How long is a marathon, in kilometres?

How to get close to this number

Most people can recall the distance in miles even if the kilometre figure will not come. Converting is then straightforward, and it is a better route than trying to remember an awkward three-decimal number directly. Whenever a figure is famously precise, the reliable move is to recall the round version in the other unit and convert.

The precision is the whole story, and it is the bit people find hard to believe. A distance this specific looks like it must be derived from something — a Greek road, a measured route, an athletic standard — and it is not. As the explanation says, it comes from a course lengthened in 1908 to finish in front of a royal box, and that accident was frozen into the rulebook in 1921.

It is also a genuinely fixed number, which is rarer on this site than it sounds. Course measurement is now standardised and certified, so a marathon is the same distance in every city, and a record set on a short course simply does not count. That stability is why this is a fair thing to ask: the answer has been identical for over a century and will be identical in a century more.