What a US ZIP code is (and what it is not)

ZIP codes are postal delivery routes, not legal boundaries. What the five digits mean, why ZIPs and counties do not line up, and how to resolve one.

A US ZIP code is a five-digit postal code: the first digit identifies a group of states, the next two a sectional centre facility, and the last two a delivery area. ZIP+4 adds four digits for a specific block or building.

ZIP codes are routes, not places

ZIP codes describe mail delivery, so they do not map cleanly onto cities or counties. One ZIP can span several counties; a city can contain dozens of ZIPs; and some ZIPs cross state lines. That is why address databases and census tools often disagree — they are reconciling two different geographies.

Resolving a ZIP

The free US ZIP Codes directory shows each ZIP's primary city, county, time zone, coordinates and neighbouring ZIPs, and links to the places and area codes that overlap it. A compact JSON index is served at /data/zips.json.

Why it matters

• Sales tax and insurance rates are set by address, not ZIP — two houses in one ZIP can pay different rates.
• School districts and voting precincts ignore ZIP boundaries entirely.
• For population and county data, use Census places and counties, not ZIPs.

Search ZIP codes · US places and counties