The difference between normal ZIP codes and ZIP+4 codes is that ZIP+4 codes contain four extra digits that more accurately define the delivery area. The extra digits are not required, but they do help in routing mail more efficiently.
ZIP is an acronym for Zone Improvement Plan, and the normal five-digit ZIP code program was started by the U.S. Postal Service in 1963. In a five-digit ZIP code, the first digit corresponds to one of nine geographic areas across the country, with each area covering several states. The next two digits refer to a Sectional Center Facility in the area that serves as a central distribution hub for several post offices. The final two digits refer to a specific post office.
A ZIP+4 code adds an additional four numbers to the normal ZIP code. These four numbers are used to breakdown the delivery area even further and can refer to a sector, a city block, a specific building or any specific mail destination that is required. A properly formatted ZIP+4 code must have a hyphen between the first five digits and the last four digits. The use of ZIP+4 codes is not mandatory, and they are primarily used by businesses and other bulk mailers.
To help locate ZIP code information, including ZIP+4 codes, the Postal Service provides a ZIP code tool at the Postal Service website, USPS.com.