The Last 4 Digits on ZIP Codes Explained

(Last Updated On: June 1, 2018)

The Last 4 Digits on ZIP Codes Explained
You probably learned your ZIP code in kindergarten when you learned your address, how to tie your shoes, and how to tell time.

It’s an important number. It’s how you get your mail. In some cases, it distinguishes your address from another address in the city with the same street name. Maybe it’s a number that’s been even more significant for you because it gives you solidarity with other people in your area.

Regardless, you know your 5-digit ZIP code. But what’s up with those random 4 digits you sometimes find on the end of it?

Ready to figure out what the last 4 digits on ZIP codes mean? Let’s go.

First, the ZIP Code Explained

Before the last 4 digits on ZIP codes are explained, let’s have a refresher on the ZIP code itself.

The ZIP code came to be in the early 1960s when mail volume was increasing substantially in urban and suburban areas. Rapid growth put the U.S. Postal Service under massive pressure, but it couldn’t grind to a halt to figure out what to do next.

To help deal with the increased amount of mail, the US. Postal Service announced the introduction of the Zone Improvement Plan (ZIP). Hence, the ZIP code.

5-digit codes were assigned to all addresses in the United States. The first digit relates to a large area, like a state or group of states. For example, ZIP codes in Minnesota and Wisconsin start with “5” and New York, Delaware, and Pennsylvania all share a “1”.

The second digit refers to a city or region, and the final two numbers represent a small group of addresses within the region.

Last 4 Digits on ZIP Codes

The United States continued to grow and cities continued to sprawl. By the 1980s, the ZIP system wasn’t enough to keep the mail moving.

Naturally, the Postal Service decided to add more numbers.

These are the 4 numbers you see today on some pieces of mail that represent the ZIP+4 codes.

The sixth and seventh number, which are added after a hyphen, indicate a group of buildings, streets, boxes, or even a large building with a lot of delivery points. This, of course, narrows down the ZIP code further, making it easier than ever to sort and deliver mail.

But the sixth and seventh number weren’t enough. The final two digits (eight and nine) break it down even further. These digits represent what is called a “delivery segment.” A delivery segment is a micro-representation of an area; it can represent an area as small as a specific department in a big office building.

Why Are We Just Learning About This?

As with most developments in efficiency, change is slow.

People who didn’t work for the Post Service didn’t care about ZIP codes to begin with and found the addition of another critical number to their lives intrusive. So, when the last 4 digits on ZIP codes were launched, people never bothered to learn why.

Around the same time as the launch of ZIP+4, the Postal System was availing of new technologies that helped them sort and deliver mail faster. Because USPS could use Postnet barcode technology to avoid human error, Americans were saved from having to remember even more numbers just to send a letter or pay their bills.

Have any USPS trivia to share? Leave it in the comments below.



Mark Heald
About Mark Heald 216 Articles
Mark Heald is the Managing Editor of He enjoys spending time with his family, traveling, and bemoaning the fact the Sonics left Seattle.