Convert mi to km

A mile is 1,760 yards — a number that comes from a Roman thousand paces, adjusted by an Elizabethan statute to fit the furlong. It is exactly 1,609.344 metres.

Kilometres are the road distance nearly everywhere outside the United States and the United Kingdom, which is why converting them is usually about reading a map or a race entry.

The mile has been defined in metric terms since the international yard and pound agreement of 1959, and in this page's units that comes out at 1.609344 km to one. The definition is exact by stipulation — it was not measured, and it cannot be refined by measuring it better. The decimal above is the whole of it, with nothing rounded away at the end.

Before that agreement the US and the British versions differed slightly, and old survey figures still carry the older definition. For anything outside land surveying the difference is far below the precision anybody is working to.

The mile is not measured against the metre; it is built from the inch. Since 1959 an inch has been exactly 25.4 millimetres, a foot exactly twelve of those, a yard exactly three feet, and a mile exactly 1,760 yards. Multiply the chain out and you get 1,609.344 metres with nothing trailing — a definition arrived at by stipulation, not a constant somebody refined with better instruments.

That is worth knowing because it settles a question this conversion invites. There is no more accurate value waiting to be discovered, and a converter showing 1.60934 or 1.6093 has rounded rather than disagreed. The digits stop after six decimal places in kilometres, which is a millimetre, and no measurement of any real mile has ever been that good.

Road running is the commonest reason an American number has to become a metric one, because the events themselves are metric almost everywhere. A 5K is 3.107 miles and a 10K is 6.214 miles; a half marathon is 21.0975 km, which is 13.11 miles; a marathon is 42.195 km. A training log kept in miles has to be converted at the entry form and then again at every kilometre marker on the course.

The direction matters for pace. Somebody who runs eight-minute miles is running 4:58 per kilometre, and the split boards on a European course will show kilometre times, not mile times. A runner expecting eight-minute miles who sees 4:58 at the first marker and takes it for a mile split concludes they have gone out far too fast, and slows down to correct a mistake they did not make.

Rounding 1.609344 to 1.6 is a good deal for short distances. A three-mile commute becomes 4.8 km against a true 4.83, and the twenty-eight metres of error is smaller than the difference between two routes through the same junction. Up to about ten miles the shortcut is invisible against how imprecisely the original distance was known.

It stops being free at race and route scale. A marathon converted with 1.6 comes out 245 metres short, which is more than half a lap of a track; a hundred-mile route loses nearly a kilometre. The rule that survives is to use the shortcut when the figure was itself approximate and the exact factor when the figure was a definition — a marathon, a course length, a legal boundary.

Road distance in miles now survives in the United States and the United Kingdom and in very few other places. British road signs give distances in miles and speed limits in mph while everything else in the country went metric decades ago, which is why a UK driver buys fuel in litres, checks tyre pressures in bar or psi depending on the gauge, and measures the journey in miles.

American roads take it further: mileposts run continuously across a state and exit numbers in most states are the milepost, so exit 137 sits 137 miles into the route. Converted to kilometres, those become 220.5 — a number that carries no meaning to anybody on the ground. Convert the distance you are travelling, not the sign number, because the sign number is an address rather than a measurement.

The nautical mile is exactly 1,852 metres, about fifteen per cent longer, and it is what aviation and shipping mean. It was chosen to be one minute of latitude, which makes sixty of them one degree on any chart, and a speed in knots is one nautical mile per hour. A flight distance quoted in miles is usually nautical, and putting it through the factor above understates it by more than a tenth.

A Scandinavian mil is ten kilometres and is still used in conversation in Sweden and Norway, so a Swedish colleague saying the drive is two miles means twenty kilometres rather than three. The Roman mille passus, a thousand paces, is where the word came from at roughly 1,480 metres; the modern figure is the result of an Elizabethan statute fitting the mile to eight furlongs of forty rods instead.

The factor is the same and the sentence around it is not. Seventy miles per hour is 112.65 km/h, and the conversion works because the hour on both sides cancels — but the useful figures are the posted limits rather than the arithmetic. A 70 mph motorway limit sits between the 110 and 120 km/h that most of Europe posts, and a 30 mph urban limit is 48 km/h against a typical 50.

Where this goes wrong is fuel economy, which is a rate with distance in the wrong place. Miles per gallon does not convert into litres per hundred kilometres by multiplying: 30 mpg (US) is 7.84 L/100 km, and the two scales run in opposite directions, so a bigger number is better on one and worse on the other. Convert the distance and the volume separately, then divide.

The factor is exact, which tempts a converter into printing 8.04672 km for five miles. That answer claims a precision of a centimetre for a distance that was almost certainly read off an odometer with a tenth-of-a-mile resolution, or estimated from a map. Carry the digits through the arithmetic and cut them at the end, at whatever the original figure honestly held.

A practical rule: an odometer reading is good to about a tenth of a mile, so quote the kilometres to a tenth as well. A surveyed course length or a legal distance is exact by definition and can take every digit. A distance somebody paced or estimated should be rounded harder in kilometres than it was in miles, because the metric figure looks more precise for the same information.