Ada County is located in the southwestern part of

Idaho Idaho ( ) is a landlocked U.S. state, state in the Pacific Northwest and Mountain states, Mountain West subregions of the Western United States. It borders Montana and Wyoming to the east, Nevada and Utah to the south, and Washington (state), ...

, United States. As of the 2020 census, the

county A county () is a geographic region of a country used for administrative or other purposesL. Brookes (ed.) '' Chambers Dictionary''. Edinburgh: Chambers Harrap Publishers Ltd, 2005. in some nations. The term is derived from the Old French denoti ...

had a population of 494,967, which by 2024 was estimated to have risen to 535,799. Ada County is by far the state's most populous county; it is home to 26.8% of the state's population. The

county seat A county seat is an administrative center, seat of government, or capital city of a county or parish (administrative division), civil parish. The term is in use in five countries: Canada, China, Hungary, Romania, and the United States. An equiva ...

and largest city is

Boise Boise ( ) is the capital and most populous city of the U.S. state of Idaho. As of the 2020 census, there were 235,685 people residing in the city. Located on the Boise River in southwestern Idaho, it is east of the Oregon border and nor ...

, which is also the state capital. Ada County is included in the

Boise metropolitan area The Boise, Idaho Metropolitan Statistical Area (MSA) (commonly known as the Boise Metropolitan Area or the Treasure Valley) is an area that encompasses Ada County, Idaho, Ada, Boise County, Idaho, Boise, Canyon County, Idaho, Canyon, Gem County, ...

. The Ada County Highway District has jurisdiction over all the local county and city streets, except for private roads and state roads. In the interior

Pacific Northwest The Pacific Northwest (PNW; ) is a geographic region in Western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though no official boundary exists, the most common ...

east of the

Cascade Range The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington (state), Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as m ...

, Ada County ranks second in population, behind

Spokane County, Washington Spokane County is a county located in the U.S. state of Washington. As of the 2020 census, its population was 539,339, making it the fourth-most populous county in Washington. The largest city and county seat is Spokane, the second largest ...

History

Ada County was created by the

Idaho Territory The Territory of Idaho was an organized incorporated territory of the United States that existed from March 3, 1863, until July 3, 1890, when the final extent of the territory was admitted to the Union as Idaho. History 1860s The territory ...

legislature on December 22, 1864, partitioned from

Boise County Boise County is a rural mountain county in the U.S. state of Idaho. As of the 2020 United States census, the population was 7,610. The county seat is historic Idaho City, which is connected through a series of paved and unpaved roads to Lowm ...

. It is named for Ada Riggs, the daughter of H. C. Riggs, a member of the legislature; he established the county and was a co-founder of

. Canyon County, which originally included Payette County and most of Gem County, was partitioned from western Ada County in 1891.

Geography

According to the

United States Census Bureau The United States Census Bureau, officially the Bureau of the Census, is a principal agency of the Federal statistical system, U.S. federal statistical system, responsible for producing data about the American people and American economy, econ ...

, the county has a total area of , of which is land and (0.7%) is water. The

Boise River The Boise River is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. , accessed May 3, 2011 tributary of the Snake River in the Northwestern United States. It drains a rugged portion of the Sawtooth Range in so ...

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 The Snake River is a major river in the interior Pacific Northwest region of the United States. About long, it is the largest tributary of the Columbia River, which is the largest North American river that empties into the Pacific Ocean. Begin ...

Adjacent counties

* Gem County - northwest *

- northeast * Elmore County - east *

Owyhee County Owyhee County ( ) is a county in the southwestern corner of the U.S. state of Idaho. As of the 2020 census, the population was 11,913. The county seat is Murphy, and its largest city is Homedale. In area it is the second-largest county in I ...

- south * Canyon County - west

Major highways

* - Interstate 84 * -

Interstate 184 Interstate 184 (I-184) is a short auxiliary Interstate Highway in Boise, Idaho, United States. The only auxiliary Interstate in Idaho, it is a spur route of I-84 that connects the freeway to Downtown Boise, terminating at US Highway&n ...

* -

US 20 U.S. Route 20 or U.S. Highway 20 (US 20) is an east–west United States Highway, United States Numbered Highway that stretches from the Pacific Northwest east to New England. The "0" in its route number indicates that US 20 is a major ...

