There are 5 US Post Offices in Calhoun County serving a total of 4,052 residents equating to 810 residents per post office. It's estimated that approximately 6,467 packages are handled across the 5 county post offices annually. Calhoun County is a county located in the U.S. state of Illinois. As of the 2010 census, the population was 5,089, making it the third-least populous county in Illinois. Its county seat is Hardin. Calhoun County is located at the tip of the peninsula formed by the confluence of the Mississippi and Illinois rivers and is almost completely surrounded by water. Often referred to as 'The Island,' Calhoun County is sparsely populated; it has just five municipalities, all of them classified as villages. Calhoun County is part of the Metro-East portion of the St. Louis, MO-IL Metropolitan Statistical Area."