How this instrument works
The kilometre arrived with a planetary definition attached. In 1791 the French Academy of Sciences set the metre at one ten-millionth of a quadrant of Earth's meridian, measured from North Pole to equator through Paris, which made that quadrant almost exactly 10,000 km. A survey between Dunkirk and Barcelona pinned that value down, and got it slightly wrong. No survey is needed now: since 1983 a metre is how far light travels in vacuum in 1⁄299,792,458 second, so a kilometre is light's path in about 3.34 microseconds.
A mile is older and considerably messier. Roman soldiers counted mille passus, a thousand double paces, roughly 1,479 m. English farming then bent it out of shape: a furlong was one furrow-length of 220 yards, and Elizabeth I's statute of 1593 set a mile at eight furlongs, or 1,760 yards, so that field measure and road measure would finally agree. That awkward 1,760 survived every later reform.
Both units met in 1959, when standards bodies across six English-speaking countries fixed the international yard at 0.9144 m. Multiply by 1,760 and a statute mile becomes 1609.344 m, exact by decree rather than by measurement. Divide 1,000 by that number and you have this page's factor, 15625⁄25146, a fraction whose decimal form never settles. Four national road networks still sign distances in miles — America, Britain, Liberia and Myanmar — while everyone else drives in kilometres, which puts this particular conversion on car dashboards, race entry forms and airline route maps more often than almost any other.
- Type your distance into the Kilometres (km) field; it opens at 5 km, the standard parkrun lap.
- Read the answer on the Miles (mi) line, which recalculates on every keystroke with no button to press.
- Keep one decimal for road distances, three for running splits, and all twelve if you are checking a printed table.
- Need miles turned into kilometres instead? Multiply your mileage by 1.609344 — that direction is exact and stops.
Worked example — a 5 km parkrun in miles
You have entered a Saturday parkrun and want to know what it amounts to in miles. Put 5 into Kilometres (km) and the Miles (mi) line reads 3.10685596119. Organisers advertise that as a 3.1-mile run, and that shorthand is honest enough: lopping off the tail discards about 11 metres, roughly three seconds of running at club pace.
The same factor scales without argument. A 10 km race is 6.21371 miles, and a 42.195 km marathon works out at 26.218758 miles, which unpacks as 26 miles and 385 yards. That peculiar imperial figure came first: our metric marathon distance is a rounding of an old 1908 London course, not vice versa.
Questions
Is 0.621371192237 an exact factor?
No — it is a twelve-digit rounding of a number that never ends. One kilometre is 1,000 ÷ 1609.344 miles, or the fraction 15625⁄25146, whose digits do repeat but only on a cycle far too long to write out. What is exact is its parent definition: 1 mi = 1609.344 m. The rounding used here is faithful to about one part in a trillion, which leaves the error on a 100 km drive smaller than a micrometre.
Why do miles-to-kilometres terminate when kilometres-to-miles does not?
Because that definition points one way. The 1959 agreement defined a mile in metres — 1609.344 of them — so multiplying miles by 1.609344 merely restates that definition and stops after six decimals. Travelling backwards means dividing by 1609.344, and its reduced denominator 25146 carries prime factors 3, 11 and 127 alongside its 2. Any fraction with primes other than 2 and 5 downstairs refuses to become a finite decimal. Identical relationship, different arithmetic.
Is an American mile the same as a British one?
Yes, and has been since 1959 — both are 1609.344 m, with no daylight between them at any decimal place. Imperial and US customary part company over volume rather than length: their gallons differ by about 20%, their miles not at all. One footnote for surveyors. American plane coordinate work long used a mile built on the US survey foot, 1609.347219 m, some 3.2 mm longer per mile and metres adrift across a whole state grid. NIST and NOAA retired it on 31 December 2022.
Does the Fibonacci trick for kilometres and miles really work?
Surprisingly well. Consecutive Fibonacci numbers approach a ratio of 1.6180, and a mile is 1.609344 km, roughly half a percent apart. So 5 km sits near 3 miles, 8 km near 5, 13 km near 8, 21 km near 13. The approximation drifts high as figures grow, because the golden ratio overshoots: 89 km supposedly gives 55 miles when the honest answer is 55.30. Good enough for reading road signs, not for fuel calculations.
Can I use this page for km/h and mph?
Yes. Dividing both sides by an hour leaves that ratio untouched, so type the speed figure into Kilometres (km) and read the result as miles per hour. A 100 km/h motorway limit is 62.14 mph, 120 km/h is 74.56 mph, and a 30 km/h urban zone is 18.64 mph. The same shortcut fails for fuel economy: litres per 100 km and miles per gallon are reciprocals of each other, so they need their own calculation rather than this factor.
Is a nautical mile the same thing?
No. A nautical mile is exactly 1852 m, picked to approximate one minute of latitude, and it governs sea and air navigation rather than road distance. One kilometre is 0.539957 nautical miles, so a statute-mile figure from this page runs about 15% larger than nautical figures for that same journey. Charts and marine plotters usually mark NM or nmi to keep them apart.