ZIP plus 4 | Structure and meaning of 4 last digits of ZIP

5-digit ZIP code is the most familiar format which was introduced in 1963 by the US Postal Service (USPS). However, in addition to it, there is also the extended ZIP + 4 code, appeared in 1983.

What is ZIP plus 4?

ZIP plus 4 code is a 9-digit combination, which can be also called “add-on codes”, “add-ons”, “+4 codes” or something like that.

It consists of two parts:

  • Usual ZIP code. It is the first 5 digits, which are enough for routing purposes.
  • 4-digit code continuation, which may or may not be required to identify the delivery destination.

A simple 5-digit ZIP code is easy to identify with our tool. Go to the section “What’s my ZIP code” to find out yours.

The last 4 digits of ZIP code help to identify geographic segment within the 5-digit delivery area more accurately. For example they might mean a post office box, a group of apartments, an individual high-volume receiver of mail, a city block or a specific delivery route. In addition, ZIP plus four codes provides an opportunity to avoid misdeliveries.

Why may 4 last digits of ZIP code change?

One more feature of ZIP code plus four is that 4 last digits may be changed. Unlike 5-digit ZIP codes, which are based on permanent boundaries, ZIP + 4 is based on delivery routes.

In fact 5-digit ZIP codes also change, but it happens really rarely, while ZIP code plus 4 can be changed as frequently as once a month. It is one of the reasons why ZIP plus 4 lookup might lose its relevance.

What are the last four digits of a ZIP code?

The most reliable way is to use official software of delivery companies, such as USPS, which updates a ZIP + 4 Code as frequently as once in two months approximately.

The list of US postal codes without last 4 digits can be found on our website in the section “All ZIP codes”. Choose your state, county and city to know more.

What’s “ZIP plus four” structure?

We’ve already told about ZIP codes structure in our article “What is a ZIP code?”, so let’s take a closer look at the structure of the last 4 digits:

  • The sixth and seventh digits of ZIP codes plus four often mean a specific street or a large building.
  • The last two digits (eighth and ninth) show one of the following reference point: side of the street, large building or its floor.

ZIP + 4 code format

It is possible not to use last 4 digits of ZIP codes?

Yes, most of clients just need to know the address or 5-digit ZIP-code. However, there were some attempts to promote universal use of the new format in USA, its creators met with public resistance, so nowadays the 4 + ZIP is not required.

In addition postal companies are able to determine the correct ZIP+4 code from the address on the mail on theirs’s own — by a multiline optical character reader (MLOCR). After identifying they form and print an Intelligent Mail barcode (IM) on the face of the mail piece that corresponds to 11 digits: nine for the ZIP plus 4 code and two for the delivery point.

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