San Mateo County area

Population and density of San Mateo County

The Census Bureau estimates that 743,568 people lived in San Mateo County on July 1, 2025. That ranks #93 of 3,222 county equivalents nationally and #15 of 58 in California. Since the April 1, 2020 estimates base of 764,508, the population has declined by 20,940 (−2.7%), compared with −0.5% for California as a whole.

With 448.6 square miles of land, that is 1,658 people per square mile (640 per km²), placing San Mateo County #103 nationally for density and #6 in California. The median county equivalent has 47 people per square mile; the California median is 112. Put another way, there are about 0.386 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 San Mateo County

20 incorporated places lie at least partly in San Mateo County. Together they are home to 682,686 people (91.8%); the other 60,882 (8.2%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is San Mateo city, with 103,337 residents in the county. The county seat, Redwood City city, has 82,178 residents. By land, Redwood City city is the largest at 19.3 mi². Among places with at least 1,000 residents in 2020, Burlingame city grew fastest since then (+1.7%).

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

Metropolitan area

San Mateo County is a central county of the San Francisco-Oakland-Fremont, CA Metropolitan Statistical Area, in the San Francisco-San Mateo-Redwood City, CA metropolitan division. Its 5 county equivalents have 4,630,041 residents on 2,469.8 square miles of land; San Mateo County accounts for 18.2% of the land and 16.1% 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

31 ZIP Code Tabulation Areas have land in San Mateo County; 30 of them have most of their land inside the county, and the others mainly cover neighboring areas.

94002940059401094014940159401894019940209402194025940279402894030940379403894044940609406194062940639406594066940709407494080941289430394401944029440394404

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 San Mateo County

San Mateo County shares a land border with 3 county equivalents. It has more land than 2 of them. The largest neighbor is Santa Clara County (1,291.1 mi²) and the smallest is San Francisco County (46.7 mi²). Together with its neighbors, San Mateo County forms a block of 2,231.5 mi² of land, of which it holds 20.1%.

How big is San Mateo County compared with U.S. states?

San Mateo County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 2.3 times as much. California’s county land could hold about 347 areas the size of San Mateo County.

Another way to picture it: the same land area would fill a square about 21.2 miles (34.1 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 San Mateo County?

San Mateo County has more land than 945 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 San Mateo County at #1,300 nationally by total area, 977 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 39.5%, compared with a national county-equivalent median of 1.2% — 38.3 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 San Mateo 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.