What marathon distance was standardized internationally in 1921?

The marathon distance standardized internationally in 1921 was 42.195 kilometres.

That distance equals 26 miles and 385 yards. It originated from the 1908 London Olympic marathon, which began at Windsor Castle and finished in front of the royal box at the White City Stadium. The route measured 26 miles and 385 yards, and that measurement later became the official standard.

Earlier Olympic marathons used varying distances, usually close to 40 kilometres. The International Amateur Athletic Federation, now World Athletics, formally standardized the distance in 1921.

The marathon is therefore longer than a simple 40-kilometre conversion might suggest. Modern road races marketed as marathons are expected to follow the 42.195-kilometre standard.