Ada County area

Population and density of Ada County

The Census Bureau estimates that 546,141 people lived in Ada County on July 1, 2025. That ranks #134 of 3,222 county equivalents nationally and #1 of 44 in Idaho. Since the April 1, 2020 estimates base of 494,966, the population has grown by 51,175 (+10.3%), compared with +10.4% for Idaho as a whole.

With 1,052.3 square miles of land, that is 519 people per square mile (200 per km²), placing Ada County #331 nationally for density and #1 in Idaho. The median county equivalent has 47 people per square mile; the Idaho median is 10. Put another way, there are about 1.2 acres of land per resident.

U.S. Census Bureau, Vintage 2025 population estimates. Estimates are revised each year and are not decennial census counts. Density divides the whole population by the whole county’s land area, so it describes an average, not how settlement is distributed within the county. Population sources and methods.

Cities and towns in Ada County

6 incorporated places lie at least partly in Ada County. Together they are home to 481,451 people (88.2%); the other 64,690 (11.8%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is Boise City city, with 238,429 residents in the county. Among places with at least 1,000 residents in 2020, Star city grew fastest since then (+84.6%).

Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside Ada County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.

Metropolitan area

Ada County is a central county of the Boise City, ID Metropolitan Statistical Area. Its 5 county equivalents have 864,243 residents on 11,766.6 square miles of land; Ada County accounts for 8.9% of the land and 63.2% of the residents. By combined land it ranks #6 of 393 metropolitan areas, and #75 by population. It belongs to the Boise City-Mountain Home-Ontario, ID-OR Combined Statistical Area.

ZIP codes

19 ZIP Code Tabulation Areas have land in Ada County; 16 of them have most of their land inside the county, and the others mainly cover neighboring areas.

83616836348364183642836468366983687837028370383704837058370683709837128371383714837168372083725

ZIP Code Tabulation Areas are the Census Bureau’s approximations of USPS ZIP Code service areas, from the 2020 Census relationship files. Lighter codes have less than half of their land in this county. Post-office-box and single-address ZIP Codes have no tabulation area.

Counties bordering Ada County

Ada County shares a land border with 5 county equivalents. It has more land than 2 of them. The largest neighbor is Owyhee County (7,668.2 mi²) and the smallest is Gem County (559.8 mi²). Together with its neighbors, Ada County forms a block of 14,841.7 mi² of land, of which it holds 7.1%.

How big is Ada County compared with U.S. states?

Ada County has more land than 1 of the 50 states. It is about 1 times the land area of Rhode Island (1,033.9 mi²), while Delaware, the next larger state, has 1.9 times as much land (1,948.6 mi²). Idaho’s county land could hold about 79 areas the size of Ada County.

Another way to picture it: the same land area would fill a square about 32.4 miles (52.2 km) on each side. Real county shapes are rarely square, so this is a scale reference rather than a width or driving distance.

State figures are the sums of each state’s county-equivalent land measurements in the same Census release, so states and counties are compared on the same basis. Published state totals can differ slightly because of rounding. Compare all states · Counties bigger than entire states.

What stands out about Ada County?

Ada County has more land than 2,602 of the other 3,221 county equivalents in the dataset.

A median is the middle county-equivalent measurement, not the average of the largest and smallest. All three bars use the same linear scale. Very small areas may appear as a thin sliver; the labels retain their values.

Does including water change its rank?

Using land plus water places Ada County at #708 nationally by total area, 88 places farther from the top than its land rank. Other county equivalents gain more area when their water measurements are included.

Its water share is 0.8%, compared with a national county-equivalent median of 1.2% — 0.4 percentage points lower. This describes the statistical geography’s land/water mix, not flood exposure or developable land.

Closest positions in the national land ranking

These entries sit immediately beside Ada County in the national ranking. They can be in different states and are not necessarily geographic neighbors.

Calculated from the same Census release as the area table. Total-area order uses original ALAND + AWATER; water shares and chart labels use published square miles. Calculation details.