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What is Zip Code and How Do Zip Codes Work?

We live in a world of differences – a world filled with all kinds of people belonging to different countries and cultures. There are very few things that transcend these differences. The ZIP-code system is possibly one of them.

Although the name may vary from country to country, you will find a postal code system similar to the US’s ZIP codes wherever you go. It makes the ZIP-code system one of the very few universally accepted systems in the world.

But what makes the ZIP-code (Postal Code) system so special? Why does almost every country in the world adopt it? Why are we still using it even after so many years with only minor changes made to it? The simplest answer to the question “What is zip code?” is that they are the postal code system used in the US.

However, that does not answer the question. ZIP codes help businesses of all kinds with their day-to-day operations. Product or mail delivery, in particular, is dependent on ZIP codes for accurate and fast delivery.

To understand what a ZIP code, is and the different cases where we can use it, you should begin with its origin. Reading further on will take you through the history. Furthermore, we will also attempt to understand the ZIP code from a global perspective.

As we move along, we will discuss the significance of the ZIP code in business operations. In addition to that, the article also elaborates on the US ZIP codes list and how you can get all 41,000+ ZIP codes. Finally, we also look at how you read a ZIP code and use it effectively for your business. 

Why Are ZIP Codes Important?

ZIP Codes are not just data that postal service providers use to optimize their postal delivery process. Even though some of you may find it hard to believe it, the ZIP Codes play a significant role in business operations in the US.

Sending Bills And Invoices

Businesses of all kinds have to send bills and invoices to their customers. These are essential documents. If the customer receives it late or does not receive it at all, they could miss the payment.

Having a full ZIP+4 code for the addresses ensures that the mail is delivered without any delays. This is why businesses employ address verification tools like PostGrid. You can get access to the full ZIP+4 Code of all your addresses using PostGrid’s bulk address verification.

Fraud Detection

ZIP codes enable businesses to conduct a quick check on their background. You can see which addresses are accurate and deliverable by pushing them through address verification tools like PostGrid. So, you can sort the real and fake addresses and detect any fraud in the address data provided by the customer.

Demographic Information

ZIP codes can be used by businesses to get access to demographic information. This data can prove vital for your business’s marketing effort. For example, you may determine specific demographics likely to buy a product or service from you. Tools like PostGrid even come equipped with geocoding capabilities to target your campaigns for higher response.

History of ZIP Codes

As we have mentioned before, the ZIP-code system first came into existence back in the 1960s. However, the first postal code system in the US was employed decades earlier, in 1943. In the beginning, postal zones were used instead of ZIP codes. However, these zones were only created for large cities.

It was only in the early 60s that it became evident that a more organized postal coding system was required. This was when they first introduced the ZIP code – a 5-digit ZIP code implemented throughout the nation.

State Abbreviations

The US Post Office Department or USPOD also published a list of abbreviations with ZIP codes. So, the capitalized abbreviations that we have for US states are a result of ZIP code implementation. All of us are familiar with the two-letter abbreviations that are used to denote states.

Initially, the plan was to use abbreviations that ranged between two and five letters. But, it was later decided to use capitalized two-letter abbreviations. The new standard abbreviations are based on a maximum 23-position line. Shorter abbreviations further made the ZIP codes easier to understand and sort.

Early ZIP Code System

Postal inspector Robert Moon first proposed the early ZIP code system in 1944. Although the early ZIP codes are five digits, Moon is only credited for the first three digits of the IP code that we now use.

The first three digits of the ZIP code are used to identify or describe a Sectional Center Facility, also known as SCF. It acts as the central mail processing facility, which sorts the mail to different post offices that fall under it.

The last two digits of the five-figure ZIP code were proposed by Henry Bentley Hahn Sr. Fourth and fifth digits further narrow the location of the address within the SCF. Hence, the postal authority used these two digits to sort the mail once it reached the SCF.

