According to the United States Census Bureau, the city has a total area of 15.5 square miles (40.1 km2), of which 15.0 square miles (39 km2) is land and 0.4 square miles (1.0 km2) (2.8%) is water. Alexandria is bounded on the east by the Potomac River, on the north and northwest by Arlington County, and on the south by Fairfax County. The western portions of the city were annexed from those two entities beginning in the 1930s.
The addressing system in Alexandria is not uniform and reflects the consolidation of several originally separate communities into a single city. In Old Town Alexandria, building numbers are assigned north and south from King Street and west (only) from the Potomac River. In the areas formerly in the Town of Potomac, such as Del Ray and St. Elmo, building numbers are assigned east and west from Commonwealth Avenue and north (only) from King Street. In the western parts of the city, building numbers are assigned north and south from Duke Street.
The ZIP code prefix 223 uniquely identifies the Alexandria postal area. However, the Alexandria postal area extends into Fairfax County and includes addresses outside of the city. Delivery areas have ZIP codes 22301, 22302, 22304, 22305, 22311, 22312, 22314, and 22315, with other ZIP codes in use for post office boxes and large mailers.