Contra Costa County area

Population and density of Contra Costa County

The Census Bureau estimates that 1,170,070 people lived in Contra Costa County on July 1, 2025. That ranks #41 of 3,222 county equivalents nationally and #9 of 58 in California. Since the April 1, 2020 estimates base of 1,166,035, the population has grown by 4,035 (+0.3%), compared with −0.5% for California as a whole.

With 716.9 square miles of land, that is 1,632 people per square mile (630 per km²), placing Contra Costa County #106 nationally for density and #7 in California. The median county equivalent has 47 people per square mile; the California median is 112. Put another way, there are about 0.392 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 Contra Costa County

19 incorporated places lie at least partly in Contra Costa County. Together they are home to 993,853 people (84.9%); the other 176,217 (15.1%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is Concord city, with 123,261 residents in the county. The county seat, Martinez city, has 36,849 residents. Among places with at least 1,000 residents in 2020, Oakley city grew fastest since then (+9.8%).

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

Metropolitan area

Contra Costa County is an outlying county of the San Francisco-Oakland-Fremont, CA Metropolitan Statistical Area, in the Oakland-Fremont-Berkeley, CA metropolitan division. Its 5 county equivalents have 4,630,041 residents on 2,469.8 square miles of land; Contra Costa County accounts for 29% of the land and 25.3% of the residents. By combined land it ranks #133 of 393 metropolitan areas, and #13 by population. It belongs to the San Jose-San Francisco-Oakland, CA Combined Statistical Area.

ZIP codes

49 ZIP Code Tabulation Areas have land in Contra Costa County; 43 of them have most of their land inside the county, and the others mainly cover neighboring areas.

94505945069450794509945119451394514945169451794518945199452094521945239452594526945289453094531945479454894549945519455394556945619456394564945659456994572945759458294583945889459594596945979459894611947069470794708948019480394804948059480695219

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 Contra Costa County

Contra Costa County shares a land border with 4 county equivalents. Each of them has more land than Contra Costa County. The largest neighbor is San Joaquin County (1,392.3 mi²) and the smallest is Alameda County (737.5 mi²). Together with its neighbors, Contra Costa County forms a block of 4,633.8 mi² of land, of which it holds 15.5%.

How big is Contra Costa County compared with U.S. states?

Contra Costa County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.4 times as much. California’s county land could hold about 217 areas the size of Contra Costa County.

Another way to picture it: the same land area would fill a square about 26.8 miles (43.1 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 Contra Costa County?

Contra Costa County has more land than 1,961 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 Contra Costa County at #1,172 nationally by total area, 89 places closer to the top than its land rank. The water assigned to this geography moves it ahead of some county equivalents with more land.

Its water share is 10.8%, compared with a national county-equivalent median of 1.2% — 9.7 percentage points higher. 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 Contra Costa 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.