Introduction of ZIP+4 Code

The USPS introduced a more elaborate ZIP code system known as ZIP+4 Codes in 1983. They are also known by the names “plus-four codes,” “add-on codes,” or “add-ons.” When writing an address with a full nine-digit code, the ZIP+4 codes follow the original five-digit ZIP code.

It is also worth noting that each Post Office Box has its own unique ZIP+4 Code. Generally, the last four digits of the PO Box number are used as the ZIP+4 Code for PO Box addresses. Similarly, they could also use zero plus the last three or two digits of the PO Box number.

Postal Bar Code

The ZIP code written on a mailpiece is converted into an Intelligent Mail Barcode to efficiently process mail. Barcodes enable automated machines to sort the mail, saving them valuable time and effort. In addition, the sender may choose to print the barcode themselves via some word-processing programs.

However, it is not preferred because the information the sender has may be out of date. So, it is best to leave the postal barcode printing to the post office. They use OCR technology for printing the postal bar codes and also use a manual process if necessary.

Businesses or organizations that send bulk mails can get a discount on postage if they print the barcode themselves. However, to do this accurately, you’d need the help of a CASS-certified direct mail service provider like PostGrid.

Solutions such as PostGrid can access the USPS’s official address database and print accurate postal barcodes on your mailers. This way, not only does your direct mail get processed faster, but you also get to save money on postage.

How do ZIP Codes Work?

We know that ZIP codes are five-digit numbers that help the USPS to make fast and timely deliveries. The five-digit ZIP code represents post offices all across the US. There are different names by which the ZIP codes are known worldwide, but the concept remains the same everywhere.

The five-digit ZIP codes can be split into three major parts, and they are as follows:

  • National Area
  • Region or City
  • Delivery Area

Furthermore, it is also worth noting that the USPS has segmented the whole country into 10 ZIP Code areas. These ZIP Code areas start from the northeast and are numbered 0-9. This means that the first digit can tell you which state the mailer is supposed to go to.

The following table can help you understand the destination of a mailer based on the first digit of its ZIP Code.

ZIP Codes starting with 0

Connecticut, Massachusetts, Maine, New Hampshire, New Jersey, Puerto Rico, Rhode Island, Vermont, Virgin Islands, Army Post Office Europe, Fleet Post Office Europe

ZIP Codes starting with 1

Delaware, New York, Pennsylvania

ZIP Codes starting with 2

District of Columbia, Maryland, North Carolina, South Carolina, Virginia, West Virginia

ZIP Codes starting with 3

Alabama, Florida, Georgia, Mississippi, Tennessee, Army Post Office Americas, Fleet Post Office Americas

ZIP Codes starting with 4

Indiana, Kentucky, Michigan, Ohio

ZIP Codes starting with 5

Iowa, Minnesota, Montana, North Dakota, South Dakota, Wisconsin

ZIP Codes starting with 6

Illinois, Kansas, Missouri, Nebraska

ZIP Codes starting with 7

Arkansas, Louisiana, Oklahoma, Texas

ZIP Codes starting with 8

Arizona, Colorado, Idaho, New Mexico, Nevada, Utah, Wyoming

ZIP Codes starting with 9

Alaska, American Samoa, California, Guam, Hawaii, Marshall Islands, Federated States of Micronesia, Northern Mariana Islands, Oregon, Palau, Washington, Army Post Office Pacific, Fleet Post Office Pacific

As you are already aware, the ZIP Code is not just random numbers thrown together. Each digit in the ZIP Code number has a specific role to play. We have already discussed how the first digit in the ZIP code denotes the state to which the direct mail is addressed.

Second Two Digits of ZIP Codes Indicate the City

The USPS assigns the second two digits in a ZIP Code to denote a city. If a specific geographical region has a central town or city, then the first ZIP Codes are assigned to that city. Once that is done, the ZIP codes get assigned in alphabetical order.

