Marion is located in Williamson county Illinois with a geographic area of 14.4 square miles. The total population in Marion is 17,539 with 49.0% male and 51.0% female. Children under age 5 represent 5.7% of the population, under age 18 equal 23.3% of the population and over age 65 is 21.3% of the population. The median household income is $44,929, per capita income is $29,475. There are a total of 7,873 housing units with 57.8% owner occupied. Education includes 27.1% of residents that are high school graduates and 27.8% with a bachelor's degree or higher. The average temperature in Marion in the summer is 74 degrees, and in the winter 27 degrees.