# Ada County, Idaho

Ada County is located in the southwestern part of Idaho, United States. As of the 2020 census, the county had a population of 494,967, which by 2022 was estimated to have risen to 518,907. 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 and largest city is Boise, which is also the state capital. Ada County is included in the Boise metropolitan area. 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 east of the Cascade Range, Ada County ranks second in population, behind Spokane County, Washington.

## History

Ada County was created by the Idaho Territory legislature on December 22, 1864, partitioned from Boise County. 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 Boise. 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 county has a total area of 1,060 square miles (2,700 km$^2$), of which 1,053 square miles (2,730 km$^2$) is land and 7.9 square miles (20 km$^2$) (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

* Gem County - northwest
* Boise County - northeast
* Elmore County - east
* Owyhee County - south
* Canyon County - west

### Major highways

* - Interstate 84
* - Interstate 184
* - US 20
* - US 26
* - US 30
* - SH-16
* - SH-21 - Ponderosa Pine Scenic Byway
* - SH-44
* - SH-55 - Payette River Scenic Byway
* - SH-69
County roads and highways are maintained by the Ada County Highway District.

### National protected areas

* Boise National Forest (part)
* Snake River Birds of Prey National Conservation Area (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 372.8 inhabitants per square mile (143.9/km$^2$). There were 159,471 housing units at an average density of 151.5 per square mile (58.5/km$^2$). 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.

### 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 was 285/sq mi (110/km$^2$). There were 118,516 housing units at an average density of 112/sq mi (43/km$^2$). 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 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 for the county was $22,519. About 5.40% of families and 7.70% of the population were below the poverty line, 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 in 1936. It rejected Lyndon B. Johnson in 1964 during his 44-state landslide. Barry Goldwater carried it by 13 points, a major reason why Idaho was Johnson's closest state. In 2008 the presidential election in Ada County was more competitive than in previous years; John McCain defeated Barack Obama 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 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 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 nominee Jerry Brady carried the county in his 2002 and 2006 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 and second 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 and includes Meridian, Eagle and Kuna, while the second district is represented by Mike Simpson and includes most of Boise proper. Both Fulcher and Simpson are Republicans.
In the Idaho Legislature, 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 races.
Ada County registered voters by political party (as of November 2, 2020) :
| Political Party | Constitution | Democratic | Libertarian | Republican | Unaffiliated | Total registered voters |
| Number of registered voters | 921 | 60,167 | 4,199 | 161,707 | 100,962 | 327,956 |
United States presidential election results for Ada County, Idaho :
| Year | Republican | Republican | Democratic | Democratic | Third party(ies) | Third party(ies) |
| Year | No. | % | No. | % | No. | % |
| 2024 | 143,759 | 53.76% | 116,116 | 43.42% | 7,544 | 2.82% |
| 2020 | 130,699 | 49.98% | 120,539 | 46.10% | 10,250 | 3.92% |
| 2016 | 93,752 | 47.91% | 75,677 | 38.68% | 26,240 | 13.41% |
| 2012 | 97,554 | 53.53% | 77,137 | 42.33% | 7,555 | 4.15% |
| 2008 | 93,328 | 51.63% | 82,236 | 45.49% | 5,196 | 2.87% |
| 2004 | 94,641 | 61.05% | 58,523 | 37.75% | 1,866 | 1.20% |
| 2000 | 75,050 | 60.78% | 40,650 | 32.92% | 7,785 | 6.30% |
| 1996 | 61,811 | 52.50% | 43,040 | 36.55% | 12,892 | 10.95% |
| 1992 | 49,000 | 44.48% | 31,941 | 28.99% | 29,225 | 26.53% |
| 1988 | 54,951 | 62.92% | 30,525 | 34.95% | 1,858 | 2.13% |
| 1984 | 60,036 | 72.40% | 21,760 | 26.24% | 1,128 | 1.36% |
| 1980 | 55,205 | 63.79% | 21,324 | 24.64% | 10,015 | 11.57% |
| 1976 | 41,135 | 64.39% | 21,125 | 33.07% | 1,629 | 2.55% |
| 1972 | 36,665 | 67.51% | 12,687 | 23.36% | 4,959 | 9.13% |
| 1968 | 30,185 | 63.04% | 11,529 | 24.08% | 6,167 | 12.88% |
| 1964 | 25,404 | 56.40% | 19,639 | 43.60% | 0 | 0.00% |
| 1960 | 27,703 | 61.95% | 17,017 | 38.05% | 0 | 0.00% |
| 1956 | 26,387 | 69.96% | 11,328 | 30.04% | 0 | 0.00% |
| 1952 | 27,415 | 72.65% | 10,281 | 27.24% | 40 | 0.11% |
| 1948 | 14,972 | 56.06% | 11,253 | 42.14% | 480 | 1.80% |
| 1944 | 13,410 | 55.54% | 10,667 | 44.18% | 67 | 0.28% |
| 1940 | 12,861 | 50.85% | 12,381 | 48.95% | 51 | 0.20% |
| 1936 | 7,581 | 36.85% | 12,027 | 58.46% | 966 | 4.70% |
| 1932 | 8,055 | 45.97% | 8,836 | 50.43% | 631 | 3.60% |
| 1928 | 10,279 | 71.84% | 3,921 | 27.40% | 108 | 0.75% |
| 1924 | 7,220 | 54.47% | 3,780 | 28.52% | 2,255 | 17.01% |
| 1920 | 8,419 | 66.72% | 4,173 | 33.07% | 27 | 0.21% |
| 1916 | 5,299 | 48.32% | 5,207 | 47.48% | 460 | 4.19% |
| 1912 | 3,198 | 32.00% | 2,569 | 25.71% | 4,227 | 42.30% |
| 1908 | 4,778 | 53.08% | 3,721 | 41.34% | 503 | 5.59% |
| 1904 | 4,536 | 69.51% | 1,466 | 22.46% | 524 | 8.03% |
| 1900 | 2,706 | 56.63% | 2,072 | 43.37% | 0 | 0.00% |
| 1896 | 851 | 35.33% | 1,531 | 63.55% | 27 | 1.12% |
| 1892 | 1,170 | 41.39% | 0 | 0.00% | 1,657 | 58.61% |

