Find Your ZIP Code in the United States

ZIP codes were introduced in 1963, when mail volumes had outgrown the older system of postal zones that covered only a limited number of cities. ZIP stands for Zone Improvement Plan, and the point of the plan was to let mail be sorted mechanically against a number rather than read as an address by a person.

That origin explains the shape of the system. The code is built outward from how mail physically moves, not from how places are drawn on a map. The first digit picks out a broad band of the country. The first three together name a sectional center facility, the regional plant that receives mail for a cluster of post offices and sorts it onward. The last two digits of the five name the particular post office or delivery area that facility feeds. Each step down is a step closer to a carrier walking a route.

This is why a ZIP code is best understood as a set of delivery routes rather than a region. The routes happen to cover a patch of ground, so the result behaves like an area most of the time, but the boundary is wherever the routes end. A code can cross a county line, and in a handful of cases a state line, because the delivery that makes operational sense does not stop where an administrative border does.

The consequence for anyone building software is that ZIP codes make poor geography. The Census Bureau does not use them directly for statistics: it publishes ZIP Code Tabulation Areas, which are approximations of ZIP code extents built from census blocks, precisely because the real thing is not a clean polygon. Any product that treats a ZIP code as a reliable shape on a map is working with an approximation, whether or not it says so.

Codes also change. When a delivery area grows, splits or is reorganised, USPS can reassign it, and a code that was correct for an address a decade ago may not be correct today. An address, not a remembered number, is always the authority.

Not every code works the same way. A few categories behave differently enough to be worth knowing about.

Some codes serve PO boxes only, with no street delivery at all. An address in one of those is a box at a particular post office, which is why a parcel carrier that does not deliver to PO boxes will reject it.

Some are unique codes, assigned to a single organisation that receives enough mail to justify its own. A government department, a large university or a company that runs a reply-paid campaign may have one, and the code effectively names the recipient rather than an area.

Military and diplomatic mail uses APO, FPO and DPO addresses with their own codes, routed through the postal system to a military or diplomatic handover point rather than to a civilian delivery area. Mail to those addresses is addressed as though it were domestic, no matter where in the world the recipient is.