What the “ZIP” in “ZIP Code” Stands For and What the Numbers Signify


Mr. ZIP: Official Mascot of the ZIP Code System

Today I found out what the “ZIP” in “ZIP Code” stands for and what the numbers signify.

ZIP Code or “Zone Improvement Plan Code” was a system put in place by the U.S. Postal Service on July 1, 1963.  The hope was that these ZIP codes would allow the postal service to more efficiently and therefore more quickly route mail.

Before the official nation-wide ZIP code system, the U.S. Postal Service had a system of postal zones for large cities as far back as 1943.  This postal zone number would be added to the address after the city name, but before the state name. (e.g. Bilbo Baggins, 3421 Bagend, Hobbiton 27, Eriador; where “27” is the postal zone within the city of Hobbiton)  This allowed for more efficient sorting of mail and, thus, quicker deliver times in high volume areas.

In 1944, post office employee, Robert Moon, submitted a proposal for a nation-wide postal zone system.  In Moon’s system, which was eventually slightly modified and then adopted, the first three digits would stand for the sectional center facility (SFC); an SFC is just a central mail processing facility in a certain region.  Specifically, the first digit ends up standing for a group of U.S. states (0-9 numbered from east to west, ascending) with the second and third digits representing a region in that group that is covered by some SFC.  The Postal System then added on to Moon’s system two extra numbers which indicate the post office or postal zone, within the region, the mail should go to.

Starting around 1983, the U.S. Postal service introduced the ZIP+4 system.  In this system, the 5 digit ZIP code number signifies the same thing as before and the extra 4 digits at the end signify an area within a certain post office’s coverage area.  Specifically, the six and seventh digits indicate a “delivery sector”, such as a group of streets; a group of office buildings; etc.  The last two numbers then indicate a “delivery segment” which could indicate a floor of an office building; the side of the street the address is on; etc.   This allows for more efficient sorting within a single post office that deals in high volumes of mail.

The new ZIP+4 system never really caught on with the general public and is somewhat superfluous as mail today is read by a multi-line optical character reader (MLOCR) that can almost always determine the correct ZIP+4 code for an address.  This machine also sprays a Postnet barcode on the face of the mail that corresponds to the full ZIP+4 code; thus, writing it in by hand before you mail something is generally no more efficient than not writing it in, because the automatic sorting systems the Postal Service has use this Postnet barcode, rather than the specific written address for sorting.

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Bonus Facts:

  • Once the ZIP code system was put in place, the U.S. Postal Service only required that bulk mailers use the ZIP codes, though they wanted everyone to use them.  The Postal Service struggled, in the beginning, to get the general public to start using the zip codes.  They eventually more or less decided to give up on convincing adults and instead they created a cartoon character “Mr. ZIP” in order to try to get kids to start using ZIP codes.  They figured that the kids would then encourage parents to use the ZIP codes when they saw their parents leaving the ZIP codes out on mail.  Further, when the kids grew older, they would teach their kids to use ZIP codes.
  • Mr. ZIP was based on a design by Howard Wilcox, who was the son of a letter carrier.  This was a design done for a New York bank that was a child-like sketch of a postman delivering a letter.  After AT&T acquired the rights to the design, they made it available to the Postal Service to use at no cost.  The Postal Service then made a few minor modifications and Mr. ZIP was born.
  • The term “ZIP code” was originally registered as a servicemark, which is a type of trademark, by the U.S. Postal Service, but the registration has since expired allowing companies like UPS and FedEx to use the term freely.
  • You can actually print the Postnet barcode directly onto your mail piece to make it slightly easier for the Postal Service to sort your mail (mainly skipping the step where they have to scan your mail and print the barcode on it).  Many word-processing programs, such as Microsoft Word, include a feature for doing this.
  • Before the 1970s, the United States Postal Service (USPS) was named the United States Post Office Department (USPOD).
  • Interestingly, the ZIP code system was first introduced because the Postal Service was beginning to be overwhelmed with the volume of mail they needed to process in a timely manner, most of which originally was processed by hand.  The ZIP code system made a handy way to increase efficiency in that way.  However, only a few years after the ZIP code system was introduced in the 1960s, the Postal Service began using the MLOCR system for automatic sorting.  Given the address, even without the ZIP code, the MLOCR system is almost always perfectly capable of assigning the ZIP+4 code to the address, with very little mail needing to be human-read to determine the correct address/ZIP code.  So in most cases, including the ZIP or ZIP+4 code with the written address doesn’t really increase mail efficiency much at all as the Postal Service’s initial scanning system can come up with those numbers for your written address on its own.
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  • Great idea for an article! I had been wondering about zip codes, but I kept forgetting to look it up.

  • RE: “So in most cases, including the ZIP or ZIP+4 code with the written address doesn’t really increase mail efficiency much at all as the Postal Service’s initial scanning system can come up with those numbers for your written address on its own.”

    My question: Wouldn’t the zip be required if the scanning system can’t read the CITY of the addressed envelope? I’ve seen mail in our office for a street that runs from Long Beach through Carson but gets determined it must be for a city in Northern CA with the same street name.

    I wouldn’t encourage people to mail anything without a zip!

    • @KaRi – yet its always good to include at least the 5 digit ZIPcode on any mail you send. While it is redundant – thats the whole point – it serves as a “backup” way to verify the mail is being sent to the proper destination, especially if there is a problem reading one or the other. Note that the house number is also used, in addition to the city, street, etc.

      The USPS OCR systems are VERY powerful – but its always a good idea to write as clearly as possible (or better, PRINT the address on a label with a computer) and leave no room for error. For a package, I would put the destination address on two different sides of the package, as well – this both adds redundancy and increases reliability.

  • Corrections. The abbreviation for “Sectional Center Facility” is SCF, not SFC.

    MLOCR’s have been obsolete for some time. The OCR systems the USPS uses today, for letters, large flat mail, as well as packages, are several orders of magnitude more powerful and faster.

    Unless you know or lookup the FULL 11 digit code for your addressee, you’re better off NOT attempting to print a postnet code on any mail you send. (Yes, theres two more digits called a “delivery point) after the 5+4 that are actually used – this 11 digit code uniquely identifies most individual addresses in the US)

    If you *are* going to print a barcode on a letter, never print it along the bottom edge where the Postal systems do – instead include it in the address block. Avoid including any other barcodes on a mailpiece, as these can sometimes be misinterpreted by sorting systems.

  • For instance, the full 11 digit barcode for the address of the registration of the “todayifoundout.com” domain is 99006-1025-25

    but the final two digits (25, in this case) are never printed as part of the address, they are only encoded in the full postnet code when printed.

    You could, if you knew EXACTLY what you were doing, mail letter (this will *only* work with a letter, and only if it can be sorted by the machines) with nothing but the person’s name and the full correct 11 digit barcode – No house number, street, or city, or human readable zipcode – and it would get there. 🙂