The US Area Code System

The United States area code system is a numerical framework employed to organize and manage telephone numbers across different geographic regions. Introduced in 1947 by AT&T and the Bell System, the system divides the U.S. into distinct Numbering Plan Areas (NPAs) and is managed by the North American Numbering Plan Administration (NANPA).

Numbering Plan Areas (NPAs)

Each Numbering Plan Area (NPA) area, also know as Area Codes, represented by a unique three-digit code. The area code format is commonly presented as "(XXX)", where XXX denotes the three-digit numerical identifier, followed by a three-digit NXX Prefix and a four-digit Line number. This gives you an (NPA)-NXX-XXXX or NPA NXX XXXX phone number format.

Number Formating

NXX Pefixes

An NXX prefix, also known as a central office code or exchange code, is the three-digit code that follows the area code in a North American telephone number. The term "NXX" is derived from the fact that the first digit can range from 2 to 9, the second digit can range from 0 to 9, and the third digit can also range from 0 to 9

The NXX prefix is associated with a specific geographic location or central office, which is a physical facility that houses telephone equipment and serves a particular area. As such, the NXX prefix helps to identify the general location of the telephone number and assists in the efficient routing of calls within the telecommunications network.

International DIaling

When dialing a long-distance number within the United States, you typically need to include the area code, even if you are in the same geographic area. For international calls, the U.S. country code is 1, followed by the three-digit area code, the three-digit prefix number, and the local phone number.

Geographic Assignment

Geographically, area codes are assigned to specific regions, ranging from entire states to parts of states or even multiple states. The allocation of area codes is designed to accommodate the varying population densities and demand for telephone numbers in different locales. Major urban centers and densely populated regions tend to have multiple area codes, while rural or less populated areas might share a single code.


The basic structure of the system involves dividing each area code into central office codes (NXX), which make up the local exchange. This hierarchical arrangement allows for efficient routing of telephone calls. Over the years, as the demand for telephone numbers has increased, overlay plans have been introduced in certain regions. These overlays involve adding a new area code to an existing geographic area, ensuring a continued supply of available phone numbers. Learn about overlays.

Number Portability

It's important to note that with the implementation of number portability, individuals can retain their phone numbers when switching service providers or moving to a different location. This means that the area code associated with a phone number may not necessarily indicate the user's current physical location.

The Future

As communication patterns evolve and population dynamics shift, the assignment and usage of area codes may change. Despite these changes, the U.S. area code system remains a vital component of the nation's telecommunications infrastructure, facilitating the effective routing of telephone calls across diverse geographic regions.