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» How are boundaries for ZIP Codes defined?


ZIP code boundaries come in all shapes and sizes and are developed to increase mail delivery efficiency. Several of these factors that are considered are as follows:

  • mail volume
  • size of the delivery area
  • geographic location
  • topography
  • ZIP codes are not necessarily defined by community perceived boundaries or municipal boundaries because of these reasons. For these listed reasons, boundaries are subject to frequent changes as the United States of America continually grows.


    In 1942, the Post Office Department started to divide larger cities into smaller delivery zones. In 1963, the United States of America was completely divided into ZIP codes. This created for increased efficiency in the mail delivery system.

    Since a hearing in 1990, the USPS has continually developed a reviewing process to asses ZIP code boundaries. Postmasters in individual areas are given charge of their areas in which they oversee.

    Changes to ZIP Codes

    With the help of locals and others who are familiar with local changes, the USPS does allow and respect requests for boundary changes through something the USPS calls a “ZIP Code Boundary Review Process.” As these changes occur on a regular basis, we offer monthly and quarterly updates to changes made to ZIP code boundaries to keep our customers up to date.

    Many ZIP codes are allowed to have multiple, acceptable city names within its boundaries due to these requests made by locals living in that ZIP code. That is why our data has multiple city names associated with certain ZIP codes. These added, acceptable names are denoted in the “CityType” column of our data by the letter “A.”


    Many ZIP code boundaries are drawn contrary to what many would intuitively think. The ZIP code 85254 is the ZIP code for Scottsdale, Arizona. However, many of the recipients of this ZIP code live in Phoenix. Therefore, it is important to have and consult the correct data for ZIP codes.