Stencycle

Remember that ZIP codes were made to make mail delivery easier. They weren’t made to correspond to existing boundaries such as cities, counties, or even states. If it is more efficient for a mail carrier to drive across a state line to deliver mail, the ZIP code “boundary” will cross the state lines. ZIP codes don’t usually cross state lines, but some do (65733, 71749, and 73949 are good examples). The complete list of ZIP codes that cross state lines is available as part of our US ZIP code list.

It gets even more complicated when trying to assign a ZIP code to a specific county (as much as 25% cross county lines), congressional district, metro area, time zone, area code, etc. The edges of the boundaries commonly overlap. For the purposes of our free zip code database by county downloads, we will commonly list either the most common region for the ZIP code or list multiple regions if several exist in the ZIP code.

For cities, the assignment is somewhat more complicated. USPS does not always use the city in which the ZIP code is located. The assignment of cities to ZIP codes is more general. The city is usually the name of the main post office. For instance, almost all ZIP codes in St. Louis County in Missouri have a city of Saint Louis when they may be more accurately described as the name of a smaller city where they are located.