessee (897,472) |
| Texas | Loving County, Texas (67) | Harris County, Texas (3,400,578) |
| Utah | Daggett County, Utah (921) | Salt Lake County, Utah (898,387) |
| Vermont | Essex County, Vermont (6,459) | Chittenden County, Vermont (146,571) |
| Virginia | Highland County, Virginia (2,536) | Fairfax County, Virginia (969,749) |
| Washington | Garfield County, Washington (2,397) | King County, Washington (1,737,034) |
| West Virginia | Wirt County, West Virginia (5,873) | Kanawha County, West Virginia (200,073) |
| Wisconsin | Menominee County, Wisconsin (4,562) | Milwaukee County, Wisconsin (940,164) |
| Wyoming | Niobrara County, Wyoming (2,407) | Laramie County, Wyoming (81,607) |