First Three Digits Together Indicate the Sectional Center Facility

Together, the first three digits of the ZIP Code indicate the Sectional Center Facility of the mail piece. The Sectional Center Facility is where mail pieces get sorted and distributed for a specific area or zone.

It is worth keeping in mind that some Sectional Center Facilities may have more than one three-digit code assigned to them. So, if you do happen to notice a Sectional Center Facility like that, do not get alarmed. However, it does not hurt to double-check the ZIP codes in such cases.

Fourth & Fifth Digits Indicate The Area of The City/Town

The area of the city or town in which the mailer is addressed is indicated by the fourth and fifth digits of the ZIP Code. This greatly simplifies the mail sorting and delivery process at the local level and enables USPS to make optimal delivery routes.

As you can see, each number in the US ZIP code serves a purpose. The USPS effectively uses the ZIP-code system to ensure that your direct mail is delivered without any delays. Therefore, having the wrong ZIP code for your addresses can stand in the way of efficient direct mail delivery.

Businesses should use address verification tools like PostGrid to get access to the full ZIP code of addresses. This way, you can ensure that your direct mail does not encounter any unnecessary delays.

Many people assume that the ZIP Codes are drawn based on state boundaries. Although this may seem true in many cases, that is not really the case. The purpose of ZIP codes is merely to optimize the mailing efficiency, and boundaries are not a consideration in this.

ZIP codes often cross county and state boundaries. Although most ZIP codes may seem like they’re assigned to a geographic area, that is not really the case. ZIP Codes may also represent address groups or delivery routes and not geographic regions.

ZIP Codes: Special Cases

Since ZIP codes denote address groups or delivery routes, some ZIP codes may overlap with each other. For example, the US Navy has a special ZIP Code assigned to them, and they may not have a fixed geographical location.

Naval offices could be stationed within a location that has a different ZIP Code. Similarly, you must also consider rural route areas that are not assigned a ZIP code because they don’t have regular mail delivery available.

Given above is just one of the few examples of how ZIP Codes can overlap or not exist for geographic locations. There are also instances where big companies that receive and send bulk mails are given a unique ZIP code.

ZIP+4 Codes

The ZIP+4 code is used to identify geographic locations inside the five-digit delivery area. Hence, the ZIP+4 Code could narrow down the address to a city block, post office box, or apartment complex. Initially, The ZIP+4 code was met with resistance from the public. But, it has now become an effective way to ensure timely mail delivery.

The full ZIP+4 code is not mandatory for sending postal mail. In many cases, the ZIP+4 code is calculated automatically by the postal service provider when the mail gets sorted. However, providing a valid ZIP+4 code on your mailer ensures there is no delay in mail processing.

Businesses that send essential documents via postal mail need it to reach their customers on time. It is especially true if the documents you send are time-sensitive. Hence most businesses prefer to use a full ZIP+4 code for their mailers to ensure fast and accurate mail delivery.

When sending direct mail, businesses should use the full nine-digit ZIP Code number for their direct mail. You can use an address verification API like PostGrid to get the full ZIP+4 Code of addresses. This way, you can ensure that your postal mails are delivered without delay.

9 Digit Full ZIP+4 Codes

Every Post Office Box has its own unique ZIP+4 code assigned to it. In fact, it is mandated by the rule that PO Boxes must have their unique ZIP+4 codes. However, the ZIP+4 Code used for a PO Box is not always assigned in the same way.

Usually, the last four digits of the PO Box number are provided as the ZIP+4 Code of that address. However, this may vary for PO Boxes numbers that have fewer than four digits in them. In such cases, zeros are used to fill in the numbers in the front of the PO Box number.

This variance means that you have to look up each ZIP+4 code individually for every PO Box. However, a significant amount of business communications happen via PO Box mails. Hence it becomes inevitable for businesses to use tools like PostGrid to determine the ZIP+4 code for their direct mail.

