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A 5-digit number that identifies a particular postal delivery area in the United States.

A 5-Digit ZIP code represents a USPS delivery area. A 9-Digit ZIP code, also known as a ZIP+4 code or an extended ZIP code, includes the same information as the 5-Digit ZIP code, plus additional information about a specific delivery route within the delivery area.

In terms of delivery points, the USPS reported approximately 89 million delivery points by the end of the decade. The five-digit code which initially specified an area or a part of a town was no longer accurate or efficient enough. So, USPS created 9-digit ZIP Codes to ensure fast and accurate mailings.

A United States postal code, for example, might be either five digits or five digits followed a hyphen (dash) and another four digits (12345-1234). If you are validating US postal codes, allow for both conditions. The following rule validates a field that can be five digits, five digits + dash + 4 digits, or blank.

The number of digits. A 9-digit ZIP code is a further subdivision of a 5-digit ZIP code. One 5-digit ZIP code might cover a small city or up to 10,000 post office boxes. One 9-digit ZIP code might cover a few blocks with less than 100 addresses, or ONE post office box.

So what do the extra numbers mean? These last 4 digits represent specific delivery routes within delivery areas. This extra detail means an even more precise matching to a more granular level. It could represent a handful of houses on one side of a street, or even a single building that receives a high volume of mail.

Yes, when using a ZIP+4 ZIP Code, the number must consist of five digits, a hyphen (or dash), and four digits. Improperly labeled mailpieces may progress slower through the mail system and may be returned to sender if the United States Postal Service® is unable to deliver the item.