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Ada County, Idaho facts for kids

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Ada County
James A. McClure Federal Building and U.S. Courthouse in Boise, Idaho.
James A. McClure Federal Building and U.S. Courthouse in Boise, Idaho.
Official seal of Ada County
Map of Idaho highlighting Ada County
Location within the U.S. state of Idaho
Map of the United States highlighting Idaho
Idaho's location within the U.S.
Country  United States
State  Idaho
Founded December 22, 1864
Named for Ada Riggs
Seat Boise
Largest city Boise
 • Total 1,060 sq mi (2,700 km2)
 • Land 1,053 sq mi (2,730 km2)
 • Water 7.9 sq mi (20 km2)  0.7%
 • Total 494,967 Increase
Time zone UTC−7 (Mountain)
 • Summer (DST) UTC−6 (MDT)
Congressional districts 1st, 2nd
County Code: 1A

Ada County is located in the southwestern part of the U.S. state of Idaho. As of the 2021 United States Census Estimate, the county had a population of 511,931, making it by far the state's most populous county; it is home to 26.8% of the state's population. The county seat and largest city is Boise, which is also the state capital. Ada County is included in the Boise metropolitan area. Ada County is by far the state's most populous, with about 27% of the state's residents, and contains its only county highway district. The Ada County Highway District (ACHD) has jurisdiction over all the local county and city streets, except for private roads and state roads. In the interior Northwest east of the Cascade Range, Ada County ranks second in population behind only Spokane County, Washington.


Ada County was created by the Idaho Territorial Legislature on December 22, 1864, partitioned from Boise County. It is named for Ada Riggs, the first pioneer child born in the area and the daughter of H.C. Riggs, a co-founder of Boise. Canyon County, which originally included Payette County and most of Gem County, was partitioned from western Ada County in 1891.


According to the U.S. Census Bureau, the county has a total area of 1,060 square miles (2,700 km2), of which 1,053 square miles (2,730 km2) is land and 7.9 square miles (20 km2) (0.7%) is water. The Boise River flows through the northern portion of the county, and the northwest border is bounded by the foothills of the Boise Range mountains, the summits are in adjacent Boise County. The southwestern border of the county is bounded by the Snake River.

Adjacent counties

Major highways

  • I-84.svg - Interstate 84
  • I-184.svg - Interstate 184
  • US 20.svg - US 20
  • US 26.svg - US 26
  • US 30.svg - US 30
  • ID-16.svg - SH-16
  • ID-21.svg - SH-21 - Ponderosa Pine Scenic Byway
  • ID-44.svg - SH-44
  • ID-55.svg - SH-55 - Payette River Scenic Byway
  • ID-69.svg - SH-69

County roads and highways are maintained by the Ada County Highway District (ACHD).

National protected areas

  • Boise National Forest (part)
  • Snake River Birds of Prey National Conservation Area (part)


Historical population
Census Pop.
1870 2,675
1880 4,674 74.7%
1890 8,368 79.0%
1900 11,559 38.1%
1910 29,088 151.6%
1920 35,213 21.1%
1930 37,925 7.7%
1940 50,401 32.9%
1950 70,649 40.2%
1960 93,460 32.3%
1970 112,230 20.1%
1980 173,036 54.2%
1990 205,775 18.9%
2000 300,904 46.2%
2010 392,365 30.4%
2020 494,967 26.1%
US Decennial Census
1790-1960 1900-1990
1990-2000 2010-2020

2010 census

As of the 2010 United States Census, there were 392,365 people, 148,445 households, and 99,282 families in the county. The population density was 372.8 inhabitants per square mile (143.9/km2). There were 159,471 housing units at an average density of 151.5 per square mile (58.5/km2). The racial makeup of the county was 90.3% white, 2.4% Asian, 1.1% black or African American, 0.7% American Indian, 0.2% Pacific islander, 2.4% from other races, and 2.8% from two or more races. Those of Hispanic or Latino origin made up 7.1% of the population. In terms of ancestry, 19.4% were German, 15.9% were English, 11.8% were Irish, and 8.6% were American.

Of the 148,445 households, 35.6% had children under the age of 18 living with them, 52.4% were married couples living together, 10.0% had a female householder with no husband present, 33.1% were non-families, and 25.0% of all households were made up of individuals. The average household size was 2.58 and the average family size was 3.11. The median age was 34.8 years.

The median income for a household in the county was $55,835 and the median income for a family was $67,519. Males had a median income of $48,290 versus $34,875 for females. The per capita income for the county was $27,915. About 6.9% of families and 10.2% of the population were below the poverty line, including 11.8% of those under age 18 and 6.8% of those age 65 or over.


In terms of ancestry, 18.2% were English, 17.6% were German, 9.7% were Irish, 5.7% were American, 3.6% were Norwegian, 3.4% were Italian, 3.4% were Scottish, 2.8% were Swedish, 2.4% were French, 2.0% were Dutch, 1.7% were Polish, 1.6% were Danish and 1.3% were Welsh.



Census-designated place


Public school districts

  • Boise School District
  • West Ada School District
  • Kuna School District

See also

Kids robot.svg In Spanish: Condado de Ada para niños

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