Km to Miles Converter

Why a nautical mile is a different length on purpose

Cut the Earth through both poles and you get a circle. Divide that circle the way navigators have divided circles since Babylon — 360 degrees, each of 60 minutes — and you get 21,600 minutes of arc around the planet. One of those minutes of latitude is one nautical mile. That is the whole definition, and it is the reason the unit exists.

What it buys you is that a chart needs no distance scale. The latitude markings printed up the side of any sea chart are the distance scale: step off a run of the course against them with a pair of dividers, count the minutes, and you have your distance in nautical miles. One degree of latitude is sixty nautical miles, anywhere on Earth. No other unit does that.

The Earth is not a perfect sphere, so a minute of latitude is not perfectly constant — it runs from about 1,843 m at the equator to about 1,862 m at the poles. In 1929 an international hydrographic conference in Monaco settled on a single figure of exactly 1,852 metres, which is close to the average, and that is the definition in force today. A knot is one nautical mile per hour, which is why aircraft and ships use knots and cars do not.

Where the ordinary mile comes from, and why it is 5,280 feet

The Roman mille passus was a thousand paces, counting a pace as two steps, and came to about 1,479 m. England inherited it and then spent centuries failing to make it agree with the furlong — the "furrow long", the length of a ploughed field, fixed at 220 yards and baked into every land record in the country. When Parliament finally legislated the mile in 1593 it chose to keep the farmland and bend the mile: eight furlongs, 1,760 yards, 5,280 feet. The mile is an awkward number because changing it would have meant re-measuring England.

The last loose end closed in 1959, when the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed to define the yard as exactly 0.9144 metres. That single line makes the foot exactly 0.3048 m and the mile exactly 1,609.344 m. Before it, the US and Imperial miles differed by about two parts per million. The one survivor of that old divergence, the US survey foot, was retired for new work at the end of 2022; it differs from the international foot by about 3 mm in a mile, which matters to a surveyor and to nobody else.

The mistake almost everyone makes

A 5K is not five miles. It is 3.107 miles. A 10K is not ten miles, it is 6.214. The mistake runs the other way on a rental car, where a dashboard reading 100 is 62 mph if the car is metric and 161 km/h if it is not. If you are driving somewhere new, learn two numbers and you can do the rest in your head: 100 km/h is 62 mph, and 60 mph is 97 km/h.

The race distances, and where the odd ones came from

• The marathon is 42.195 km because the 1908 London Olympic course ran 26 miles 385 yards, from Windsor Castle to a finish in front of the royal box at the White City stadium. It was one course, on one day. The IAAF adopted it as the standard distance in 1921 and it has been stuck there ever since.
• A half marathon is 21.0975 km, which is 13.109 miles — hence "13.1" on the sticker, and hence the fact that a half marathon is not a round number in either system.
• 5K and 10K are metric all the way down. If a race is advertised in miles it is a different race: a 5-mile race is 8.047 km, well over a 5K, and people do get caught out by that.

The conversions worth committing to memory

Kilometres to miles, multiply by 0.62 — or take five-eighths, which is 0.625 and accurate to within half a percent. Miles to kilometres, multiply by 1.6. The elegant version is the Fibonacci trick: consecutive Fibonacci numbers approximate the ratio, so 5 km is about 3 miles, 8 km is about 5, 13 km is about 8, 21 km is about 13, 34 km is about 21. That is not numerology — the golden ratio is 1.618 and a mile is 1.609 km, so the two happen to agree to within half a percent.