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What are Zip codes

What Are ZIP + 4 Codes Background

A ZIP (Zone Improvement Plan) Code is assigned to specific geographic regions in the United States. This code is used by the United States Postal Service (USPS) to sort and deliver mail more effectively.

ZIP Codes are tied to addresses and, like addresses, are distinct. This means that where one ZIP Code ends, another begins without any transition area. Also, there are some significant areas in the U.S. without any addresses, so those regions are not applied to any ZIP Code. These areas without ZIP Codes are primarily located in Alaska and desert regions in the Southwest region of the U.S.

Most people are familiar with the basic five-digit ZIP Code – you use this when you enter your address into a website to order a pizza or get a delivery from Amazon. These five-digit ZIP Codes are usually applied to one specific post office. This makes the sorting process much easier for USPS employees; they don’t need to spend time looking for which Post Office in Atlanta delivers to a particular street because they have a ZIP Code to tell them. Using a ZIP Code is not strictly required to deliver a piece of mail, but it will substantially speed up the delivery.

The basic ZIP code is broken into five digits:

  • The first digit represents the National Area – each state is part of a National Area

Ex: All Zip Codes in Georgia, Tennessee, Florida, Alabama, and Mississippi start with a 3

  • The following two digits represent the Sectional Area – a smaller region inside the National Area, usually a city or county

Ex: All ZIP Codes in Atlanta, Georgia have 303 as the first three digits

  • The final two digits represent a specific Post Office inside of the Sectional Area

Ex: The Post Office that services Downtown Atlanta is 30303

While most countries utilize some type of postal code, the ZIP Code system is specific to the U.S. When sending a letter or package outside of the U.S., the shipping label will need to follow that country’s postal standard to avoid delays. Remembering to include the country in the address will let any US-based mail carrier know where to send it.

ZIP + 4 Codes

It is possible to use a more specific ZIP Code that will allow you to estimate the location of an address to a few square blocks: the ZIP + 4 Code. Addresses validated and standardized have these additional four numbers that typically refer to a specific delivery route within the ZIP Code. Generally speaking, ~20 addresses will be grouped into a ZIP + 4 Code. However, it is not uncommon for large apartment buildings or businesses to have their own +4 Codes. Specific P.O. Boxes will also have their own ZIP + 4 Codes: the four digits matching the P.O. Box Number.

While certainly not required, using the ZIP + 4 Code can result in your mail being delivered significantly faster. The fact that only verified addresses get +4 codes provides an additional benefit of peace of mind that your package has been sent to an actual address.

Because these last four digits refer to a delivery route, they change much more often than an address’s five-digit ZIP Code – whenever delivery routes change for reasons like the season, number of postal employees, and the number of packages to be delivered. While it is unlikely that your address’s five-digit ZIP Code will change while you live there, your ZIP + 4 Code can change as often as once a month! 


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Written by Austin Proctor 


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