County outline
Where St. Charles County is located
Area breakdown
Land accounts for 94.6% of St. Charles County’s measured area and water for 5.4%. Its combined area converts to 1,534.2 square kilometers or approximately 379,113 acres.
Green: land · blue: water
| Measurement | Square miles | Square kilometers | Acres |
|---|---|---|---|
| Land | 560.309 | 1,451.194 | 358,597.8 |
| Water | 32.055 | 83.022 | 20,515.2 |
| Total | 592.364 | 1,534.216 | 379,113 |
How St. Charles County compares
By original Census land area, St. Charles County ranks #65 of 115 in Missouri and #1848 of 3,222 nationally. National rankings include the 50 states, District of Columbia, and Puerto Rico.
Its land area is 0.946 times the Missouri county-equivalent median of 592 square miles. It accounts for 0.8% of the combined county land area in the state.
See the complete Missouri ranking · What counts as a county equivalent?
Population and density of St. Charles County
The Census Bureau estimates that 426,499 people lived in St. Charles County on July 1, 2025. That ranks #180 of 3,222 county equivalents nationally and #3 of 115 in Missouri. Since the April 1, 2020 estimates base of 405,266, the population has grown by 21,233 (+5.2%), compared with +1.9% for Missouri as a whole.
With 560.3 square miles of land, that is 761 people per square mile (294 per km²), placing St. Charles County #237 nationally for density and #4 in Missouri. The median county equivalent has 47 people per square mile; the Missouri median is 31. Put another way, there are about 0.841 acres of land per resident.
| July 1 | 2020 | 2021 | 2022 | 2023 | 2024 | 2025 |
|---|---|---|---|---|---|---|
| Estimate | 406,407 | 410,568 | 413,991 | 417,442 | 422,590 | 426,499 |
| Change | — | +4,161 | +3,423 | +3,451 | +5,148 | +3,909 |
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. Charles County
17 incorporated places lie at least partly in St. Charles County. Together they are home to 332,806 people (78%); the other 93,693 (22%) live outside any incorporated place, in unincorporated communities and rural areas.
The largest is O'Fallon city, with 96,101 residents in the county. The county seat, St. Charles city, has 72,561 residents. Among places with at least 1,000 residents in 2020, St. Paul city grew fastest since then (+28.1%).
| Incorporated place | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| O'Fallon city | 96,101 | +5.2% | 31 |
| St. Charles cityCounty seat | 72,561 | +2.7% | 25.5 |
| St. Peters city | 61,479 | +6.5% | 22.5 |
| Wentzville city | 49,495 | +10.1% | 22.2 |
| Lake St. Louis city | 19,129 | +14.4% | 8.566 |
| Dardenne Prairie city | 15,032 | +17.8% | 5.337 |
| Cottleville city | 6,717 | +19.6% | 4.743 |
| Weldon Spring city | 5,301 | −0.5% | 7.647 |
| St. Paul city | 3,849 | +28.1% | 7.223 |
| Flint Hill city | 566 | +13.9% | 3.032 |
| New Melle city | 550 | −0.5% | 1.789 |
| Josephville village | 529 | +12.3% | 2.865 |
| Foristell cityAlso in 1 other county | 466 | +70.1% | 6.043 |
| West Alton city | 354 | −0.8% | 28.8 |
| Portage Des Sioux city | 321 | −2.7% | 0.575 |
Show the other 2 places
| Name | Population 2025 | Since 2020 | Land mi² |
|---|---|---|---|
| Augusta town | 265 | −1.5% | 1.03 |
| Weldon Spring Heights town | 91 | −3.2% | 0.096 |
Population: U.S. Census Bureau Vintage 2025 subcounty estimates, counting only the part of each place inside St. Charles County. Land: 2026 Census Gazetteer, for the whole place even where it extends into other counties. How communities are listed.
