The standard distance of a marathon is 42.195 kilometres (26.2 miles). This is the official distance used for modern marathon races, including Olympic and World Athletics competitions.
Early Olympic marathons were not identical in length and were generally about 40 kilometres. The modern distance traces back to the 1908 London Olympics, where the course ran from Windsor Castle to the Olympic stadium, finishing in front of the royal box.
The distance was formally standardized in 1921. It is often rounded to 42.2 kilometres in casual conversation, but 42.195 kilometres is the precise official measurement. A half marathon is exactly half that distance: 21.0975 kilometres.