* -

US 26 U.S. Highway 26 (US 26) is an east–west United States Numbered Highway that runs from Seaside, Oregon, to Ogallala, Nebraska. When the U.S. Numbered Highway System was first defined, it was limited to Nebraska and Wyoming Wyoming ...

* -

US 30 U.S. Route 30 or U.S. Highway 30 (US 30) is an east–west main route of the United States Numbered Highway System, with the highway traveling across the Northern U.S. With a length of , it is the third-longest U.S. Highway, afte ...

* - SH-16 * - SH-21 - ''Ponderosa Pine Scenic Byway'' * - SH-44 * - SH-55 - ''Payette River Scenic Byway'' * -

SH-69 Route 69, or Highway 69, may refer to: International *European route E69 Australia * Appin Road Canada * Alberta Highway 69 * Ontario Highway 69 China * G69 Yinchuan–Baise Expressway, G69 Expressway Finland * Finnish national road 69 ...

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

National protected areas

Boise National Forest Boise National Forest is a United States National Forest, National Forest covering of the U.S. state of Idaho. Created on July 1, 1908, from part of Sawtooth National Forest, it is managed by the United States Forest Service, U.S. Forest Se ...

(part) *

Snake River Birds of Prey National Conservation Area The Morley Nelson Snake River Birds of Prey National Conservation Area has one of the densest populations of nesting raptors. The National Conservation Area (NCA) is located south of Boise, Idaho along of the Snake River, and is managed by th ...

(part)

Demographics

2010 census

As of the 2010 census, there were 392,365 people, 148,445 households, and 99,282 families in the county. The population density was . There were 159,471 housing units at an average density of . 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 German(s) may refer to: * Germany, the country of the Germans and German things **Germania (Roman era) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizenship in Germany, see also Ge ...

, 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.

2000 census

As of the 2000 census, there were 300,904 people, 113,408 households, and 77,344 families in the county. The

population density Population density (in agriculture: Standing stock (disambiguation), standing stock or plant density) is a measurement of population per unit land area. It is mostly applied to humans, but sometimes to other living organisms too. It is a key geog ...

was . There were 118,516 housing units at an average density of . The racial makeup of the county was 92.86% White, 0.65% Black or African American, 0.69% Native American, 1.74% Asian, 0.15% Pacific Islander, 1.67% from other races, and 2.24% from two or more races. Hispanic or Latino of any race were 4.48% of the population. There were 113,408 households, out of which 36.20% had children under the age of 18 living with them, 55.10% were

married couples Marriage, also called matrimony or wedlock, is a culturally and often legally recognised union between people called spouses. It establishes rights and obligations between them, as well as between them and their children (if any), and b ...

living together, 9.40% had a female householder with no husband present, and 31.80% were non-families. 23.80% of all households were made up of individuals, and 6.07% had someone living alone who was 65 years of age or older. The average household size was 2.59 and the average family size was 3.11. The county population contained 27.30% under the age of 18, 10.30% from 18 to 24, 32.50% from 25 to 44, 20.80% from 45 to 64, and 9.10% who were 65 years of age or older. The median age was 33 years. For every 100 females, there were 100.6 males. For every 100 females age 18 and over, there were 98.9 males. The median income for a household in the county was $46,140, and the median income for a family was $54,416. Males had a median income of $37,867 versus $26,453 for females. The

per capita income Per capita income (PCI) or average income measures the average income earned per person in a given area (city, region, country, etc.) in a specified year. In many countries, per capita income is determined using regular population surveys, such ...

for the county was $22,519. About 5.40% of families and 7.70% of the population were below the

poverty line The poverty threshold, poverty limit, poverty line, or breadline is the minimum level of income deemed adequate in a particular country. The poverty line is usually calculated by estimating the total cost of one year's worth of necessities for ...

, including 9.20% of those under age 18 and 5.70% of those age 65 or over.

Government and politics

Ada County has traditionally been rather conservative for an urban county. Like Idaho as a whole, it has long been a Republican Party stronghold. The last victory in a presidential election by a Democrat in Ada County was by

Franklin D. Roosevelt Franklin Delano Roosevelt (January 30, 1882April 12, 1945), also known as FDR, was the 32nd president of the United States, serving from 1933 until his death in 1945. He is the longest-serving U.S. president, and the only one to have served ...

