Postal Code Encyclopedia

A ZIP code (Zone Improvement Plan code) is a five-digit postal code used by the United States Postal Service (USPS) to identify specific geographic delivery areas. The system was introduced in 1963 to improve mail sorting and delivery efficiency.

ZIP code is the term used specifically in the United States. Postal code is the generic international term for similar systems worldwide. Countries like the UK use 'postcode,' Canada uses 'postal code,' and Germany uses 'Postleitzahl' (PLZ). All serve the same purpose: routing mail efficiently.

There are approximately 41,000 active ZIP codes in the United States. This number changes slightly as the USPS creates new codes or retires old ones. ZIP codes range from 00501 (Holtsville, NY — IRS) to 99950 (Ketchikan, AK).

ZIP codes are organized geographically from east to west. The first digit represents a broad region (0 = Northeast, 9 = West Coast). The second and third digits narrow to a sectional center facility (SCF), and the last two digits identify a specific post office or delivery area.

A ZIP+4 code adds four extra digits to the standard five-digit ZIP code, separated by a hyphen (e.g., 90210-1234). The additional digits identify a specific delivery route, building, or group of addresses, enabling more precise mail sorting and faster delivery.

While rare, some ZIP codes do cross state boundaries. This typically occurs in rural areas where a single post office serves communities in adjacent states. Examples include ZIP codes along the borders of Virginia/West Virginia and other neighboring states.