Metropolitan area
St. Charles 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. Charles County accounts for 7.1% of the land and 15.2% 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 County | 990,911 | 508.5 |
| St. Charles County | 426,499 | 560.3 |
| St. Louis city | 278,144 | 61.7 |
| Madison CountyIllinois | 263,110 | 715.5 |
| St. Clair CountyIllinois | 250,708 | 657.7 |
| Jefferson County | 233,132 | 656 |
| Franklin County | 108,090 | 922.5 |
| Lincoln County | 66,676 | 626.6 |
| Macoupin CountyIllinois | 43,748 | 863 |
| Warren County | 39,016 | 428.6 |
| Clinton CountyIllinois | 37,128 | 473.9 |
| Monroe CountyIllinois | 34,840 | 385.3 |
| Jersey CountyIllinois | 21,160 | 369.6 |
| Bond CountyIllinois | 16,959 | 380.3 |
| Calhoun CountyIllinois | 4,300 | 253.8 |
ZIP codes
15 ZIP Code Tabulation Areas have land in St. Charles County; 14 of them have most of their land inside the county, and the others mainly cover neighboring areas.
633016330363304633326334163348633576336563366633676336863373633766338563386
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. Charles County
St. Charles County shares a land border with 7 county equivalents, including 3 in Illinois. It has more land than 4 of them. The largest neighbor is Franklin County (922.5 mi²) and the smallest is Calhoun County (253.8 mi²). Together with its neighbors, St. Charles County forms a block of 4,385.4 mi² of land, of which it holds 12.8%.
| Bordering county equivalent | Land mi² | Size vs. St. Charles | Compare |
|---|---|---|---|
| Franklin County | 922.5 | 1.6× larger | Compare |
| Madison CountyIllinois | 715.5 | 1.3× larger | Compare |
| Lincoln County | 626.6 | 1.1× larger | Compare |
| St. Louis County | 508.5 | 1.1× smaller | Compare |
| Warren County | 428.6 | 1.3× smaller | Compare |
| Jersey CountyIllinois | 369.6 | 1.5× smaller | Compare |
| Calhoun CountyIllinois | 253.8 | 2.2× smaller | Compare |
How big is St. Charles County compared with U.S. states?
St. Charles County has less land than any of the 50 states. Rhode Island, the smallest state by land area, has 1,033.9 mi², about 1.8 times as much. Missouri’s county land could hold about 123 areas the size of St. Charles County.
Another way to picture it: the same land area would fill a square about 23.7 miles (38.1 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 St. Charles County?
St. Charles County has more land than 1,374 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. Charles County at #1,768 nationally by total area, 80 places closer to the top than its land rank. The water assigned to this geography moves it ahead of some county equivalents with more land.
Its water share is 5.4%, compared with a national county-equivalent median of 1.2% — 4.2 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. Charles County in the national ranking. They can be in different states and are not necessarily geographic neighbors.
| Land rank | County equivalent | Land mi² | Comparison |
|---|---|---|---|
| #1847 | Darlington County, South Carolina | 560.6 | Compare |
| #1848 | St. Charles County, Missouri | 560.3 | This county |
| #1849 | Cleburne County, Alabama | 560.1 | 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 38.781102, -90.674915. 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: 1,451,193,708 m²; AWATER: 83,022,114 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. Charles County?
St. Charles County, Missouri has 560.3 square miles (1,451.2 km²) of land and 32.1 square miles of water, for a total area of 592.4 square miles. It ranks #1848 of 3,222 U.S. county equivalents by land area.
Is St. Charles County the largest county in Missouri?
No. St. Charles County ranks #65 of 115 in Missouri by land area. The largest is Texas County, with 1,177.3 square miles of land.
What counties border St. Charles County?
St. Charles County shares a land border with St. Louis County, Warren County, Lincoln County, Calhoun County, Illinois, Franklin County, Madison County, Illinois, and Jersey County, Illinois.
What is the population of St. Charles County?
The Census Bureau estimates St. Charles County had 426,499 residents on July 1, 2025, ranking #180 of 3,222 county equivalents. That is 761 people per square mile of land.
What is the county seat of St. Charles County?
The county seat of St. Charles County is St. Charles.
What cities and towns are in St. Charles County?
St. Charles County has 17 incorporated places. The most populous five are O'Fallon city, St. Charles city, St. Peters city, Wentzville city, and Lake St. Louis city; 93,693 residents live outside any incorporated place.
Is St. Charles 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. Charles County?
15 ZIP Code Tabulation Areas have land in St. Charles County, and 14 of them have most of their land inside the county. Census ZIP Code Tabulation Areas approximate USPS ZIP Code areas.
How does St. Charles County compare with U.S. states?
It has less land than every state. Rhode Island, the smallest, has 1.8 times as much land.
How many acres of land are in St. Charles County?
The land measurement converts to approximately 358,598 acres. Including water, total area is approximately 379,113 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.