How the page finds your ZIP code
Your device reports its position, and the page asks OpenStreetMap for the nearest mapped address to that point. The postcode attached to that address is the ZIP code shown in the white shield. The city, county and state come from the same lookup.
Because it works from where you are rather than from something you type, it helps in the situations where you cannot look a ZIP code up the usual way: you are travelling, you have just moved, you are at a friend's house, or a website wants your ZIP code to show local results and you are not sure which one applies.
If you know the address, you can type it into the search box instead and get its ZIP code without sharing your location at all.
What a ZIP code actually is
A ZIP code is not an area on a map. The United States Postal Service created ZIP codes in 1963 to organise mail delivery, and each five-digit code is really a set of delivery routes served from one post office. The first digit is a broad region, from 0 in the Northeast to 9 on the West Coast, and the next two narrow it to a sorting centre.
Because they follow delivery routes, ZIP codes do not respect city, county or even state lines. One ZIP code can cover parts of several towns, and a single city can have dozens of them. Some ZIP codes belong to a single large building, a university or a company, and some cover only PO boxes.
The Census Bureau draws approximate areas called ZIP Code Tabulation Areas to report statistics, which is why ZIP code maps exist, but the post office's own definition is the list of addresses it delivers to.
When the ZIP code shown may not be yours for mail
Most of the time the ZIP code here matches the one on your mail. It can differ in three situations. First, near the edge of a ZIP code, where your device's position may land closer to an address that belongs to the neighbouring code. Second, where you receive mail at a PO box, which has its own ZIP code separate from the street. Third, where the mapped address data is missing a postcode, in which case the shield shows a dash.
For anything that has to match postal records exactly, such as shipping labels or official forms, confirm the address with the USPS ZIP Code lookup on usps.com. It also gives the full ZIP+4 code, the extra four digits that identify a specific block or building.
- Near a ZIP code boundary, check the street address the page shows. If the house number is wrong, the ZIP code may belong to the next street.
- For a PO box, use the ZIP code printed on your box rental, not your street's.
- For ZIP+4, use the USPS lookup; open map data does not include it.
Finding the county for a ZIP code
Since ZIP codes cross county lines, "what county is this ZIP code in" does not always have one answer. The county sign on this page is based on your actual position, not on the ZIP code, so it tells you which county you are standing in even where a ZIP code is split between two. The county page puts that answer first.