County Maps US

San Mateo County, California

A county in California, ranked 15 of 58 by population.

CountyFIPS 06081

Within California
San Mateo County, highlighted, with the areas around it. Boundaries generalised from the 2025 Census cartographic boundary file; neighbour fills carry no data.

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Population
743,568
-19,071 (-2.5%) since 2020
Land area
448.6
square miles. A further 39.5% of the total area is water
Population density
1,658
people per square mile, computed rather than published
County seat
Redwood City
Seat of county government.
Established
1856
169 years ago
Largest city
San Mateo city
Population 103,337.

Sources: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026; US Census Bureau, Gazetteer Files, 2025 Gazetteer Files, released 10 September 2025; Wikidata (CC0), Wikidata. Retrieved 30 August 2026.

Satellite view

Aerial imagery of San Mateo County with its boundary drawn on top. A map shows you the shape; this shows you what is inside it, and which side of the line a place falls on.

Aerial view of San Mateo County, California from the USGS National Map, with its boundary outlined
Imagery: USGS National Map, USGS Imagery Only, public domain, requested from USGS by your browser when you reach this point on the page, at 468 by 800 pixels. The boundary is the 2025 Census cartographic boundary file, reprojected to Web Mercator to match the imagery. You can also open the USGS National Map viewer.

Where San Mateo County ranks

Position among the 58 counties of California, and among all 3,144 US counties and county-equivalents. Computed from the figures on this page.

MeasureValueIn CaliforniaNationally
Population743,56815th of 5893rd of 3,144
Land area448.6 sq mi56th of 582,277th of 3,144
Population density1,658 per sq mi6th of 5894th of 3,144
Median household income$158,8552nd of 583rd of 3,143

Full data for San Mateo County

Every figure below is attributed to the Census Bureau product it came from, with the vintage of that product.
MeasureValueSource and vintage
Population743,568Vintage 2025 Population Estimates
Population in 2020762,639Vintage 2025 Population Estimates
2020 estimates base?764,508Vintage 2025 Population Estimates
Land area448.6 sq mi2025 Gazetteer Files
Water area292.4 sq mi2025 Gazetteer Files
Total area741 sq mi2025 Gazetteer Files
Population density?1,658 per sq miComputed: Vintage 2025 estimates divided by 2025 Gazetteer land area
Median household income$158,855American Community Survey 2020-2024 5-Year Estimates
Median home value$1,559,600American Community Survey 2020-2024 5-Year Estimates
Median age40.9American Community Survey 2020-2024 5-Year Estimates
Households264,419American Community Survey 2020-2024 5-Year Estimates
Housing units287,184American Community Survey 2020-2024 5-Year Estimates

Sources: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026; US Census Bureau, Gazetteer Files, 2025 Gazetteer Files, released 10 September 2025; US Census Bureau, American Community Survey, American Community Survey 2020-2024 5-Year Estimates, released 29 January 2026. Population density is computed here, not published: it divides the Vintage 2025 population estimate by the 2025 Gazetteer land area, which are two different Census products. Retrieved 30 August 2026.

How the population changed

The Census Bureau publishes the components behind an estimate, not just the total. For the year to July 2025:

Births minus deaths
+2,279
7,131 births, 4,852 deaths
Net migration
+627
More people moved in than moved out.

Source: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026. Retrieved 30 August 2026.

The 4 areas bordering San Mateo County

Ordered by the length of the border actually shared, which the Census Bureau publishes in its county adjacency file and which no other county reference presents.

Bordering areaStateShared borderPopulation
Santa Cruz CountyCalifornia25 mi258,852
Santa Clara CountyCalifornia24 mi1,914,391
Alameda CountyCalifornia19 mi1,636,630
San Francisco CountyCalifornia17 mi826,079

Source: US Census Bureau, County Adjacency File, 2025 County Adjacency File, released 15 September 2025. Retrieved 30 August 2026.

Side by side

All 58 counties of California

Open the interactive California county map, or jump straight to any of them.

Data on this page last rebuilt 30 August 2026. How these figures are produced.