Middlesex County area

Population and density of Middlesex County

The Census Bureau estimates that 1,669,979 people lived in Middlesex County on July 1, 2025. That ranks #20 of 3,222 county equivalents nationally and #1 of 14 in Massachusetts. Since the April 1, 2020 estimates base of 1,631,912, the population has grown by 38,067 (+2.3%), compared with +1.7% for Massachusetts as a whole.

With 817.9 square miles of land, that is 2,042 people per square mile (788 per km²), placing Middlesex County #82 nationally for density and #2 in Massachusetts. The median county equivalent has 47 people per square mile; the Massachusetts median is 673. Put another way, there are about 0.313 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 Middlesex County

Middlesex County is divided into 54 cities and towns. In Massachusetts, county subdivisions are local governments, and together they cover all of the county’s land and residents.

The most populous is Cambridge city, with 122,588 residents (7.3% of the county). The largest by land is Groton town, at 32.8 mi², or 4% of the county’s land. Among those with at least 1,000 residents in 2020, Ayer town grew fastest since then (+9.2%).

Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each subdivision inside Middlesex County. Land: 2026 Census Gazetteer. How communities are listed.

Metropolitan area

Middlesex County is a central county of the Boston-Cambridge-Newton, MA-NH Metropolitan Statistical Area, in the Cambridge-Newton-Framingham, MA metropolitan division. Its 7 county equivalents have 5,034,221 residents on 3,486.3 square miles of land; Middlesex County accounts for 23.5% of the land and 33.2% of the residents. By combined land it ranks #84 of 393 metropolitan areas, and #11 by population. It belongs to the Boston-Worcester-Providence, MA-RI-NH Combined Statistical Area.

ZIP codes

83 ZIP Code Tabulation Areas have land in Middlesex County; 81 of them have most of their land inside the county, and the others mainly cover neighboring areas.

0143101432014340145001460014630146401469014740170101702017180171901720017210173001731017410174201746017480174901752017540176001770017730177501776017780180101803018210182401826018270185001851018520185401862018630186401867018760187901880018860188701890021380213902140021410214202143021440214502148021490215502176021800242002421024510245202453024580245902460024610246202464024650246602467024680247202474024760247802493

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 Middlesex County

Middlesex County shares a land border with 5 county equivalents, including 1 in New Hampshire. It has more land than 3 of them. The largest neighbor is Worcester County (1,510.7 mi²) and the smallest is Suffolk County (58.3 mi²). Together with its neighbors, Middlesex County forms a block of 4,152 mi² of land, of which it holds 19.7%.

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

Middlesex County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.3 times as much. Massachusetts’s county land could hold about 9.5 areas the size of Middlesex County.

Another way to picture it: the same land area would fill a square about 28.6 miles (46 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 Middlesex County?

Middlesex County has more land than 2,188 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 Middlesex County at #1,092 nationally by total area, 58 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 3.4%, compared with a national county-equivalent median of 1.2% — 2.3 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 Middlesex 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.