What is a ZIP Code?

ZIP is an acronym for Zone Improvement Plan, and refers to the USPS’ initiative in the 1960’s to improve the sorting and delivery of mail.  Units with roughly the same number of deliverable addresses were created and termed ZIP Code.

ZIP Codes are comprised of 5 numbers. An additional four, referred to as ZIP +4, are appended by the USPS to allow mail to not only reach the postal town associated with a ZIP Code but also be sorted the city block, office building, or individual high-volume receiver of mail level. 

How frequently and why do ZIP Codes change?

There are approximately 45,000 ZIP Codes in the US, although the exact number is constantly in flux. The USPS estimates that there are approximately 25,000 ZIP Code changes every month. It is important to keep in mind that the majority of these are quite subtle changes, while a few are dramatic.

ZIP Codes change for one of a two reasons:

• A new ZIP Code is added because population has expanded in a given region. This is a fairly common occurrence in areas of the country that are high growth, such as Phoenix, AZ and Las Vegas, NV.  This time of population growth can also result in the addition of new Sectional Center Facilities to aid in the distribution of mail.
• The USPS has decided to reorganize existing ZIP Codes to improve the ease of delivery.

What is a 3 Digit ZIP Code?

A three digit ZIP Code is the first three digits in a 5 digit ZIP Code. So take for example the ZIP Code 10011.  The three digit ZIP Code would be 100.  The first number a ZIP Code refers generally to the region of the country a ZIP Code covers.  ZIP Codes in Maine start with 0 and those in California start with 9 for example.  The next two digits specifies a Sectional Center Facility.  There are just over 900 three digit ZIP Codes in the US.

What is a Sectional Center Facility or SCF

This term refers to one of the approximately 450 regional centers where mail is sorted and then passed on for delivery to each five digit ZIP Code center. A single facility can serve more than one three digit ZIP Code.

Does the USPS provide ZIP Code Maps?

No.  This is because ZIP Codes are not technically polygons but rather collections of deliverable addresses. As such boundaries have to be designated based on these address clusters, and then these boundaries have to be given latitude and longitude coordinates. This is an extensive process and outside the mission of the USPS.   There are a few data compilers in the US that do this, including Tele Atlas North America, which supplies Maponics.

Do ZIP Codes cross county and state boundaries?

ZIP Codes can cross state, place, county, census tract, block group and census block boundaries.  In reality, ZIP Codes only cross county boundaries about 10% of the time.

How do ZIP Codes relate to area codes?

ZIP Codes and area codes were developed independent of one another.  For this reason, they do not sync although in many cases there may be a primary area code for a given ZIP Code.

What are the different ZIP Code types?

The USPS differentiates between unique, standard and PO Box ZIP Codes.

Unique: assigned to a single, high volume mailer such as a university, large business, city apartment building or military base.

PO Box: used only for a location where mailers have Post Office boxes.

Standard: All other ZIP Codes

Which ZIP Code Types are included in ZIP Code Maps?

ZIP Code maps traditionally show only the standard ZIP Codes.

Is there a unit smaller than the ZIP Code?

Yes. To aid in mail delivery, the USPS divides ZIP Codes into carrier routes. As the phrase implies, these are determined as the actual physical area an individual mail person can cover every day. Carrier routes always sync within their corresponding ZIP Code.  The deepest discounts the USPS gives people conducting direct mail campaigns are to those people who mail by carrier route, in which case carrier route maps may be helpful.

ZIP Code Maps - Frequently Asked Questions

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