A marathon is 42.195 km. The exact figure comes from the definition: one international mile is defined as exactly 1.609344 km, so 26.2 miles × 1.609344 = 42.164 km... which is the rounded "26.2" figure. The official distance uses the full 26 miles plus 385 yards: 26 × 1.609344 = 41.843 km, plus 385 yards × 0.0009144 km = 0.352 km, for a total of 42.195 km.
In practice, most people round to 42.2 km. The gap between 26.2 miles and 42.195 km is small, but it matters for race measurement, which is why official courses are certified by distance rather than by a simple conversion. The 26.2-mile figure itself dates from the 1908 London Olympics, when the course was adjusted to finish in front of the royal box; the distance was standardized by the IAAF (now World Athletics) in 1921.
If you'd like to check the definition, World Athletics publishes its road-racing rules, and the International Bureau of Weights and Measures documents the mile's exact kilometre equivalent.
