How Many Miles Is a 10K?

The answer

10 km = 6.2137 miles

The conversion is one division: 10 ÷ 1.609344.

Why 1.609344 is exact

It is tempting to treat this as an approximation that someone measured carefully. It is not. Under the 1959 international yard and pound agreement, the yard was defined as exactly 0.9144 metres, which makes the mile exactly 1,609.344 metres by definition.

That means the constant carries no measurement uncertainty. Any imprecision in a conversion comes from where you round the answer, never from the number you divided by.

The race distances

Worth having these to hand, because race entries and training plans routinely mix the two units:

5K — 3.1069 miles
10K — 6.2137 miles
Half marathon (21.0975 km) — 13.1094 miles
Marathon (42.195 km) — 26.2188 miles

The famous 26.2 is just that last number rounded to one decimal. The marathon distance was fixed at 42.195 km in 1921, taking its length from the 1908 London Olympic route rather than from any attempt at a round figure.

Turning distance into pace

The conversion becomes genuinely useful when you are planning a race, because pace targets are usually quoted per mile and course markers are usually per kilometre.

A 55 minute 10K works out at 5:30 per kilometre. In the other unit that is 8:51 per mile, or an average of 10.91 km/h — 6.78 mph.

The gap between those two per-unit numbers catches people out. 5:30 and 8:51 sound like different efforts, but they describe exactly the same run. If you train against mile splits and race against kilometre markers, converting the target once beforehand saves a lot of unhelpful mental arithmetic at 7 km.

The quick mental version

Multiply kilometres by 0.6 and add a small amount. For 10 km that gives 6, and the true answer is 6.21 — close enough to judge whether a route is the right length. For anything you are actually going to race, use the exact figure.