There are 23 US Post Offices in Merrimack County serving a total of 123,402 residents equating to 5,365 residents per post office. It's estimated that approximately 196,963 packages are handled across the 23 county post offices annually. Merrimack County is a county in the U.S. state of New Hampshire. As of the 2010 census, the population was 146,445, making it the third-most populous county in New Hampshire. Its county seat is Concord, the state capital. The county was organized in 1823 from parts of Hillsborough and Rockingham counties, and is named for the Merrimack River. Merrimack County comprises the Concord, NH Micropolitan Statistical Area, which is also included in the Boston-Worcester-Providence, MA-RI-NH-CT Combined Statistical Area. In 2010, the center of population of New Hampshire was located in Merrimack County, in the town of Pembroke."