County outline
Where St. Clair County is located
Area breakdown
Land accounts for 97.6% of St. Clair County’s measured area and water for 2.4%. Its combined area converts to 1,745.4 square kilometers or approximately 431,290 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 657.748 | 1,703.559 | 420,958.7 |
| Water | 16.143 | 41.81 | 10,331.5 |
| Total | 673.891 | 1,745.37 | 431,290.2 |
How St. Clair County compares
By original Census land area, St. Clair County ranks #28 of 102 in Illinois and #1428 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 1.288 times the Illinois county-equivalent median of 510.7 square miles. It accounts for 1.2% of the combined county land area in the state.
See the complete Illinois ranking · What counts as a county equivalent?
Population and density of St. Clair County
The Census Bureau estimates that 250,708 people lived in St. Clair County on July 1, 2025. That ranks #295 of 3,222 county equivalents nationally and #9 of 102 in Illinois. Since the April 1, 2020 estimates base of 257,617, the population has declined by 6,909 (−2.7%), compared with −0.8% for Illinois as a whole.
With 657.7 square miles of land, that is 381 people per square mile (147 per km²), placing St. Clair County #425 nationally for density and #9 in Illinois. The median county equivalent has 47 people per square mile; the Illinois median is 44. Put another way, there are about 1.7 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 257,057 | 255,194 | 252,830 | 252,232 | 251,375 | 250,708 |
| Change | — | −1,863 | −2,364 | −598 | −857 | −667 |
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
30 incorporated places lie at least partly in St. Clair County. Together they are home to 201,106 people (80.2%); the other 49,602 (19.8%) live outside any incorporated place, in unincorporated communities and rural areas.
The largest is Belleville city, with 40,237 residents in the county. Among places with at least 1,000 residents in 2020, Mascoutah city grew fastest since then (+5.1%).
| Incorporated place | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Belleville cityCounty seat | 40,237 | −5% | 23.2 |
| O'Fallon city | 32,656 | +0.7% | 16 |
| East St. Louis city | 17,682 | −4.4% | 13.9 |
| Cahokia Heights city | 16,874 | −5.6% | 15.9 |
| Fairview Heights city | 16,175 | −3.4% | 11.5 |
| Shiloh village | 14,604 | +3.5% | 13.6 |
| Swansea village | 14,356 | −0.1% | 6.553 |
| Mascoutah city | 9,239 | +5.1% | 9.64 |
| Lebanon city | 4,884 | +2.4% | 2.695 |
| Freeburg village | 4,507 | −2.1% | 6.824 |
| Caseyville village | 4,402 | +0.3% | 7.312 |
| Millstadt village | 3,950 | −3.5% | 3.562 |
| Smithton village | 3,868 | −3.5% | 4.51 |
| Dupo villageAlso in 1 other county | 3,766 | −5.7% | 6.179 |
| Collinsville cityAlso in 1 other county | 2,555 | −5.5% | 15.4 |
Show the other 15 places
| Name | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Washington Park village | 2,461 | −4.6% | 2.543 |
| Fairmont City villageAlso in 1 other county | 2,132 | −4.8% | 6.205 |
| New Athens village | 1,854 | −5.3% | 2.112 |
| Marissa village | 1,724 | −5.8% | 7.214 |
| Brooklyn village | 626 | −4.4% | 0.811 |
| St. Libory village | 598 | −4.9% | 0.97 |
| Lenzburg village | 436 | −4.8% | 1.155 |
| East Carondelet village | 370 | −5.1% | 1.221 |
| Summerfield village | 336 | −5.1% | 0.522 |
| Fayetteville village | 285 | −3.1% | 0.296 |
| New Baden villageAlso in 1 other county | 207 | −5% | 1.847 |
| Columbia cityAlso in 1 other county | 191 | −3.5% | 10.6 |
| Sauget village | 129 | −3.7% | 4.539 |
| Hecker villageAlso in 1 other county | 1 | 0% | 0.268 |
| Madison cityAlso in 1 other county | 1 | 0% | 14.1 |
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 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; St. Clair County accounts for 8.4% of the land and 8.9% 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.
| County equivalent | Population 2025 | Land mi² |
|---|---|---|
| St. Louis CountyMissouri | 990,911 | 508.5 |
| St. Charles CountyMissouri | 426,499 | 560.3 |
| St. Louis cityMissouri | 278,144 | 61.7 |
| Madison County | 263,110 | 715.5 |
| St. Clair County | 250,708 | 657.7 |
| Jefferson CountyMissouri | 233,132 | 656 |
| Franklin CountyMissouri | 108,090 | 922.5 |
| Lincoln CountyMissouri | 66,676 | 626.6 |
| Macoupin County | 43,748 | 863 |
| Warren CountyMissouri | 39,016 | 428.6 |
| Clinton County | 37,128 | 473.9 |
| Monroe County | 34,840 | 385.3 |
| Jersey County | 21,160 | 369.6 |
| Bond County | 16,959 | 380.3 |
| Calhoun County | 4,300 | 253.8 |
ZIP codes
34 ZIP Code Tabulation Areas have land in St. Clair County; 27 of them have most of their land inside the county, and the others mainly cover neighboring areas.
