ZIP+4 Codes™ | 9-Digit ZIP Codes – Definition and FAQs

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Looking for how to find your own extended ZIP Code for an address? See our article on ZIP+4 Code™ Lookup tools.

ZIP+4 Definition

ZIP+4™ is the US Postal Service’s extended ZIP code format that includes an additional four digit code. The four extra numbers in the extended ZIP code represent specific delivery routes within the delivery area.


What is the difference between a standard 5-Digit Zip Code vs. a 9-Digit ZIP Code?

A 5-Digit ZIP code represents a USPS delivery area. A 5-digit ZIP code can be augmented with ZIP+4 Codes (also known as “plus-four codes”, “add-on codes” or simply “zip plus 4”) to specify delivery route information within the ZIP code 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.

What do the numbers in a ZIP Plus 4 code mean?

Keep in mind that plus-four codes are the 4-digit codes appended to the original 5-digit USPS ZIP Codes. Using full ZIP+4 Code when mailing with the USPS ensures more accurate and efficient delivery – because these codes indicate a how mail should be sorted and delivered on a specific delivery route taken by a mail truck when making a delivery. This mail truck delivery route might consist of a city block, a group of apartments, an individual high-volume mail recipient, a post office box, or some other unit within a five-digit delivery area.

Zipcode ZIP+4 meaning and digit reference
An example of the data represented by each digit of a ZIP+4 code.

The following list includes some of the data that can be encoded into a plus-four code:

  • Specific Street or Large Building that makes up a route (first two digits)
  • Side of the Street or Floor of a Large Building (last two digits)

Another encoding of a plus-four code exists for PO boxes. Each PO Box gets its own ZIP+4 Code, which often matches the box number.

How often do ZIP codes change?

5-digit ZIP codes rarely change. ZIP+4 codes change often.

Because ZIP+4 codes are based on delivery routes, they change when the postal delivery routes are updated by the USPS. That’s why ZIP+4 codes can change as often as once a month, since there are several factors that can result in the USPS updating its delivery routes. The 5-digit ZIP code is more stable and changes less frequently.

The USPS can provide current postal code data by subscription via their Address Information Systems (AIS) Products if you have a need to maintain a current list in your database/application. USPS National Customer Support Center.

USPS also provides free access to their web services which provide this data via real time HTTP requests, if you want to validate an address, lookup a zip, or do a city/state lookup in your application or site. Web Tools APIs | USPS®.