What ZIP code am I in?
Your position never leaves your device. The postal table is downloaded to your browser and the lookup happens there. Outside the United States, pick your country and it answers in that system.
Your code can only come from your position, so this needs your browser's location permission. The lookup then runs here, in this page — your coordinates are never sent anywhere.
How this works
- Your browser asks whether you want to share your position. Nothing happens until you say yes.
- The page downloads the slice of the postal table covering your latitude — tens of kilobytes, not a whole country.
- It finds the nearest centre point, and measures how much to trust that.
Your coordinates take part only in step three, which happens on your own machine. There is no request that carries them, so there is nothing for anyone to log.
Outside the United States
The same question has a different name in every country, and the same method answers it wherever the published data is good enough to check. Pick your country above; the page tells you which code system it is answering in, and how often that answer is right.
The United Kingdom is a special case worth stating plainly: the open data
carries only the outward code — the part before the space,
like SW1A. The full postcode belongs to Royal Mail and is not in
any source this page can use, so the page gives the outward code and says so
rather than implying it has the rest.
How accurate this is, honestly
None of these sources publishes boundaries in a form small enough to send to a browser. They publish centre points. So this finds the nearest centre — which is not quite the same question as which area you are standing in.
It would be the same question if postal areas were tidy circles around their centres. They are not: they are built from delivery routes, so they are long, thin and interlocking. Near an edge, the nearest centre can belong to the code next door.
So the page measures its own doubt and shows it, every time:
- How much closer the winner is than the runner-up. When a second code is almost as close, you are near a boundary and the page names the other one instead of hiding it.
- Whether the distance is plausible for that code's size. A point 40 km from the centre of an area the size of a neighbourhood is probably not in it, whatever the ranking says.
Every figure below was measured, not estimated, by
measure-postal-accuracy.mjs driving the code that runs on this
page. They are floors rather than scores: the test compares against a
deliberately crude notion of ground truth, so some of the "misses" are the
method being right about a point the test had already placed outside.
| Country | Answers in | Right about | From |
|---|---|---|---|
| United States | ZIP code | about 82% | US Census ZCTA gazetteer 2024, 33,791 areas |
| United Kingdom | postcode, outward part only | about 87% | GeoNames postal export 2026-09-18, 3,002 codes at 27,450 places |
| Australia | postcode | about 73% | GeoNames postal export 2026-09-18, 3,171 codes at 18,521 places |
The two are not measured the same way and should not be read as one column. For the United States, points are drawn inside each ZIP area's own published footprint. Elsewhere there is no published area, so a real locality is removed from the table and the page is asked what it says standing on it — which is the situation it actually faces, since nobody stands exactly on a gazetteer point.
Why the ZIP on your mail might not appear here
ZIP codes are delivery routes; the Census turns them into areas only where mail goes to streets. A ZIP assigned to one building or a bank of PO boxes has no area at all — 20500, the White House, is a real ZIP code that no area-based lookup can return. Stand next to one of those and you will get the ZIP of the surrounding neighbourhood, which is the best answer this kind of data can give and is not what is printed on the envelope.
Questions
Why does it need my location?
A postal code is a property of a place, and the only thing that knows your place is your device. Unlike your country, it cannot be inferred from your timezone. The permission is asked for when you press the button, never when the page loads.
Is my location sent anywhere?
No. The postal table for your latitude is downloaded to your browser and the comparison runs there. Your coordinates are never part of any request, so there is nothing for this site or anyone else to log.
Does this work outside the United States?
For the United Kingdom and Australia, yes — choose the country at the top and the page answers in that system, with its own measured accuracy. Elsewhere it says so rather than naming the nearest foreign code, which would be a real number and a meaningless answer. Countries are added only where the open data can be checked: where a source lists one point per code there is nothing to test the method against, and a figure nobody can verify is not worth printing.
Why does the UK answer stop before the space?
Because that is all the open data has. The full postcode is Royal Mail’s, and the freely licensed source carries only the outward code — the SW1A of SW1A 1AA. The page gives that and says what it is, rather than implying a precision it does not have.
Why is the code different from the one on my mail?
Two reasons. If you are near the edge of an area, the nearest centre point can belong to the code next door — the page tells you when you are in that situation and names the alternative. And some codes are assigned to a single building or a bank of PO boxes rather than to streets; those have no area in the data and cannot be returned by any area-based lookup.
How accurate is it?
About 82% for the United States, 87% for the United Kingdom and 73% for Australia — each measured by running the code on this page against a deliberately crude notion of ground truth, so each is a floor rather than a score. It is most reliable in the middle of a large area and least reliable in a dense downtown, where areas are small and packed together.