Are Nine-Digit ZIP Codes Required in the USA?

Letter in hand image by Oleg Tarasov from

The U.S. Postal Service's Zone Improvement Plan code, better known as the ZIP code, is the key component in an address to ensure mail makes it to its proper destination. You usually memorize only a five-digit ZIP code when learning addresses, but in the past several decades, the USPS has added an extra four digits to the codes to help mail reach its intended destination faster. You generally do not need to use the extra digits when sending mail, however.


The USPS introduced the five-digit ZIP code in 1963 in order to better process mail as its volume shifted increasingly toward business mail rather than personal letters. The first number designates the general region in the U.S., such as New England, which uses the number zero. The next two numbers designate a sectional center within that region. The fourth and fifth numbers designate specific post offices or zones. In 1983, the USPS introduced a four-digit extension to the ZIP code called ZIP+4. The extra digits designate specific locations within a ZIP code, such as an office building, a city block or another location that receives a high volume of mail. The first two numbers represent a specific sector or group of blocks, and the final two indicate a specific segment or side of the street.


While the USPS requires the five-digit ZIP Code for second- and third-class mail, it does not require you to use the ZIP +4. Business mailers primarily use the extra four digits for quicker processing. When prepared in a typed or computerized format, USPS automated scanners can read the codes when the mail is processed. Additionally, businesses can use bulk-mailing software to translate these nine-digit codes into bar codes the USPS uses for automated mail processing.


Using the ZIP+4 code reduces the chances of human error and incorrect delivery, as it cuts down on the number of human hands your mail must pass through to be delivered, according to the USPS. Additionally, usage of the ZIP+4 code, translated into bar code form, is part of the requirement for businesses to receive bulk mailing discount rates. While you generally do not need to know your own ZIP+4 code for personal reasons, you might need it in a few situations. Some ZIP code areas, for example, contain multiple Congressional districts, and the ZIP+4 code could facilitate a quick look-up to determine which district a certain address is in.


Printing an address clearly is more important than determining the nine-digit ZIP code to ensure quick and accurate mail delivery. When you send mail, post office workers feed it into a machine that automatically reads addresses and marks the mail with the corresponding bar code for that address regardless of whether you used the five- or nine-digit ZIP Code. If the machine cannot read the address, it sends it to a human worker for deciphering, which will delay your delivery. If you use the ZIP+4 Code, always include a hyphen after the fifth number, or your mail might be deemed undeliverable.


About the Author

Michael Baker has worked as a full-time journalist since 2002 and currently serves as editor for several travel-industry trade publications in New York. He previously was a business reporter for "The Press of Atlantic City" in New Jersey and "The [Brazoria County] Facts" in Freeport, Texas. Baker holds a Master of Science in journalism from Quinnipiac University in Hamden, Conn.

Photo Credits

  • Letter in hand image by Oleg Tarasov from