### County offices

The county government is administered by the Ada County Board of Commissioners, a three-member legislative body. Other elected officials include clerk, treasurer, sheriff, assessor, coroner, and prosecutor.
County elected offices :
| Office | Office holder | Party |
| Assessor | Robert H. McQuade | Republican |
| Clerk of the Circuit Court | Trent Tripple | Republican |
| County Commissioner District 1 | Ryan Davidson | Republican |
| County Commissioner District 2 | Rod Beck (chair of the board) | Republican |
| County Commissioner District 3 | Tom Dayley | Republican |
| Coroner | Richard Riffle | Republican |
| Prosecutor | Jan M. Bennetts | Republican |
| Sheriff | Matthew Clifford | Republican |
| Treasurer | Elizabeth Mahn | Republican |
The Idaho Department of Correction operates the South Boise Prison Complex, a correctional facility located in unincorporated Ada County, south of the Boise Airport 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 (Meridian Joint School District 2)
Residents are in the area (and the taxation zone) for College of Western Idaho.

## Communities
### Cities

* Boise (county seat and state capital)
* Eagle
* Garden City
* Kuna
* Meridian
* Star

### Census-designated places

* Avimor
* Hidden Springs

### Unincorporated communities

* Mora
* Pleasant Valley
* Sonna

### Population ranking

The population ranking of the following table is based on the 2020 census of Ada County.
† county seat
| Rank | Cities | Municipal type | Population (2022 Estimate) | Population (2020 Census) |
| 1 | † Boise | City | 236,634 | 235,684 |
| 2 | Meridian | City | 129,736 | 117,635 |
| 3 | Eagle | City | 32,399 | 30,346 |
| 4 | Kuna | City | 27,229 | 24,011 |
| 5 | Star | City | 14,646 | 11,117 |
| 6 | Garden City | City | 12,927 | 12,316 |
| 7 | Hidden Springs | CDP | — | 3,076 |
| 8 | Avimor | CDP | — | 1,255 |