Introduction
In the United States, an area code is a three-digit Numbering Plan Area (NPA) code. It’s the first part of most 10-digit phone numbers and helps the telephone network determine where a number is assigned.
Area codes are part of a larger system designed to make numbering predictable, scalable, and routable—especially as new phone lines, mobile devices, and services are added.
The North American Numbering Plan (NANP)
The North American Numbering Plan (NANP) is the numbering framework used across multiple countries and territories, including the US, Canada, and parts of the Caribbean. The NANP defines the structure and rules for numbers formatted as NPA-NXX-XXXX.
Numbering plan administration and public reporting is commonly associated with NANPA (the North American Numbering Plan Administrator). For transparency about what we use and how we process it, see our sources page.
How area codes work
Number structure
A typical 10-digit number follows:
NPA - NXX - XXXX- NPA: the area code
- NXX: the central office / exchange code
- XXXX: the line number
Routing concept
When a call is placed, switching and routing systems use the digits to identify how to deliver the call to the correct destination network. Area codes are not a GPS location, but they are a strong hint about where a number was originally assigned.
In many cases, especially with mobile numbers and number portability, a person can keep a number while moving. That’s why area codes are best interpreted as numbering assignments—not proof of a caller’s current physical location.
Area code assignment
New area codes are introduced when existing numbering resources approach exhaustion. Relief planning typically involves regulators and industry coordination, with NANPA publishing reference data and reports.
Two common approaches are overlays and splits. If you want the practical differences, read Overlay vs Split.
10-digit dialing
10-digit dialing means dialing the full NPA-NXX-XXXX even for local calls. It becomes common when multiple area codes serve the same geographic region (an overlay), because the 7-digit local part alone is no longer sufficient to uniquely identify the destination.
In other situations, dialing rules can change due to regulatory requirements or network transitions. Local guidance from providers is the best source for the dialing rules that apply to a specific region.
The future of area codes
Over time, demand for numbers can lead to additional relief plans. This can mean introducing new overlays, creating new geographic NPAs, or changing dialing requirements. Public NANPA reports help track these changes.
For a transparent view of where our data comes from and how we build it, see Data Sources.