Furthermore, every USPS deliverable address doesn’t have to have a ZIP+4 code assigned. This is because address verification can’t give you a ZIP+4 code if it does not exist for the address. Such addresses may require advanced geocoding capabilities like the one you get in PostGrid.

Many of you may not know this, but the US is one of the member countries of the Universal Postal Union (UPU). It means that whatever postal delivery standards developed and followed by the US align with UPU’s guidelines. Doing so ensures the free exchange of global mail with minimal hiccups.

Universal Postal Union

The Universal Postal Union or UPU is a regulatory body that functions under the United Nations. One of the crucial functionalities of UPU is to build new postal systems for developing countries. UPU accomplishes this by utilizing the Geographic Information Systems (GIS) technology from the US.

Countries like the United States primarily fund UPU. However, the benefits UPU receives from its top funding members like the US goes beyond monetary ones. For example, UPU also gets access to the ZIP-code system used by the US and its expert knowledge regarding postal service.

Canada Post Postal Codes

The UPU provides specific guidelines to maintain universal standards for postal services around the world. However, that is only possible to a certain extent because, in real-life situations, the postal codes of two countries may look nothing alike. You can see this if you compare the postal codes in Canada and the US.

ZIP codes are created explicitly for addresses in the United States. Furthermore, the ZIP codes are managed by the official postal authority of the country, which is USPS. When it comes to postal codes in Canada, the system is controlled by Canada Post.

Early History

Similar to the US, Canada also maintains a well-refined postal code system that enables easy and fast mail processing. The first postal system in Canada evolved in the 50s and 60s and initially used a three-digit postal code system. However, the system later evolved to become a six-digit system in 1969.

Understanding Canada Postal Code

Canada Post uses an alphanumeric postal code. The first three digits in the postal code are called Forward Sortation Area or FSA. Canada Post corresponds geographical regions like a province to an FSA. The remaining three digits of the six-digit postal code correspond to an LSU or Local Delivery Unit.

The LDU usually includes a group of addresses, and they act as the local level of address for making mail deliveries. Direct mail first gets sent to the FSA, from where the mail gets sorted and forwarded to LDUs, where the final sorting happens. Finally, the mail is then delivered to the addressee.

Similarities to ZIP Codes

The postal code employed by Canada Post is very similar to the ZIP code used by the USPS. Consequently, the major mailing processes of both Canada Post and the USPS are more alike than different. The major mail operations that are classified similarly by both postal service providers are listed below.

  • Mail sorting
  • Automation
  • Demographic data management

What is the US ZIP Codes List?

As the name suggests, the US ZIP code list lists ZIP codes of all the addresses inside the United States.

As the name suggests, the US ZIP code list lists ZIP codes of all the addresses inside the United States. The USPS maintains the US ZIP codes list because it is the official postal authority of the United States. Furthermore, USPS updates its ZIP code lists every month to ensure their accuracy.

The monthly changes made to the US ZIP Code list can add up to 5000 changes every year. ZIP codes of US addresses can frequently change due to several reasons. Some of the most common reasons behind ZIP code changes include the following:

  • Opening of new Post Offices
  • Closing of existing Post Offices
  • Boundary changes

How Can You Get Access to All 41000+ ZIP Codes?

There are over 41000 different ZIP codes available in the US. You can not rely on poor websites that update their ZIP code list every six months or even annually. It is best to go for a CASS-certified service provider to access a reliable ZIP Codes list.

There are tools and services available online that provide you with a US ZIP Codes list. All you need to do is make sure that they are CASS-certified service providers that can access the official database of USPS. Some of them even allow you to download them as a .txt file and use them freely.

Finding the full ZIP Code for all the addresses in your database can be challenging if you do it manually. Thankfully you now have access to advanced address verification tools like PostGrid that enable you to run bulk address verifications.

PostGrid’s bulk address verification uses a simple CSV file containing your address database. The system then returns you with verified addresses, including the full ZIP+4 code for those addresses. This way, you can easily ensure which addresses are real and deliverable in your database.