62059620606220162203622046220562206622076220862220622216222362225622266223262234622366223962240622436225462255622576225862260622646226562269622826228562289622936229462298
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, including 2 in Missouri. It has more land than 6 of them. The largest neighbor is Madison County (715.5 mi²) and the smallest is St. Louis city (61.7 mi²). Together with its neighbors, St. Clair County forms a block of 3,940.7 mi² of land, of which it holds 16.7%.
| Bordering county equivalent | Land mi² | Size vs. St. Clair | Compare |
|---|---|---|---|
| Madison County | 715.5 | 1.1× larger | Compare |
| Randolph County | 575.4 | 1.1× smaller | Compare |
| Washington County | 562.6 | 1.2× smaller | Compare |
| St. Louis CountyMissouri | 508.5 | 1.3× smaller | Compare |
| Clinton County | 473.9 | 1.4× smaller | Compare |
| Monroe County | 385.3 | 1.7× smaller | Compare |
| St. Louis cityMissouri | 61.7 | 10.7× smaller | Compare |
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 1.6 times as much. Illinois’s county land could hold about 84 areas the size of St. Clair County.
Another way to picture it: the same land area would fill a square about 25.6 miles (41.3 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 1,794 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 #1,510 nationally by total area, 82 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 2.4%, compared with a national county-equivalent median of 1.2% — 1.2 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.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #1427 | Pulaski County, Kentucky | 658.1 | Compare |
| #1428 | St. Clair County, Illinois | 657.7 | This county |
| #1429 | Allen County, Indiana | 657.1 | Compare |
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.
Representative coordinates
The Census internal point is 38.470198, -89.928546. It represents a location inside the geography; it is not necessarily the centroid or the address of a courthouse.
Source and precision
U.S. Census Bureau, 2026 Gazetteer Files, Counties. Original ALAND: 1,703,560,251 m²; AWATER: 41,809,675 m². Rankings use ALAND; exact ties use ascending GEOID. Tables preserve Census-published square miles to three decimals, with conversions based on those values.
Common questions
How big is St. Clair County?
St. Clair County, Illinois has 657.7 square miles (1,703.6 km²) of land and 16.1 square miles of water, for a total area of 673.9 square miles. It ranks #1428 of 3,222 U.S. county equivalents by land area.
Is St. Clair County the largest county in Illinois?
No. St. Clair County ranks #28 of 102 in Illinois by land area. The largest is McLean County, with 1,183.2 square miles of land.
What counties border St. Clair County?
St. Clair County shares a land border with Monroe County, Madison County, Clinton County, Washington County, Randolph County, St. Louis city, Missouri, and St. Louis County, Missouri.
What is the population of St. Clair County?
The Census Bureau estimates St. Clair County had 250,708 residents on July 1, 2025, ranking #295 of 3,222 county equivalents. That is 381 people per square mile of land.
What is the county seat of St. Clair County?
The county seat of St. Clair County is Belleville.
What cities and towns are in St. Clair County?
St. Clair County has 30 incorporated places. The most populous five are Belleville city, O'Fallon city, East St. Louis city, Cahokia Heights city, and Fairview Heights city; 49,602 residents live outside any incorporated place.
Is St. Clair County part of a metropolitan area?
Yes. It is a central county of the St. Louis, MO-IL Metropolitan Statistical Area, which has 15 county equivalents and 2,814,421 residents.
How many ZIP codes are in St. Clair County?
34 ZIP Code Tabulation Areas have land in St. Clair County, and 27 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does St. Clair County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 1.6 times as much land.
How many acres of land are in St. Clair County?
The land measurement converts to approximately 420,959 acres. Including water, total area is approximately 431,290 acres. One square mile equals 640 acres.
Why might another source show a different area?
Check whether it reports land or total area, its Census vintage, and its rounding. Our guide explains how to reconcile county area figures.
Can this establish a property boundary?
No. Statistical measurements and generalized maps do not establish property lines. Consult the relevant local surveying or land-records authority.