ZIP Code FAQ, via www.carrierroutes.com

What does “ZIP Code” refer to?

A ZIP Code is a five digit number that identifies a specific geographic mail delivery area. The acronym ZIP refers to Zone Improvement Plan, a plan implemented by the USPS in the early 1960’s to improve the sorting and delivery of mail.

How many ZIP Codes are there in the United States?

There are approximately 43,000 ZIP Codes in the United States. This number can fluctuate by a few thousand ZIP Codes annually, depending on the number of changes made

How many changes are made to ZIP Codes monthly?

According to the USPS, there are an average of 25,170 5-digit ZIP Code changes on a monthly basis.

Where did ZIP Codes come from?

In the 1900’s and in the first half of the 20th century, mail was primarily made up of social letters, which were transported by train throughout the country. Rapid industrialization changed the country and by 1963 business mail constituted 80% of the total volume. This was a direct result of the invention of the computer, which allowed companies to centralize accounts and led to an explosion of bills, statements etc. that were delivered by mail.



On July 1 in 1963 the Post Office began implementing a coding program aimed at taking advantage of new transportation systems now available (namely highway and air in addition to rail). Sectional Center Facilities (SCFs) were created, each of which was the central distribution point for between 40 and 150 surrounding post offices.



A five-digit code was assigned to every address in the country. The first digit designated a general area, from zero for the Northeast to nine for the far West. The next two digits referred to one of the Sectional Center Facilities accessible to common transportation networks. The final two digits designated small post offices or postal delivery zones.

What is a three digit ZIP Code?

This refers to the first three digits of any ZIP Code. The first digit, 0-9 designates the general area of the country with numbers starting lower in the east and increasing as you move west. For example 0 covers Maine while 9 refers to California. The next two digits referred to one of the 455 Sectional Center Facilities (SCFs) in the US.



0 = Connecticut (CT), Massachusetts (MA), Maine (ME), New Hampshire (NH), New Jersey (NJ), Puerto Rico (PR), Rhode Island (RI), Vermont (VT), Virgin Islands (VI)

1 = Delaware (DE), New York (NY), Pennsylvania (PA)

2 = District of Columbia (DC), Maryland (MD), North Carolina (NC), South Carolina (SC), Virginia (VA), West Virginia (WV)

3 = Alabama (AL), Florida (FL), Georgia (GA), Mississippi (MS), Tennessee (TN)

4 = Indiana (IN), Kentucky (KY), Michigan (MI), Ohio (OH)

5 = Iowa (IA), Minnesota (MN), Montana (MT), North Dakota (ND), South Dakota (SD), Wisconsin (WI)

6 = Illinois (IL), Kansas (KS), Missouri (MO), Nebraska (NE)

7 = Arkansas (AR), Louisiana (LA), Oklahoma (OK), Texas (TX)

8 = Arizona (AZ), Colorado (CO), Idaho (ID), New Mexico (NM), Nevada (NV), Utah (UT), Wyoming (WY)

9 = Alaska (AK), American Samoa (AS), California (CA), Guam (GU), Hawaii (HI), Oregon (OR), Washington (WA)

What is the difference between a 3-digit ZIP Code and an SCF?

An SCF (Sectional Center Facility) is a postal facility that serves as the distribution and processing center for post offices in a designated geographic area, which is defined by the first three digits of the ZIP Code of those offices. This facility may serve more than one 3-digit ZIP Code range.

3-digit codes (ranges of ZIP Codes) refer to the first 3 digits of the 5-digit ZIP Code. For instance, the ZIP Code 38104 has a 3-digit of 381. Any ZIP Code starting with 381 would be grouped into this 3-digit area.

How many 3-digit ZIP Codes are there?

There are 929 3-digit ZIP Codes in the country.

Do other countries have ZIP Codes?

While other countries do have methods for distributing mail by regional code, they do not have ZIP Codes exactly as the USPS defines the term. For example, Canadians rely on “FSALDU”, where the FSA is similar to a US ZIP Code and the LDU is more akin to a US ZIP+4.

Are there different kinds of ZIP Codes?

The USPS differentiates ZIP Codes as standard and unique. A standard ZIP Code is one that is an established and usual ZIP Code, while an unique ZIP Code is one that only delivers to one specific large entity (such as a hospital or university).

What does “Unique” mean when referring to a ZIP Code?

A unique ZIP Code is a ZIP Code which includes only addresses within one specific large entity, for example a university, air force base or other large mail generating organization

Do ZIP Codes for P.O. Boxes change?

No, these generally do not change.

What is a ZIP+4 Code?

This refers to the 5-digit ZIP code plus a 4-digit add-on number which identifies a geographic segment within the 5-digit delivery area, such as a city block, office building, individual high-volume receiver of mail, or any other distinct mail unit. The purpose of +4 codes is to aid efficient mail sorting and delivery. Business mailers are the primary users of ZIP+4’s, because mailers who mail in bulk on the ZIP+4 or carrier route level can receive rate discounts.

Can ZIP Codes cross State, County, political jurisdictions (cities, congressional district), and metro areas?

In short, the answer is yes to all of the above. ZIP Codes rarely cross state lines, but cross county lines as often as 10%. You can see this yourself by viewing a sample ZIP code map


Visit these pages for more ZIP Code FAQs: ZIP Code Mapping and Mailing by ZIP Code.


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