There are 13 US Post Offices in Calvert County serving a total of 88,984 residents equating to 6,845 residents per post office. It's estimated that approximately 142,028 packages are handled across the 13 county post offices annually. Calvert County is a county located in the U.S. state of Maryland. As of the 2010 census, the population was 88,737. Its county seat is Prince Frederick. The county's name is derived from the family name of the Barons of Baltimore, the proprietors of the English Colony of Maryland. Calvert County is included in the Washington-Arlington-Alexandria, DC-VA-MD-WV Metropolitan Statistical Area. It occupies the Calvert Peninsula, which is bordered on the east by Chesapeake Bay and on the west by the Patuxent River. Calvert County is part of the Southern Maryland region. The county has one of the highest median household incomes in the United States."