How US ZIP codes work

A ZIP code is a mail delivery route, which is why it does not line up neatly with city or county borders.

The five digits

The first digit divides the country into groups of states, the next two narrow that to a sectional centre facility, and the final two identify a delivery area — a set of carrier routes served from one post office. Because it is a route, a single ZIP can cover parts of two cities, and a city can be split across a dozen ZIPs.

ZIP+4

The four extra digits after a hyphen identify a smaller segment: one side of a street, a building, a floor, or a group of PO boxes. ZIP+4 improves mail sorting accuracy but is optional for most personal mail. The five-digit code is what address forms and websites normally ask for.

ZIPs are not census areas

The Census Bureau builds ZCTAs (ZIP Code Tabulation Areas) by snapping ZIP delivery areas to census blocks so population can be estimated. A ZCTA approximates a ZIP but is not identical, which is why population figures are published per ZCTA and labelled as estimates.

What ZIPs do not tell you

They do not define municipal boundaries, school districts, or tax jurisdictions. Two houses on the same street can share a ZIP but sit in different cities, counties or even states. When a ZIP appears to be in two places at once, both are usually correct — the ZIP follows mail routes, the city follows legal boundaries.