ZIP+4 Codes are the last 4 digits of a nine-digit ZIP Code. A complete, nine-digit ZIP Code consists of two parts. The first five digits indicate the destination post office or delivery area. The last 4 digits represents a specific delivery route within that overall delivery area. All 9 digits of a full zip code assist the USPS in effectively sorting the mail.
(Want to find your own ZIP+4 code?)
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 divide the country into different "zones" 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.
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 gameplan 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.
Using a complete 9-digit ZIP Code ensures the fastest, most accurate mailing possible. They're only provided for an address once it has been validated and proven real. These codes indicate a specific delivery route, meaning the actual path the mail truck would travel in a single drop-off. Usually this comprises ten to twenty homes or locations. ZIP+4 Codes are also assigned to PO Boxes. Typically, each PO Box gets its own +4 Code, which often matches the box number.
Because they are based on delivery routes instead of more permanent boundaries, the last 4 digits of a complete ZIP Code can change often. Five-digit ZIP Codes also change, but they do so infrequently; it's a lot less likely that you will be living in a ZIP Code when it changes. Not so for ZIP+4. The +4 on a ZIP Code can be changed as frequently as once a month, based on things like how many postal employees are working, or who is working what route, etc.
There are a number of reasons that ZIP+4 Codes are good omens for your shipping. For one, ZIP+4 Codes require validation; that means you know for sure an address is real if it has a ZIP+4 attached to it. For another thing, they can help get you those bulk mailing discounts.
Another important benefit to using ZIP+4 codes is delivery speed. Using ZIP+4 on your mail can speed up processing and delivery, sometimes by as much as two days. That's right, your mail can show up faster if you label things right. Bet you're scrambling for those codes now, huh?
Three things will be on the quiz: (1) ZIP+4 Codes indicate delivery routes, (2) using ZIP+4 Codes gets your mail there faster and more accurately, and (3) we at SmartyStreets can get you those codes. Our blindingly fast USPS address verification API provides the appropriate ZIP+4 Code for every address we process. Go ahead and try it out. Or you could call us instead if you'd rather talk to a real person. (We'd offer a fake person, but we don't have one of those on staff.) Either way, we can help you ZIP your address, and ZIP it good.