Ada County is a county in Idaho, United States, home to the state capital and centred on the Boise River valley in the southwestern part of the state. It had a population of 494,967 at the 2020 census, making it by far Idaho’s most populous county, and covers 1,052 square miles (2,725 km2) of land plus 7.7 square miles (20 km2) of water, centred near 43°27′N 116°14′W.
Boise is the county seat, and its downtown postal code is 83702. The county contains six incorporated cities, one census-designated place, and a scatter of unincorporated rural communities. Idaho has no civil townships. Ada County sits in the Mountain time zone and carries FIPS code 16001.
Towns & Villages in Ada County
Ada County has six incorporated cities — Boise, Meridian, Eagle, Kuna, Star, and Garden City — plus the census-designated place of Hidden Springs and the planned community of Avimor. “CDP” below means census-designated place (unincorporated but tracked separately by the Census Bureau); “Unincorp.” means an unincorporated community with no defined census boundary.
| Place | Type | Pop. | ZIP |
|---|---|---|---|
| Boise | City · seat | 235,684 | 83702–83716 |
| Meridian | City | 117,635 | 83642, 83646 |
| Eagle | City | 30,346 | 83616 |
| Kuna | City | 24,011 | 83634 |
| Star | City | 11,117 | 83669 |
| Garden City | City | 12,316 | 83714 |
| Hidden Springs | CDP | — | 83714 |
| Avimor | Unincorp. | — | 83714 |
Several other place names are attached to Ada County in older records: Barber, Beatty, Blacks Creek, Boise Hills Village, Cloverdale, Collister, Highlands, Hillcrest, Leone, Mora, Orchard, Owyhee, Perkins, Pleasant Valley, Regina, Sonna, Swan Falls, Ustick, and Vernon. Most of these are unincorporated rural localities or former Boise neighborhoods absorbed by the city rather than standalone census places, so no independent population or ZIP figure is published for them.
ZIP Codes
- 83702, 83703, 83704, 83705, 83706, 83709, 83712, 83713, 83716 — Boise (city proper, multiple carrier routes)
- 83642, 83646 — Meridian
- 83616 — Eagle
- 83634 — Kuna
- 83669 — Star
- 83714 — Garden City (also serves Hidden Springs and Avimor)
Several additional Boise-area ZIP codes (including 83708, 83711, and a set of PO-Box-only and single-entity codes serving Boise State University, the state capitol, and large corporate mailers) are not tied to a distinct town.
Geography
Ada County straddles the Boise River as it flows northwest out of the foothills of the Boise Mountains toward the Snake River plain. The Boise Front foothills rise along the city’s northeastern edge, while the southern half of the county flattens into high desert rangeland toward the Owyhee canyonlands. The Boise River and the Lucky Peak and Arrowrock reservoirs upstream (in neighboring Boise County) supply irrigation water to the valley’s farmland.
Ada County borders Gem County and Boise County to the north, Elmore County to the east, Owyhee County to the south, and Canyon County to the west. Live neighboring county pages: Gem County, Boise County, Elmore County, Owyhee County, and Canyon County.
Demographics
Ada County’s 494,967 residents at the 2020 census made it home to roughly one in four Idahoans. Boise (235,684) and Meridian (117,635) together hold more than 70% of the county’s population; Eagle, Kuna, Garden City, and Star make up most of the remainder. Household income in the county runs above the Idaho median of $74,942, reflecting Boise’s concentration of state government, healthcare, technology, and finance jobs. Population density across the county’s 1,052 square miles of land works out to roughly 470 residents per square mile, though density is far higher inside Boise and Meridian and much lower in the rural south.
Economy & Local Life
Boise anchors Ada County’s economy as Idaho’s seat of state government and the headquarters city for Micron Technology, Albertsons, and several regional banks and healthcare systems, including St. Luke’s Health System and Saint Alphonsus. Idaho’s state government and Boise State University, home to the blue-turfed Albertsons Stadium, are both major employers and cultural anchors. Meridian has grown into a retail and healthcare hub in its own right, while the county’s southern reaches around Kuna and the Snake River canyon stay agricultural, producing hay, mint, and dairy. The Western Idaho Fair, held each August at the fairgrounds in Garden City, is the county’s largest annual event.
History
Ada County was created on December 22, 1864, carved out of Boise County by the Idaho territorial legislature. It was named for Ada Riggs, daughter of legislator H. C. Riggs, one of the county’s founders. Boise, already established as a supply town on the Oregon Trail and as Idaho’s territorial capital since 1864, became the new county’s seat. The completion of the Oregon Short Line Railroad through the valley in the 1880s and the New York Canal irrigation project in the 1900s turned the surrounding desert into farmland, setting the pattern of agriculture and small-city growth that defined the county through the 20th century before its recent boom as a technology and finance center.
Getting There
Interstate 84 runs east-west through the southern part of the county, connecting Boise and Meridian to Twin Falls and Utah to the east and Oregon to the west. U.S. Routes 20, 26, and 30 and Idaho State Highways 16, 21, 44, 55, and 69 fan out from Boise to serve the rest of the county and the surrounding region.
- Meridian — 11 miles west of downtown Boise
- Eagle — 12 miles northwest of downtown Boise
- Kuna — 20 miles southwest of downtown Boise
- Star — 18 miles northwest of downtown Boise
Boise Airport (BOI), Idaho’s largest commercial airport, sits just south of downtown Boise inside the county and offers direct flights to major West Coast, Mountain, and Midwest hubs.
Frequently Asked Questions
Where is Ada County?
It is in southwestern Idaho, on the Boise River, and contains the state capital, Boise.
What is the population of Ada County?
494,967 people at the 2020 census, the largest of any Idaho county.
How big is Ada County?
1,052 square miles (2,725 km2) of land plus 7.7 square miles (20 km2) of water.
What is the county seat of Ada County?
Boise, which is also the seat of Idaho’s state government.
How many towns are in Ada County?
Six incorporated cities — Boise, Meridian, Eagle, Kuna, Star, and Garden City — plus the CDP of Hidden Springs and the community of Avimor.
What ZIP codes are in Ada County?
Boise uses 83702 through 83716; Meridian uses 83642 and 83646; Eagle is 83616; Kuna is 83634; Star is 83669; Garden City is 83714.
What is Ada County’s biggest employer?
Boise-based Micron Technology and Albertsons are among the largest private employers, alongside Idaho’s state government and Boise State University.
More counties in this state: Towns and villages in Idaho.