Miles to kilometers is the conversion most people reach for coming from a US or UK context — driving distances, road signs, and race distances quoted in miles that need translating for a metric audience. A half marathon, commonly described as “13.1 miles,” converts to 21.0824 km — close to, but not identical to, the official half-marathon distance of 21.0975 km, because “13.1” is itself a rounded figure runners use for convenience.
That gap matters when precision counts: “26.2 miles,” the familiar marathon shorthand, converts to about 42.165 km, while the marathon’s actual fixed distance is 42.195 km. The difference comes entirely from rounding 26.2 miles to one decimal place before converting — the true marathon distance in miles is 26.2188, not a clean 26.2.
For anything other than a fixed one-off distance — converting a running pace between minutes-per-mile and minutes-per-kilometer, for instance — use the running pace calculator, since pace conversion involves time as well as distance.