Jefferson County area

Population and density of Jefferson County

The Census Bureau estimates that 233,132 people lived in Jefferson County on July 1, 2025. That ranks #311 of 3,222 county equivalents nationally and #7 of 115 in Missouri. Since the April 1, 2020 estimates base of 226,573, the population has grown by 6,559 (+2.9%), compared with +1.9% for Missouri as a whole.

With 656 square miles of land, that is 355 people per square mile (137 per km²), placing Jefferson County #448 nationally for density and #7 in Missouri. The median county equivalent has 47 people per square mile; the Missouri median is 31. Put another way, there are about 1.8 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 Jefferson County

16 incorporated places lie at least partly in Jefferson County. Together they are home to 66,409 people (28.5%); the other 166,723 (71.5%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is Arnold city, with 20,994 residents in the county. The county seat, Hillsboro city, has 3,493 residents. Among places with at least 1,000 residents in 2020, Herculaneum city grew fastest since then (+15.9%).

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

Metropolitan area

Jefferson County is a central county of the St. Louis, MO-IL Metropolitan Statistical Area. Its 15 county equivalents have 2,814,421 residents on 7,863.5 square miles of land; Jefferson County accounts for 8.3% of the land and 8.3% of the residents. By combined land it ranks #19 of 393 metropolitan areas, and #23 by population. It belongs to the St. Louis-St. Charles-Farmington, MO-IL Combined Statistical Area.

ZIP codes

25 ZIP Code Tabulation Areas have land in Jefferson County; 16 of them have most of their land inside the county, and the others mainly cover neighboring areas.

63010630126301563016630196302063023630256302663028630306304163047630486304963050630516305263053630696307063072630876362763628

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 Jefferson County

Jefferson County shares a land border with 6 county equivalents, including 1 in Illinois. It has more land than 4 of them. The largest neighbor is Franklin County (922.5 mi²) and the smallest is Monroe County (385.3 mi²). Together with its neighbors, Jefferson County forms a block of 4,183.3 mi² of land, of which it holds 15.7%.

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

Jefferson County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.6 times as much. Missouri’s county land could hold about 105 areas the size of Jefferson County.

Another way to picture it: the same land area would fill a square about 25.6 miles (41.2 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 Jefferson County?

Jefferson County has more land than 1,788 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 Jefferson County at #1,530 nationally by total area, 96 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 1.2%, compared with a national county-equivalent median of 1.2%. The two percentages are within 0.1 percentage points of each other. 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 Jefferson 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.