1936 Events January–February * January 20 – The Prince of Wales succeeds to the throne of the United Kingdom as King Edward VIII, following the death of his father, George V, at Sandringham House. * January 28 – Death and state funer ...

. It rejected

Lyndon B. Johnson Lyndon Baines Johnson (; August 27, 1908January 22, 1973), also known as LBJ, was the 36th president of the United States, serving from 1963 to 1969. He became president after the assassination of John F. Kennedy, under whom he had served a ...

1964 Events January * January 1 – The Federation of Rhodesia and Nyasaland is dissolved. * January 5 – In the first meeting between leaders of the Roman Catholic and Orthodox churches since the fifteenth century, Pope Paul VI and Patria ...

during his 44-state landslide.

Barry Goldwater Barry Morris Goldwater (January 2, 1909 – May 29, 1998) was an American politician and major general in the United States Air Force, Air Force Reserve who served as a United States senator from 1953 to 1965 and 1969 to 1987, and was the Re ...

carried it by 13 points, a major reason why Idaho was Johnson's closest state. In

2008 2008 was designated as: *International Year of Languages *International Year of Planet Earth *International Year of the Potato *International Year of Sanitation The Great Recession, a worldwide recession which began in 2007, continued throu ...

the presidential election in Ada County was more competitive than in previous years;

John McCain John Sidney McCain III (August 29, 1936 – August 25, 2018) was an American statesman and United States Navy, naval officer who represented the Arizona, state of Arizona in United States Congress, Congress for over 35 years, first as ...

defeated

Barack Obama Barack Hussein Obama II (born August 4, 1961) is an American politician who was the 44th president of the United States from 2009 to 2017. A member of the Democratic Party, he was the first African American president in American history. O ...

by six percentage points. Obama became the first Democrat to garner as much as 40 percent of the county's vote since Lyndon B. Johnson. In 2016,

Donald Trump Donald John Trump (born June 14, 1946) is an American politician, media personality, and businessman who is the 47th president of the United States. A member of the Republican Party (United States), Republican Party, he served as the 45 ...

won the county by only a plurality due to high third party performance. In 2020, whereas the state of Idaho voted very strongly for Trump, he carried Ada County by a slim majority, only beating

Joe Biden Joseph Robinette Biden Jr. (born November 20, 1942) is an American politician who was the 46th president of the United States from 2021 to 2025. A member of the Democratic Party (United States), Democratic Party, he served as the 47th vice p ...

by around 3 percentage points. The election was close due to Biden's strength in Boise's downtown urban core, which gave him 59.2% of the vote to Trump's 37.4%. This is the narrowest election in decades for Ada County, and the closest a Democrat has come to carrying the county since 1940. Yet, in 2024, this trend reversed, with Trump once again taking the majority in the county with 53.76% of the vote. This was the best performance for a Republican presidential candidate in Ada county since George W. Bush in 2004. Democratic

gubernatorial A governor is an administrative leader and head of a polity or political region, in some cases, such as governors-general, as the head of a state's official representative. Depending on the type of political region or polity, a ''governor'' ma ...

nominee Jerry Brady carried the county in his 2002 and

2006 2006 was designated as the International Year of Deserts and Desertification. Events January * January 1– 4 – Russia temporarily cuts shipment of natural gas to Ukraine during a price dispute. * January 12 – A stampede during t ...

races, despite losing statewide in both contests. Another prominent Democrat, Boise mayor David Bieter, was elected in 2007, 2011 and 2015. Ada County is split between the

first First most commonly refers to: * First, the ordinal form of the number 1 First or 1st may also refer to: Acronyms * Faint Images of the Radio Sky at Twenty-Centimeters, an astronomical survey carried out by the Very Large Array * Far Infrared a ...

and

second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...

congressional districts; it is the only county in the state that is not located entirely within one district. As of 2022, the first district is represented by

Russ Fulcher Russell Mark Fulcher (born March 9, 1962) is an American businessman and politician serving as the U.S. representative for Idaho's 1st congressional district since 2019. A member of the Republican Party, he represented the 21st district in the ...

