Arnold is the largest city in Jefferson County, Missouri, United States. The population was 20,808 at the 2010 census.
Arnold is located at 38°25′58″N 90°22′10″W / 38.43278°N 90.36944°W / 38.43278; -90.36944 (38.432753, -90.369393). According to the United States Census Bureau, the city has a total area of 11.94 square miles (30.92 km), of which 11.58 square miles (29.99 km) is land and 0.36 square miles (0.93 km) is water.
The city is located at the confluence of the Meramec and Mississippi rivers, just south of St. Louis County.
As of the census of 2010, there were 20,808 people, 8,090 households, and 5,695 families residing in the city. The population density was 1,796.9 inhabitants per square mile (693.8/km). There were 8,547 housing units at an average density of 738.1 per square mile (285.0/km). The racial makeup of the city was 96.3% White, 0.6% African American, 0.2% Native American, 0.9% Asian, 0.6% from other races, and 1.4% from two or more races. Hispanic or Latino of any race were 2.2% of the population.
There were 8,090 households of which 34.1% had children under the age of 18 living with them, 52.3% were married couples living together, 12.9% had a female householder with no husband present, 5.2% had a male householder with no wife present, and 29.6% were non-families. 23.9% of all households were made up of individuals and 9.7% had someone living alone who was 65 years of age or older. The average household size was 2.55 and the average family size was 3.02.
The median age in the city was 39.2 years. 23.3% of residents were under the age of 18; 8.6% were between the ages of 18 and 24; 26.3% were from 25 to 44; 27.4% were from 45 to 64; and 14.4% were 65 years of age or older. The gender makeup of the city was 48.7% male and 51.3% female.
As of the census of 2000, there were 19,965 people, 7,550 households, and 5,564 families residing in the city. The population density was 1,775.0 inhabitants per square mile (685.2/km²). There were 7,913 housing units at an average density of 703.5 per square mile (271.6/km²). The racial makeup of the city was 97.91% White, 0.30% African American, 0.21% Native American, 0.38% Asian, 0.02% Pacific Islander, 0.27% from other races, and 0.92% from two or more races. Hispanic or Latino of any race were 1.04% of the population.
There were 7,550 households out of which 34.3% had children under the age of 18 living with them, 58.2% were married couples living together, 11.1% had a female householder with no husband present, and 26.3% were non-families. 22.0% of all households were made up of individuals and 7.7% had someone living alone who was 65 years of age or older. The average household size was 2.61 and the average family size was 3.05.
In the city, the population was spread out with 25.5% under the age of 18, 8.4% from 18 to 24, 30.4% from 25 to 44, 23.6% from 45 to 64, and 12.1% who were 65 years of age or older. The median age was 37 years. For every 100 females, there were 94.7 males. For every 100 females age 18 and over, there were 90.3 males.
The median income for a household in the city was $47,188, and the median income for a family was $53,664. Males had a median income of $37,972 versus $27,222 for females. The per capita income for the city was $20,378. About 3.0% of families and 4.4% of the population were below the poverty line, including 6.1% of those under age 18 and 3.6% of those age 65 or over.