St. Clair County area

Population and density of St. Clair County

The Census Bureau estimates that 9,976 people lived in St. Clair County on July 1, 2025. That ranks #2,483 of 3,222 county equivalents nationally and #85 of 115 in Missouri. Since the April 1, 2020 estimates base of 9,285, the population has grown by 691 (+7.4%), compared with +1.9% for Missouri as a whole.

With 675 square miles of land, that is 15 people per square mile (5.7 per km²), placing St. Clair County #2,480 nationally for density and #93 in Missouri. The median county equivalent has 47 people per square mile; the Missouri median is 31. Put another way, there are about 43 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 St. Clair County

7 incorporated places lie at least partly in St. Clair County. Together they are home to 3,045 people (30.5%); the other 6,931 (69.5%) live outside any incorporated place, in unincorporated communities and rural areas.

The largest is Appleton City city, with 1,096 residents in the county. The county seat, Osceola city, has 1,012 residents. By land, Roscoe village is the largest at 1.531 mi².

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

Metropolitan area

St. Clair County is not part of any metropolitan or micropolitan statistical area in the Office of Management and Budget’s July 2023 delineation; 1,306 other county equivalents are also outside one. These areas are built around an urban core of at least 10,000 people plus the counties tied to it by commuting, so counties outside them are generally rural.

ZIP codes

16 ZIP Code Tabulation Areas have land in St. Clair County; 6 of them have most of their land inside the county, and the others mainly cover neighboring areas.

64724647386474064744647506476364770647766478064781647836535565674657356577465785

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 St. Clair County

St. Clair County shares a land border with 7 county equivalents. It has more land than 3 of them. The largest neighbor is Bates County (836.7 mi²) and the smallest is Hickory County (398.8 mi²). Together with its neighbors, St. Clair County forms a block of 5,247.8 mi² of land, of which it holds 12.9%.

How big is St. Clair County compared with U.S. states?

St. Clair County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.5 times as much. Missouri’s county land could hold about 102 areas the size of St. Clair County.

Another way to picture it: the same land area would fill a square about 26 miles (41.8 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 St. Clair County?

St. Clair County has more land than 1,827 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 St. Clair County at #1,432 nationally by total area, 37 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 3.9%, compared with a national county-equivalent median of 1.2% — 2.7 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 St. Clair 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.