ZIP Code FAQs
We have compiled this list of frequently asked questions based on conversations that we have each day with customers and prospects. Some of the information provided here may not apply directly to your mapping needs but we think you will find it interesting nonetheless. We do.
What does the term “ZIP Code” mean?
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 fluctuates from month to month, depending on the number of changes made.
How many changes are made to ZIP Codes monthly?
It varies from month to month and from year to year. Because of the schedule that the USPS uses to implement ZIP Code changes we see a tendency for some months to consistently have more changes than others. ZIP Code changes are reported to us by the USPS in the form of addresses that have new or changed delivery information. It is then our job to determine how these changes impact the ZIP Code boundaries. While there are certainly many changes to ZIP Codes at the carrier route level many of these will have no impact on the 5-Digit ZIP Code boundary.
Where did ZIP Codes come from?
In the 19th and first half of the 20th century, mail was primarily transported by rail throughout the country and consisted mostly of personal correspondence. The growth of business and the use of mail as a sales and marketing tool rapidly 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 purchase and maintain large databases of customers and prospects. This eventually lead to an explosion of advertising, bills, statements etc. that were delivered by mail.
On July 1 in 1963 the Post Office implemented a new program aimed at taking advantage of new transportation systems now available (namely Interstate highways and air in addition to rail). Sectional Center Facilities (SCFs) were created, each of which was a central distribution point for 40-150 surrounding post offices.
A five-digit code was assigned to every address in the country. The first digit designated a general postal area (see map below), beginning in the northeast (0) to west coast (9). The next two digits referred to one of the Sectional Center Facilities accessible to common transportation networks. The final two digits designated individual post offices or postal delivery zones.
Postal Area Map ©Copyright TTG, Inc., 2010
What is a three digit ZIP Code?
This refers to the first three digits of a 5-Digit 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 450+ Sectional Center Facilities (SCFs) in the US.
Postal Areas (first digit of the 5-Digit ZIP Code)
|Connecticut (CT), Massachusetts (MA), Maine (ME), New Hampshire (NH), New Jersey (NJ), Puerto Rico (PR), Rhode Island (RI), Vermont (VT), Virgin Islands (VI)|
|Delaware (DE), New York (NY), Pennsylvania (PA)|
|District of Columbia (DC), Maryland (MD), North Carolina (NC), South Carolina (SC), Virginia (VA), West Virginia (WV)|
|Alabama (AL), Florida (FL), Georgia (GA), Mississippi (MS), Tennessee (TN)|
|Indiana (IN), Kentucky (KY), Michigan (MI), Ohio (OH)|
|Iowa (IA), Minnesota (MN), Montana (MT), North Dakota (ND), South Dakota (SD), Wisconsin (WI)|
|Illinois (IL), Kansas (KS), Missouri (MO), Nebraska (NE)|
|Arkansas (AR), Louisiana (LA), Oklahoma (OK), Texas (TX)|
|Arizona (AZ), Colorado (CO), Idaho (ID), New Mexico (NM), Nevada (NV), Utah (UT), Wyoming (WY)|
|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 refer to the first 3 digits of the 5-digit ZIP Code. For instance, the ZIP Code 55144 has a 3-digit of 551. Any ZIP Code starting with 551 would be grouped into this 3-digit area.
How many 3-digit ZIP Codes are there?
There are approximately 900 3-digit ZIP Codes in the United States.
Do other countries have ZIP Codes?
Other countries do have methods for distributing mail using various coding schemes (often referred to using the more generic term "Post Codes") they do not have "ZIP" Codes as defined by the USPS. For example, Canadians use a 6 digit coding scheme with a mix of characters and numbers called “FSALDU” where the FSA is similar to a US 5-Digit ZIP Code and the LDU is similar 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, university or other unique delivery location).
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.
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 and metro areas?
Yes they can and do, however, this is not the norm. ZIP Codes rarely cross state lines but do more frequently cross county lines. You can see this yourself by viewing a ZIP code map. The reason for this is that ZIP Codes are service delivery areas and do not necessarily need to adhere to other geopolitical boundaries. For example, it may be more efficient to service a particular area from one post office even though it is in a different State or County.
How are boundaries for ZIP Codes defined?