Furthermore, you can automate the entire direct mail process using verified addresses. Everything from postcards to brochures and even checks can be printed and mailed using PostGrid. Therefore, automated tools like PostGrid effectively ensure that your direct mails are delivered on time with a minimum return rate.


ZIP Codes are an integral part of several business operations, and their application is not limited to postal mail delivery. For example, businesses can use them for accurately sending essential documents to their customers. They can even use them to get a better understanding of their target demographics.

It is vital to understand the answers to questions like how long is a zip code? How do zip codes work? Doing so allows you to use ZIP Code data to its full potential and optimize several of your business operations. 

Learning small facts like how many numbers are in a zip code and understanding what those numbers mean helps you understand the process better. A better understanding will make you realize the significance of automated tools like PostGrid for your business.

The CASS certification of PostGrid enables it to access the official address database of USPS. In addition, it allows tools like PosGrid to supply you with the full ZIP+4 code for your address database. As a result, you can optimize your business operations, especially your direct mail process.

Writing Zip Codes — The Blue Book of Grammar and Punctuation

We at times receive questions concerning the proper way to address outbound mail and, in particular, use zip codes. Because your goal and ours is to achieve precision in writing, we’ll take a closer look at zip codes and how you can treat them in your correspondence.

What Is a ZIP Code?

Zip codes today are five-digit numbers at the end of U.S. mailing addresses to help ensure more-efficient delivery in the American postal system. Without them, mail can be delayed because of extra manual handling.

According to the Smithsonian National Postal Museum, what we now know as zip codes were introduced in 1943 as a result of World War II. In addition to more mail being sent during the war, the postal service was hiring many new workers to replace those who had left and were leaving for battle. To help the large number of inexperienced employees, the postal service created a new system of numerical postal zones that would simplify the sorting and sending of mail (zip stands for zone improvement plan).

In the beginning, the numbered postal zones were used only in 124 of America’s largest urban areas. An original address might have looked like this, with the number identifying the city:

Gary Grammar
1234 Apostrophe Way

Indianapolis 24, Indiana

By the early 1960s, the American post needed a more detailed and organized system, so non-mandatory five-digit zip codes were introduced along with two-letter state abbreviations (e. g., Chicago, IL 60601).

In 1983, the zip + 4 format we now use (e.g., 60601-1635) was introduced to support delivery to densely populated areas. The first three digits before the hyphen identify the sectional center or large city. The second two classify the post office facility or delivery area. After the hyphen, the first two digits indicate the sector or group of blocks, and the last two identify the segment or one side of a street.


Zip Code: A Matter of Style

Sources differ concerning how to write the phrase for the U.S. mail’s identifying numerical system. The U.S. Postal Service uses ZIP Code. It also includes a trademark symbol (ZIP Code) even though available information suggests the mark expired in 1997.

The Associated Press Stylebook advises using ZIP code. The Chicago Manual of Style favors zip code, which is the style that observes.

You would be correct to use any one of these choices unless directed otherwise by an established preference or style guideline.

Writing Zip Codes: Content vs. Mail

If you are writing an address with a zip code in content, style guides often agree on including a comma between the city and state (e.g., Chicago, IL) in the city-state-zip code line.

AP would use the two-letter state code in both content and outbound mail:

Chicago, IL 60601 Tulsa, OK 74120
Santa Clara, CA 95053 St. Petersburg, FL 33731

CMOS would spell out the state unless it appears in lists, tabular matter, notes, bibliographies, indexes, or outbound mail (in which case it would follow AP style):

Chicago, Illinois 60601 Tulsa, Oklahoma 74120
Santa Clara, California 95053 St. Petersburg, Florida 33731

When we are writing zip codes for the U.S. mail, the postal service prefers that punctuation not be used. It also prefers that lettering be in all CAPS; the address be printed on the center of the envelope with a flush left margin; and the + 4 number be included.


