St. Louis is the principal city of the largest metropolitan area in Missouri, composed of 17 counties and the independent city of St. Louis; eight of its counties are in Illinois. As of 2019, St. Louis was the 21st-largest metropolitan area in the nation with 2.91 million people. However, if ranked using Combined Statistical Area, it is 20th-largest with 2.91 million people in 2019. Some of the major cities making up the St. Louis metro area in Missouri are O'Fallon, St. Charles, St. Peters, Florissant, Chesterfield, Wentzville, Wildwood, University City, and Ballwin.