Why ZIP codes and area codes don't line up
Published
ZIP codes follow mail delivery routes and area codes follow telephone rate centers, so their edges rarely match. Why that is, and how this site links the two.
Two maps drawn for two jobs
ZIP codes and area codes both seem to name a place, but they were drawn by different bodies for different work. The Postal Service draws ZIP codes to deliver mail. Area codes come from the telephone industry, through NANPA and state regulators, and are built around the areas used to rate phone calls.
Neither map was made to match the other, and neither follows town lines closely. A town, a ZIP code and an area code can each have a different edge.
Sources: USPS: postal ZIP Code boundaries; NANPA: area code relief frequently asked questions
How ZIP codes are drawn
The Postal Service gives ranges of ZIP codes to regional post offices, and those post offices assign ZIP codes to delivery routes. The Postal Service says its ZIP code assignments follow mail volume, the size of the delivery area, location and terrain, but not necessarily town or community boundaries.
ZIP code boundaries also shift from time to time as delivery grows and neighborhoods change.
Sources: U.S. Census Bureau: ZIP Code Tabulation Areas (ZCTAs); USPS: postal ZIP Code boundaries
How area codes are drawn
Area codes are built from rate centers, also called rate areas. A rate center is the area phone companies use to decide which calls are local and which are toll calls. When rate centers were first set up, each one was placed at the center of a community, often its post office.
When a new area code is planned, the industry has to follow rate center boundaries. So an area code line runs along the edges of rate centers, not along ZIP codes or town limits.
What that means for a ZIP code
Because the two maps were drawn separately, a ZIP code can sit across an area code boundary. Landlines on one side of it can have one area code and landlines on the other side another.
A town can also have several ZIP codes and more than one area code, from a boundary running through it or an overlay covering it. Mobile phones add one more layer: a mobile number keeps the area code it started with, wherever its owner lives now.
How this site matches them
This site starts from the Census Bureau's ZIP code areas, called ZCTAs, which give each ZIP code a shape on the map. Each of those areas is matched to the telephone rate centers that serve it, and the area codes of those rate centers are the ones listed for the ZIP code.
A ZIP code with no mapped area of its own, such as one used only for post office boxes, takes the area codes of the nearest ZIP code area in the same state. The result is the set of area codes local to that place; any one phone there can still carry a code from somewhere else.
Source: U.S. Census Bureau: ZIP Code Tabulation Areas (ZCTAs)