What ZIP code am I in?

Why the address comes first

ZIP areas vary a lot in size. A dense city ZCTA can cover well under a square mile, while a rural one can span hundreds. Near the edge of a large ZCTA, the internal point of a small neighbor can be closer than the internal point of the area you are actually in, so the nearest centroid alone can name the wrong ZIP.

A street address carries the ZIP the Postal Service delivers to, so the tool uses it whenever one is close by. The centroid answer only appears in fields, forests and open water, and is labeled as such.

How to use it

1. Tap Use GPS for your current position, or search an address, or click the map.
2. The tool asks the geocoder for the nearest street address and reads its ZIP code.
3. Read the card: the ZIP code, the address it came from, the matching Census ZCTA with its land area, and the method used.
4. Far from any address the card shows the nearest ZCTA centroid instead, with its distance. Move the point to a building to get the postal ZIP.

ZIP codes and ZCTAs are different things

A ZIP code is a set of delivery routes that the Postal Service can change at any time. It has no official boundary. The Census Bureau builds ZIP Code Tabulation Areas from census blocks: each block gets the ZIP code most common among its addresses, and blocks with the same code form a ZCTA.

Most ZCTA codes match a ZIP code, but not every ZIP has a ZCTA. Codes used only for PO boxes or single large mail recipients have no area of their own. That is why this tool can never return a PO box ZIP.

How the ZIP is found

Your point goes to the Photon geocoder, which returns the nearest OpenStreetMap address and its postcode. The tool then looks that code up in a file with one row per ZCTA (code, internal point and land area) to add the land area, or notes that the ZIP has no ZCTA, as with PO box and single-recipient ZIPs.

If there is no address nearby or the geocoder is unavailable, the tool measures the great-circle distance to every ZCTA internal point and keeps the smallest. ZCTA polygons are not shipped, because the national set is large.

When to use it and when not to

It works well for a quick answer in town, for filling a form field when you are unsure, or for checking which ZCTA a survey point falls near. For mailing, use the Postal Service's own lookup with a full street address.

To list every ZCTA around a point, use ZIP codes within a radius. To measure between two codes, use the ZIP code distance calculator.