Convert miles to kilometers

One mile equals exactly 1.609344 kilometers. A quick estimate: add 60 percent. Sixty miles is about 96 km, and the real answer is 96.6.

Consecutive Fibonacci numbers convert miles to kilometers with under 0.5 percent error. The sequence runs 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 — each number is the sum of the two before it. Take any number in it as miles and the next one is the kilometers: 5 miles is about 8 km, 8 miles is about 13 km, 55 mph is about 89 km/h.

It works because the ratio between consecutive Fibonacci numbers converges on the golden ratio, about 1.618, and the mile-to-kilometer factor is 1.609. The two agree to within half a percent by coincidence, not design — one comes from Roman marching and the other from the size of the Earth.

From mille passus, Latin for a thousand paces, where a pace was two steps — left foot to left foot. The Roman mile ran about 1,480 m. England later fixed the statute mile at 5,280 feet under Elizabeth I, a number that looks arbitrary because it is: it was chosen to keep the mile compatible with the furlong used in farming.

Nautical miles. A nautical mile is exactly 1,852 meters, about 15 percent longer than a statute mile, and it is not a rounding of anything — it is one minute of arc of latitude, tied to the size of the planet. Aviation and shipping use it, and a speed in knots means nautical miles per hour. Converting a boat's speed as if knots were mph understates it by 15 percent. Speed in knots follows from it — see mph to km/h.

The statute mile is 5,280 feet, which NIST Special Publication 811, the US guide to the SI gives as exactly 1.609344 km. The United Kingdom defines the same mile, as 1,760 yards, in its Weights and Measures Act 1985. The nautical mile is a separate unit of exactly 1,852 m, adopted at the International Hydrographic Conference in Monaco in 1929.