21791 -- U.S. ZIP code Municipality (1990): UNIONVILLE, Maryland All ZIPs for this municipality (1990): 21791 Population (2000): 5413 Housing Units (2000): 1985 Land area (2000): 48.814197 sq. miles (126.428184 sq. km) Water area (2000): 0.000358 sq. miles (0.000928 sq. km) Total area (2000): 48.814555 sq. miles (126.429112 sq. km) Located within: Maryland (MD) Location: 39.554900 N, 77.181764 W