What is my ZIP Code?

Zip Code Search (Lookup) provides correct and detailed information about your city, area, county or state zip codes.

Knowing what the last four digits of a ZIP Code are all about requires knowing what ZIP Codes themselves are all about.


The Zone Improvement Plan was something that the USPS came up with to make it easier to ship letters and packages across the country. It helped “zone” the country according to how mail was distributed, which accelerated sorting and delivery.

As the US population has increased and scattered, it’s been necessary to expand on the system, to make room for everybody and their dog. That’s where the “+4” part comes in. We’re getting ahead of ourselves, though, so let’s start with the basics.

Five-Digit ZIP Code

These are the codes you’re familiar with. They look like this…


…and most commonly indicate a destination post office.

Here’s why:If you’re mailing a letter from Boston to Seattle, the mail carrier in Massachusetts doesn’t really care what the street address of the destination is. It’s what you might call “outside his jurisdiction.” He just needs to know which mail carrier to send it to so the other carrier can get it to that address.

A postal worker can only cover so much ground on a given day. And since the USPS has a standard of delivering in rain, sleet or snow, that rules out the possibility of doing the service of delivering in stages (some today, some tomorrow). That means that any given post office is only servicing what it can reach in a day. ZIP Codes reflect that.

Typically, a ZIP Code is tied to a post office; by that we mean, every one of the latter has one of the former.

You might think of ZIP Codes as a mailing address for a particular post office. Some do handle more than one ZIP Code, but a one-on-one game plan is the norm.

It's very important to note that ZIP Codes aren’t “boundaries.” They’re a collection of delivery routes. They don’t follow geographic or administrative boundary lines; they can cross city, county, even state lines. They follow where the delivery trucks go.

Some ZIP Codes are special cases. Among them are “military” ZIP Codes, which include everything from military bases (domestic or otherwise) to battleships at sea. Then there are “unique” codes.

Businesses and organizations sometimes get their own ZIP Codes, due to the volume of mail they send and receive.

These are frequently benefiting from bulk mailing discounts, since the organization usually has a mail department that (1) presorts mail before giving it to the USPS, and (2) distributes mail internally so the USPS doesn’t have to.

Like standard ZIP Codes, “military” and “unique” ZIP Codes circumscribe their own delivery area.

ZIP+4 Codes