What distance became the standard Olympic marathon length after the 1908 London Olympics?

The standard Olympic marathon length became 42.195 kilometers after the 1908 London Olympics.

The marathon was originally based loosely on the ancient Greek story of a messenger running from Marathon to Athens, so early Olympic races did not use one fixed distance. The 1908 race in London was planned to run from Windsor Castle to the White City Stadium, a route of about 26 miles.

Organizers extended the start so the race began at Windsor Castle and finished in front of the royal box inside the stadium. The resulting distance was 26 miles and 385 yards, equivalent to 42.195 kilometers. That measurement later became the official marathon standard.

The distance was formally standardized internationally in the early 1920s. A marathon is therefore longer than the approximate historical journey between Marathon and Athens, and the exact Olympic distance comes from the London route rather than ancient Greek practice.