There are 2 US Post Offices in Calhoun County serving a total of 10,886 residents equating to 5,443 residents per post office. It's estimated that approximately 17,375 packages are handled across the 2 county post offices annually. Calhoun County is a county in the U.S. state of South Carolina. As of the 2010 census, its population was 15,175, making it the third-least populous county in South Carolina. Its county seat is St. Matthews. The county was formed in 1908 from portions of Lexington and Orangeburg Counties and is named for John C. Calhoun, the former U.S. Vice-President, Senator, Representative and cabinet member from South Carolina. Calhoun County is part of the Columbia, SC Metropolitan Statistical Area."