The USPS does not define boundaries for ZIP Codes. Instead ZIP Codes represent groups of delivery points (addresses), designed for efficiency of delivery. In most cases ZIP Codes resemble spatial areas since they comprise contiguous streets and address ranges. However, this is not always the case. In rural areas, ZIP codes can be collections of roads (rural delivery routes) that in reality do not look much like a closed spatial area. It is our job to combine these points and lines into polygons. It may sound like a simple task to draw a perimeter boundary around a set of points to come up with the ZIP Code, however, it is more complicated than it may first appear. This is the reason that ZIP Codes from different vendors may appear differently on a map. Decisions about whether to include or exclude various topological objects such as lakes, ponds, forests and other similar unpopulated areas can radically alter the size and shape of a ZIP Code. This is why we often say that to draw ZIP Code boundaries is more an art than a science.
What is the difference between your ZIP Code data and the free data from the US Census bureau?
The USPS does not define ZIP Code boundaries. It only maintains and releases ZIP Code correspondence information in database format. TTG then takes this information provided by the USPS and creates a polygon that represents the geographic area covered by each ZIP Code. The free ZIP Code boundaries provided by the US Census Bureau are not real ZIP Code areas. They are referred to as ZIP Code Tabulation Areas or ZCTAs. While ZCTAs may be suitable for some applications they are not ZIP Codes and should not be used in place of ZIP Codes. Here is a quote from the US Census Bureau web site regarding ZCTAs that defines them and suggests that they are incomplete:
"ZCTAs are generalized area representations of U.S. Postal Service (USPS) ZIP Code service areas. Simply put, each one is built by aggregating the Census 2000 blocks, whose addresses use a given ZIP Code, into a ZCTA which gets that ZIP Code assigned as its ZCTA code. They represent the majority USPS five-digit ZIP Code found in a given area. For those areas where it is difficult to determine the prevailing five-digit ZIP Code, the higher-level three-digit ZIP Code is used for the ZCTA code."
"Based on the January 2000 list of ZIP Codes from the USPS's Delivery Type File, the ZCTA delineation process excluded 10,068 ZIP Codes in the United States and Puerto Rico (not counting overseas military ZIP Codes). These included 2,523 ZIP Codes that served specific companies or organizations with high volumes of mail and 6,419 ZIP Codes dedicated to Post Office (PO) Box and/or general delivery addresses primarily located in areas otherwise served by rural route or city style mail delivery. The remainder represents ZIP Codes that were either inactive or insufficiently represented in the MAF and therefore did not become ZCTAs."
Does your data include Are P. O. Boxes?
Yes. P.O. Box ZIP Codes are represented as points in our data within the appropriate parent ZIp Code polygon.
What is a ZIP Code centroid?
A ZIP Code centroid is a calculated center point at the center of each 5-Digit ZIP Code polygon. The centroid is calculated based on the coordinate extremes of the polygon. In cases where the polygon is irregular, the centroid may be adjusted so that it does not fall outside of the polygon. Centroids are used for labeling and other purposes by many GIS software programs.
Who provides ZIP Code boundaries? Can’t I get them directly from the USPS?
The Postal Service does not define or maintain ZIP Code boundaries. Defining ZIP Code boundaries is a difficult and challenging process. The USPS directs people looking for ZIP Code boundaries to third party companies to obtain ZIP Code boundary data.
Are there areas of the nation that are not covered by a ZIP Code?
Yes, in sparsely populated rural areas where there is no mail delivery (deserts, mountains, lakes) ZIP codes are not defined. This creates unsightly "holes" in the ZIP Code boundary data. Some vendors have created polygons to fill in these holes and have given them their own arbitrary numerical designations - typically outside the range of numbers used by valid ZIP Codes. Since most of our customers do not want holes in their data or polygons with arbitrary numbers, we take a different approach. We sew up these unpopulated areas into the surrounding ZIP Codes to create a seamless and contiguous map layer. We understand that this may not suit all applications but for the majority of our customers this is the preferred method to handle this issue.
Do you provide demographics by ZIP Code?
Yes. We have a demographic database of over 18,000 variables at the ZIP Code level. For more information please visit our companion web site . You can download a complete listing of the variables available. Or, if you prefer, contact us for a quote at .