What is the official marathon distance in kilometres?

The official marathon distance is 42.195 kilometres.

This distance equals 26 miles and 385 yards, the measurement standardized after the 1908 Olympic marathon in London. The race began at Windsor Castle and finished in front of the royal box at White City Stadium, producing the distance later adopted internationally.

A marathon is therefore slightly longer than 42 kilometres. Road races may use different local courses, but certified marathon courses are measured to the official distance, with additional tolerance allowed for measurement accuracy.

The marathon should not be confused with the half marathon, which covers 21.0975 kilometres. The unusual 42.195-kilometre figure comes from a particular Olympic route rather than an ancient Greek measurement.