There are 11 US Post Offices in Rock County serving a total of 133,435 residents equating to 12,130 residents per post office. It's estimated that approximately 212,977 packages are handled across the 11 county post offices annually. Rock County is a county in the U.S. state of Wisconsin. As of the 2010 census, the population was 160,331. Its county seat is Janesville. Rock County comprises the Janesville-Beloit, WI Metropolitan Statistical Area and is included in the Madison-Janesville-Beloit, WI Combined Statistical Area."