Mercer County area

Population and density of Mercer County

The Census Bureau estimates that 399,289 people lived in Mercer County on July 1, 2025. That ranks #196 of 3,222 county equivalents nationally and #12 of 21 in New Jersey. Since the April 1, 2020 estimates base of 387,334, the population has grown by 11,955 (+3.1%), compared with +2.8% for New Jersey as a whole.

With 224.5 square miles of land, that is 1,779 people per square mile (687 per km²), placing Mercer County #92 nationally for density and #8 in New Jersey. The median county equivalent has 47 people per square mile; the New Jersey median is 1,137. Put another way, there are about 0.36 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 Mercer County

Mercer County is divided into 12 municipalities and townships. In New Jersey, county subdivisions are local governments, and together they cover all of the county’s land and residents.

The most populous is Hamilton township, with 94,157 residents (23.6% of the county). The largest by land is Hopewell township, at 58.1 mi², or 25.9% of the county’s land. The county seat, Trenton city, has 91,506 residents. Among those with at least 1,000 residents in 2020, West Windsor township grew fastest since then (+11.4%).

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

Metropolitan area

Mercer County is a central county of the Trenton-Princeton, NJ Metropolitan Statistical Area. Mercer County is the area’s only county equivalent, so the metro area has the same 224.5 square miles of land. By combined land it ranks #391 of 393 metropolitan areas, and #143 by population. It belongs to the New York-Newark, NY-NJ-CT-PA Combined Statistical Area.

ZIP codes

26 ZIP Code Tabulation Areas have land in Mercer County; 22 of them have most of their land inside the county, and the others mainly cover neighboring areas.

0850108512085200852508530085340854008542085440855008558085600856108608086090861008611086180861908620086280862908638086480869008691

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 Mercer County

Mercer County shares a land border with 6 county equivalents, including 1 in Pennsylvania. Each of them has more land than Mercer County. The largest neighbor is Burlington County (799.3 mi²) and the smallest is Somerset County (301.9 mi²). Together with its neighbors, Mercer County forms a block of 3,135.4 mi² of land, of which it holds 7.2%.

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

Mercer County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 4.6 times as much. New Jersey’s county land could hold about 33 areas the size of Mercer County.

Another way to picture it: the same land area would fill a square about 15 miles (24.1 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 Mercer County?

Mercer County has more land than 261 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 Mercer County at #2,984 nationally by total area, 23 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 1.9%, compared with a national county-equivalent median of 1.2% — 0.7 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 Mercer 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.