St. Charles County is in the central eastern part of the U.S. state of Missouri. As of the 2010 census, the population was 360,485, making it Missouri’s third-most populous county. Its county seat is St. Charles. The county was organized October 1, 1812 and named for Saint Charles Borromeo, an Italian cardinal. The county executive is Steve Ehlmann, since January 2007.
According to the U.S. Census Bureau, the county has a total area of 593 square miles (1,540 km), of which 560 square miles (1,500 km) is land and 32 square miles (83 km) (5.4%) is water.
The highest elevation is 901 feet (275 m) northwest of Augusta near Femme Osage Creek headwaters.
As of 2010, there were 360,485 people, 132,906 households, and 77,060 families residing in the county. The population density was 643 people per square mile (1665/km²). There were 142,766 housing units at an average density of 73 persons/km² (188 persons/sq mi). The racial makeup of the county was 91.3% White, 4.4% African American, 0.3% Native American, 2.3% Asian (0.9% Asian Indian, 0.4% Chinese, 0.2% Filipino, 0.2% Vietnamese, 0.2% Korean, 0.1% Japanese, 0.2% Other), 0.1% Pacific Islander, and 1.6% from two or more races. Hispanic or Latino made up 2.5% of the population (1.8% Mexican, 0.2% Puerto Rican, 0.1% Cuban, 0.7% Other).
There were 101,663 households out of which 40.50% had children under the age of 18 living with them, 63.20% were married couples living together, 9.20% have a woman whose husband does not live with her, and 24.20% were non-families. 19.40% of all households were made up of individuals and 5.90% had someone living alone who was 65 years of age or older. The average household size was 2.76 and the average family size was 3.18.
In the county, the population was spread out with 29.00% under the age of 18, 8.20% from 18 to 24, 32.60% from 25 to 44, 21.60% from 45 to 64, and 8.80% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 97.10 males. For every 100 females age 18 and over, there were 94.10 males.
The median income for a household in the county was $71,458, and the median income for a family was $64,415. Males had a median income of $44,528 versus $29,405 for females. The per capita income for the county was $23,592. 4.00% of the population and 2.80% of families were below the poverty line. Out of the total people living in poverty, 4.90% are under the age of 18 and 5.10% are 65 or older.
St. Charles County, with an estimated population of 373,495, has been one of the fastest-growing counties in the country for decades, with 55% growth in the 1970s, 48% in the 1980s, 33% in the 1990s, and another 27% in the 2000s. The county features a cross-section of industry, as well as extensive retail and some agriculture. With the Missouri River on the south and east and the Mississippi River on the north, the county is bisected east to west by Interstate 70. St. Charles County has one small airport St. Charles County Smartt Airport ( St. Charles Airport closed in 2010 ) and two ferries that cross the Mississippi River.