Kilometers to Miles

Road races worldwide are measured in kilometers, even in the United States, which is why runners convert so often. A 5K is 3.11 miles, a 10K is 6.21 miles and a half marathon of 21.0975 km is 13.11 miles. The marathon is 42.195 km, or 26.22 miles; the odd distance dates from the 1908 London Olympics, when the course ran from Windsor Castle to the stadium, and it became standard in 1921.

Track athletes have their own version. The 1,500 m is the metric mile, but it is only 0.932 miles, about 109 m short of a real mile. That gap is why world records exist separately for the 1,500 m and the mile.

The UK and the US are the large exceptions that still put miles on road signs; nearly every other country uses kilometers. Drivers renting a car abroad, or importing one, meet both: a dashboard reading 120 km/h is about 74.6 mph, and an odometer showing 50,000 km means 31,069 miles. Fuel economy flips too, since metric countries quote liters per 100 km rather than miles per gallon.

The mile is defined as exactly 1,609.344 m, a value fixed by the 1959 international yard and pound agreement. The kilometer is 1,000 meters, and the meter has been defined by the distance light travels in 1/299,792,458 of a second since 1983, so both units are exact. Only the 0.621371 factor is rounded.

A neat trick uses Fibonacci numbers, because their ratio is close to 1.609: 8 km is about 5 miles, 13 km about 8 miles, 21 km about 13 miles. Otherwise, multiply by 6 and divide by 10: 40 km × 0.6 = 24 miles, against the exact 24.85.

The usual mistake is multiplying by 1.6 instead of dividing. A mile is longer than a kilometer, so the mile figure must always be smaller. If 50 km came out as 80, you went the wrong way; that is the miles to kilometers direction. Nautical miles, used at sea and in aviation, are a different unit again; the unit converter handles them.