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ZIP Code Basics

ZIP+4 Codes Explained: What the Extra Four Digits Mean

U.S. ZIP Codes Guide · Updated November 2026

A ZIP+4 Code adds a hyphen and four digits to the familiar five-digit ZIP Code. The additional digits help USPS narrow a delivery point to a block face, building, group of apartments, or other mail-routing segment.

A complete ZIP+4 can improve sorting and delivery efficiency, especially for businesses and organizations that send large volumes of mail. Not every address has a unique public-facing ZIP+4 that is easy to infer. Always verify the full code using the exact street address and a trusted postal lookup.

For address-specific mailing decisions, verify the complete delivery address with the United States Postal Service (USPS). This directory is provided as a general reference.

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ZIP codes are a system of postal codes used by the United States Postal Service (USPS) since 1963. The term ZIP, an acronym for Zone Improvement Plan, is properly written in capital letters and was chosen to suggest that the mail travels more efficiently, and therefore more quickly, when senders use the code in the postal address. The basic format consists of five decimal numerical digits. An extended ZIP+4 code, introduced in the 1980s, includes the five digits of the ZIP code, a hyphen, and four more digits that determine a more precise location than the ZIP code alone. The term ZIP code was originally registered as a servicemark (a type of trademark) by the U.S. Postal Service, but its registration has since expired.

ZIP codes designate only delivery points within the United States and its dependencies, as well as locations of its armed forces. There are no ZIP codes reserved for designating mail bound for foreign destinations (with the exception of U.S. military units stationed outside of the United States), and therefore, international outbound mail should not include a ZIP code in the delivery address. The last line of a foreign address must only show the name of the country of destination.

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