and includes Meridian,

Eagle Eagle is the common name for the golden eagle, bald eagle, and other birds of prey in the family of the Accipitridae. Eagles belong to several groups of Genus, genera, some of which are closely related. True eagles comprise the genus ''Aquila ( ...

and Kuna, while the second district is represented by

Mike Simpson Michael Keith Simpson (born September 8, 1950) is an American politician serving as the United States House of Representatives, U.S. representative for since 1999. The district covers most of the eastern portion of the state, including Idaho Fa ...

and includes most of Boise proper. Both Fulcher and Simpson are Republicans. In the

Idaho Legislature The Idaho Legislature is the legislative branch of the government of the 43rd State of Idaho and is bicameral, consisting of the upper chamber of the Idaho Senate and the lower chamber of the Idaho House of Representatives. The state of Idaho ...

, Ada County is split among nine districts, the most of any county. Each district elects one state senator and two state representatives. As of 2022, In the state senate, Republicans hold five seats and Democrats hold four. In the state house, Republicans hold ten seats and Democrats hold eight. Generally, Democratic strength is concentrated in Boise itself, while Republican strength is concentrated in the western suburbs. Several of the Boise seats were Democratic pickups in 2006. Each party held all of their respective legislative seats in the 2008 elections, but Republicans won two competitive

county commission A county commission (or a board of county commissioners) is a group of elected officials (county commissioners) collectively charged with administering the county government in some states of the United States. A county usually has three to fiv ...

races.

County offices

The county government is administered by the Ada County Board of Commissioners, a three-member legislative body. Other elected officials include

clerk A clerk is a white-collar worker who conducts record keeping as well as general office tasks, or a worker who performs similar sales-related tasks in a retail environment. The responsibilities of clerical workers commonly include Records managem ...

, treasurer,

sheriff A sheriff is a government official, with varying duties, existing in some countries with historical ties to England where the office originated. There is an analogous, although independently developed, office in Iceland, the , which is common ...

, assessor,

coroner A coroner is a government or judicial official who is empowered to conduct or order an inquest into the manner or cause of death. The official may also investigate or confirm the identity of an unknown person who has been found dead within th ...

, and

prosecutor A prosecutor is a legal representative of the prosecution in states with either the adversarial system, which is adopted in common law, or inquisitorial system, which is adopted in Civil law (legal system), civil law. The prosecution is the ...

. The

Idaho Department of Correction The Idaho Department of Correction (IDOC) operates nine prisons, four community release centers and 20 probation and parole offices in seven districts located throughout the state of Idaho. The agency has its headquarters in Boise. IDOC employs a ...

operates the South Boise Prison Complex, a correctional facility located in unincorporated Ada County, south of the

Boise Airport Boise Airport (Boise Air Terminal or Gowen Field) is a joint civil-military airport in the western United States in Idaho, south of downtown Boise in Ada County. The airport is operated by the city of Boise Department of Aviation, overseen ...

and east of Kuna.

Education

;Public School districts School districts include: * Boise School District * Kuna Joint School District 3 * Melba Joint School District 136 *

West Ada School District The West Ada School District #2, long known as the Meridian School District, is a school district based in Meridian, Idaho, Meridian, Idaho. In addition to Meridian, the district operates public schools in Eagle, Idaho, Eagle, Star, Idaho, St ...

(Meridian Joint School District 2) Residents are in the area (and the taxation zone) for

College of Western Idaho College of Western Idaho (CWI) is a public community college in Southwest Idaho with its primary campus locations in Boise and Nampa. CWI also offers classes at several community locations throughout the Treasure Valley. It is one of four c ...

Communities

Cities

(county seat and state capital) *

* Garden City * Kuna * Meridian *

Star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

Census-designated places

* Avimor * Hidden Springs

Unincorporated communities

* Mora * Pleasant Valley * Sonna''Sonna ID'' Google Maps (accessed January 16, 2019)
/ref>

Population ranking

The population ranking of the following table is based on the 2020 census of Ada County. † ''county seat''

References

External links

* {{coord, 43.45, -116.24, display=title, type:adm2nd_region:US-ID_source:UScensus1990 Idaho counties 1864 establishments in Idaho Territory Boise metropolitan area Populated places established in 1864