Why is a marathon exactly 42.195 km?

A marathon could easily have been 40 km. Instead, runners everywhere race 42.195 km, a distance so specific it looks engineered.

It wasn't. It is simply what one Olympic course in London measured in 1908.

A marathon wasn't always 42.195 km.

The first modern Olympic marathon, in Athens in 1896, was run over about 40 km and named after the legendary run from Marathon to Athens. Until 1908, Olympic marathons ranged from 40 to 42 km.

London started out the same way. The race agreed with the IOC was 25 miles, about 40 km.

Then London chose a very particular route.

The race would start in the grounds of Windsor Castle and finish in the new Olympic stadium at White City, in front of the Royal Box, where every race finished.

Two changes made it longer. Starting inside the castle grounds, for privacy, added about 700 yards (640 m). A detour around unsuitable final miles added roughly 1,000 yards (900 m) more. The organisers fixed the distance at exactly 26 miles to the stadium entrance.

Then the stadium supplied the last 385 yards.

The plan was for runners to come in beneath the Royal Box and run a full lap. Instead, their entrance moved to the opposite side, probably because the royal entrance had been fitted out for carriages, and they ran the track clockwise to the same finishing line: 385 yards (352 m). That is the stretch on which the leader, Dorando Pietri, collapsed, was helped over the line and was disqualified.

26 miles
On the road
Windsor Castle to the stadium entrance: 41.843 km
+ 385 yards
On the track
Part of a lap, to the finish in front of the Royal Box: 0.352 km
= 42.195 km
The 1908 marathon
Standardised in 1921; the Olympic distance since 1924

How the 1908 London course adds up. The odd decimal comes from converting miles and yards into kilometres.

The familiar version, which World Athletics also gives, says the start was moved so royal children could watch from the nursery. A 2008 study of the contemporary records found no evidence for that, and concluded that royal requests did not shape the distance.

The number stuck, eventually.

It wasn't adopted straight away. The International Amateur Athletic Federation, now World Athletics, standardised 42.195 km in 1921, and the Olympics have used it since 1924.

The next time someone says the royal family added those last 385 yards, tell them it was a change of stadium entrance. Had the runners come in under the Royal Box and run a full lap as planned, the race that set the standard would have been about 42.38 km.