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What Is ZIP Code?

A Zone Improvement Plan (ZIP) Code is the numerical code assigned by the U.S. Postal Service to designate a local area or entity for the delivery of mail. ZIP Codes may consist of 5, 7, 9, or 11 digits, and may refer to a street section, a collection of streets, an establishment, a structure, or a group of post office boxes.

From Wikipedia, the free encyclopedia.

A ZIP Code is the postal code used by the United States Postal Service, which always writes it with capital letters. ZIP is an acronym for the Zoning Improvement Plan, but was also cleverly meant to suggest that mail travels more efficiently (and therefore faster) when senders use it. The basic ZIP Code format consists of five numerical digits. An extended ZIP+4 code includes the five digits of the ZIP Code plus four digits which allow a piece of mail to be delivered to a specific address. ZIP Code was originally registered as a trademark by the U.S. Postal Service, but its registration has since expired.

The term "ZIP Code" is also used in the Philippines to name its postal codes. The Philippine ZIP Code is used by the Philippine Postal Corporation. Unlike American ZIP Codes, Philippine ZIP Codes are four-digit numbers without any extensions. While the cities of Metro Manila use more than one code, towns and cities outside Metro Manila are assigned only one code for every town and city.


In 1983, the US Postal Service began using an expanded ZIP Code system called "ZIP+4", which are often called "plus-four codes" or "add-on codes." A ZIP+4 code uses the basic 5-digit ZIP plus an additional 4-digits to identify a geographic segment within the 5-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail, or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery. Use of the plus-four code is not required, but it helps the Postal Service direct mail more efficiently and accurately. By decreasing handling and in turn, the potential for misdelivery, the plus-four code also lowers delivery cost.

For Post Office boxes, the general (but not invariable) rule is that each box has its own ZIP+4 code. The add-on code is often either the last four digits of the box number or 0 plus the last three digits of the box number. However, there is no uniform rule, so the ZIP+4 code must be looked up individually for each box.

It is common to use add-on code 9998 for mail addressed to the postmaster, 9999 for general delivery, and other high-numbered add-on codes for business reply mail and requests for special cancellation of stamps. For a unique ZIP code (explained below), the add-on code is typically 0001.

Postal bar code

The ZIP Code is often translated into a barcode called POSTNET, that is printed on the mailpiece as well, to make it easier for automated machines to sort the mail. Unlike most barcode symbologies, POSTNET uses long and short bars, not thin and thick bars. The barcode can be printed by the person who sends the mail, or the post office will put one on when they receive it. The post office generally uses OCR technology, though a human may have to read the address if absolutely necessary. (The automated machinery has the unfortunate tendency to paste the coding over the bottom half-inch of postcards, often obliterating the signature.)

People who send bulk mail can get a discount on postage if they have pre-printed the barcode themselves. This requires only a simple and often free font, and the knowledge of at least the main 5-digit code, if all 9 are not available. An additional two digits are usually used to indicate the exact delivery point, so that every single mailable point in the country has its own 11-digit number (at least in theory). These two digits are usually the last two of the street address or box number, though non-numeric points with names or letters are assigned DP numbers by the local post office. However, when house numbers differ only by a letter suffix, e.g., 120 and 120A, the delivery point may be the same. The last digit is always a check digit, which is obtained by adding up the 5-, 9-, or 11-digits, then subtracting the last digit of that result from 10. (Thus, the check digit for 10001-0001 00 would be 7, or 1+1+1=3 and 10?3=7.) The sender needs only to type something like /100010001007/ in the 12-point POSTNET font to create the code for printing. Digits are encoded as a two-out-of-five code.

On business reply mail, the FIM code indicates whether the POSTNET bar code is included.


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