To further explore written format for the U.S. mail, you can review our related topics:

Writing Addresses
Commas to Separate a City and State


If the article or the existing discussions do not address a thought or question you have on the subject, please use the «Comment» box at the bottom of this page.

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Create zip code records to automatically assign organizations to service territories. Enter the address on the organization form and the system will automatically fill in the service area if the zip code matches.

Postal codes are associated with territories. However, they are not required for the territories feature to work.

It is not possible to assign the same zip code to multiple territories.

Create zip code

  1. In Field Service , open area Settings .

  2. Under General , select Postcodes .

  3. Select Create to create a zip code entry.

    • Name : Enter the zip code.
    • Service Territory : Select an existing territory or create a new one.
  4. Select Save .

Change zip code

  1. In Field Service , open the area Settings .

  2. Under General , select Postcodes .

  3. Select the postal code you want to change and change the entry. You can also select multiple entries and select Edit if you want to batch change the territory.

  4. Select Save .

Delete zip code

  1. In Field Service , open the area Settings .

  2. Under General select Postcodes .

  3. Select the zip code you want to remove. You can also select multiple entries to delete.

  4. Select Delete and confirm deletion.


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How to find postcodes and area codes online

Postcodes and zip codes are numeric codes used to facilitate mail delivery, while area codes are used to indicate a geographic region when calling a phone number. Instead of flipping through a huge cumbersome phone book, you can easily find and check postcodes and area codes online.

How to Find a Zip Code

If you have an address or a partial address, you can find the zip code through the USPS Zip Code Lookup, a tool on the US Postal Service website. You can also search by city or state, or find all cities that are part of a specific zip code.

The International Postal Codes Site, maintained by Columbia University, contains an extensive index of domestic and international postal codes, as well as abbreviations, policies, and mailing best practices.

You can also use various search engines to find postcodes. Using Google for example, to do a postcode search for an address, enter what you know the address to see a map of the area with the postcode included.

You can even type something like South Laurel MD zip code , to see all of them for that area (and find links that show the full list of zip codes for that region). You can also enter a zip code to see the geographic location it belongs to, as well as a map and other related web results such as cities that use that zip code.

If you don’t add zip code at the end of the search, Google may show unrelated information. For example, searching for 90210 shows serial information, and for zip code 90210 shows zip code information.

You can also find the zip code with DuckDuckGo. Enter a zip code and DuckDuckGo will return a map, zip code location, weather, real estate, and other relevant web results.

10 things that might surprise you about DuckDuckGo

Bing zip code search allows you to determine which city/locale it belongs to, maps, and local points of interest such as grocery stores, hotels, and movie theaters. If you enter a partial address, Bing will complete it for you and show you the zip code.

How to find an area code

As with a postal code, you can use a search engine to find an area code by doing a city search. To do the opposite and find out which part of the country an area code belongs to, simply enter the area code into a search engine.

To find an area code using Google, enter the name of the city and state you are looking for, followed by the words in area code . Usually you will find what you need. For international listings, look up a phrase such as Kenya’s call code and you’ll get an informative response with the numbers you’ll need to call that country.

Enter city and state in the new Bing search and you’ll likely see Google-like results that show the city code at the very top of the results. The same applies to international dialing codes.

Bing Search Labels you should know

Another way to find an area code online is to use Wolfram Alpha, which bills itself as «Computational Intelligence». This data-driven search engine doesn’t display results like the others mentioned on this page, but with a little digging, you might find some unique city and state information that you might not find in another search engine.

Using Yahoo to look up an area code is very similar to using Google; just enter the city and state name followed by area code and you will get an instant result. It’s not as easy to find the international code on Yahoo as it is on Google, but you can of course use Yahoo to look for other websites that can provide you with this information.

Many sites specialize in finding area codes that a simple search query might not show. You can use these free resources to find just about any area code and/or country code in the world:

  • Area Codes: Area codes from mostly the United States are featured here, although other countries are also scattered here.

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