What’s up With Those Extra 4 Zip Code Digits—They Are Important!

April 13, 2018 4:52 pm Last Updated: April 13, 2018 4:52 pm

You may have noticed that zip codes often have an extra four digits tacked on to the end and wondered what purpose they serve.

While the first five digits correspond to a particular postal office, the four additional digits mark a specific delivery route, “meaning the actual path the mail truck would travel in a single drop-off,” SmartyStreet.com explains.

The website claims adding the four digits can speed up mail delivery by as much as two days because it makes the mail easier to process. But there’s a downside too.

“Because they are based on delivery routes instead of more permanent boundaries, the last 4 digits … can be changed as frequently as once a month, based on things like how many postal employees are working, or who is working what route, etc.,” the website states.

The correct 5+4-digit zip code for an address can be looked up on the United States Postal Service website.