What is my postcode?

Why this is not your doorstep

A postcode is an area, and how large an area varies enormously by country. A Dutch postcode like 1011 PL covers one side of one street — often a handful of front doors. A British SW1A 1AA can be a single building. A US ZIP code covers a delivery route and can hold tens of thousands of people; a five-digit French or Spanish code often covers a whole small town.

So "what is my postcode" has a different meaning depending on where you are standing when you ask it, and a page that treats all of them as equally specific is misleading in half the world.

There is a second gap underneath that one. The postcode above did not come from your position directly — nothing broadcasts a postcode. It comes from a reverse geocoder: we hand it your coordinates and it returns the nearest thing in its records that has a postcode attached. That is all it does. It is not claiming the record it found is yours, because it cannot — it knows where its records are, not where you are. Stand in a yard, on a path, or at a set-back entrance and the nearest record may be a building across the road, in a different code.

What the accuracy figure covers

Beside the answer is a radius, in metres or kilometres. It is not decoration — it decides whether there is a postcode on this page at all.

Every position fix arrives with an accuracy figure, and that figure gates what we are willing to print. A fix good to tens of metres has earned a house number, and a postcode with it. A fix good to a few hundred metres has earned the street, and choosing the street deliberately drops the postcode — because a postcode identifies a block, and in the Netherlands nearly a doorstep, so keeping it would assert precision we have just admitted we do not have.

Beyond that, a fix is a neighbourhood or a rough area, and there is no honest postcode to give.

This is why the page sometimes shows no postcode at all while still knowing perfectly well roughly where you are. That is not a failure. It is the same answer any careful person would give: near enough here, and I am not going to guess which block.

When it is masked

Between those two states there is a third, and it looks like this: 941••.

When the fix is wide enough that the full code would overstate what we know, we keep the leading half — rounded up, and never fewer than two characters — and replace the rest with dots. 94103 becomes 941••. 1011 PL becomes 1011••. SW1A 1AA becomes SW1A••.

The dots are doing real work. They say there is more to this code and we are not confident enough to show it, which is a different statement from a short postcode, and a very different statement from a confident wrong one. The characters that remain are true; the ones removed were never known well enough to print.

If you want the argument for why an app should show you any of this rather than a clean, certain-looking answer, we wrote it down.