St. Louis County is located in the far eastern portion of the U.S. state of Missouri. It is bounded by the city of St. Louis and the Mississippi River to the east, the Missouri River to the north, and the Meramec River to the south. As of the 2016 Census Bureau population estimate, the population was 998,581, making it the most populous county in Missouri. Its county seat is Clayton.
According to the U.S. Census Bureau, the county has a total area of 523 square miles (1,350 km), of which 508 square miles (1,320 km) is land and 15 square miles (39 km) (2.9%) is water.
As of the census of 2010, there were 998,954 people. There were 404,765 households with 263,423 family households. The population density was 1,966 people per square mile (773/km²). There were 423,749 housing units at an average density of 834 per square mile (322/km²). The racial makeup of the county was 68.3% White (66.9% Non-Hispanic White), 35.3% African American, 0.2% Native American, 3.5% Asian (1.1% Asian Indian, 1.0% Chinese, 0.3% Filipino, 0.3% Korean, 0.3% Vietnamese, 0.1% Japanese, 0.4% Other), 0.03% Pacific Islander, 0.9% from other races, and 1.9% from two or more races and 2.5% Hispanic or Latino (1.5% Mexican, 0.2% Puerto Rican, 0.1% Cuban, 0.8% Other).
There were 404,765 households out of which 31.0% had children under the age of 18 living with them, 51.00% were married couples living together, 12.70% had a female householder with no husband present, and 33.00% were non-families. 28.00% of all households were made up of individuals and 10.10% had someone living alone who was 65 years of age or older. The average household size was 2.47 and the average family size was 3.05.
In the county, the age distribution of the population shows 25.20% under the age of 18, 8.30% from 18 to 24, 29.00% from 25 to 44, 23.50% from 45 to 64, and 14.10% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 90.00 males. For every 100 females age 18 and over, there were 85.70 males.
The median income for a household in the county was $50,532, and the median income for a family was $61,680. Males had a median income of $45,714 versus $30,278 for females. The per capita income for the county was $27,595. About 5.00% of families and 6.90% of the population were below the poverty line, including 9.30% of those under age 18 and 5.30% of those age 65 or over. St. Louis County is the most affluent county in Missouri.