Convert mph to km/h

Miles per hour is the road speed of the United States and the United Kingdom, which is a rare pairing — the UK uses miles for road speed and metric for nearly everything else.

Kilometres per hour is the speed limit almost everywhere in the world. Dividing by 3.6 gives metres per second, because there are 3,600 seconds in an hour and 1,000 metres in a kilometre.

A speed is a distance over a time, and these two are counted over the same time. The hour on each side cancels, which leaves 1.609344 — the plain mi-to-km factor, with nothing added for the clock.

That makes this the one speed conversion you already know the number for: whatever you use to turn a road distance in mi into km works unchanged on a road speed, because the time unit never enters the arithmetic at all.

The mile per hour 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/h 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 values that make up an American road trip convert to nothing round at all. A 25 mph residential street is 40.2 km/h, a 35 mph arterial is 56.3, a 55 mph two-lane highway is 88.5, and interstates at 65, 70 and 75 mph are 104.6, 112.7 and 120.7 km/h. The highest posted limits in the country, 80 and 85 mph in parts of Texas, are 128.7 and 136.8 km/h.

The scatter is the tell that these numbers were set in miles and never reconsidered. A metric country builds its limits on 30, 50, 80, 100 and 130; the American set clusters at multiples of five miles an hour, which land wherever they land once converted. Canada sits between the two, having converted its signs to metric in 1977 and rounded to metric values rather than carrying the mile figures across.

Almost every American performance figure is quoted as 0 to 60 mph and almost every European one as 0 to 100 km/h, and the two are routinely compared as if they were interchangeable. They are not: 100 km/h is 62.14 mph, so the metric run continues for another two miles an hour after the American one has stopped.

The gap is small in seconds and not small in ranking. On a car that reaches 60 mph in six seconds the extra 2.14 mph costs roughly two to three tenths of a second, which is the difference between two cars in the same paragraph of a review. If you are comparing figures across the two conventions, the honest move is to find both numbers for one of the cars and use that as a calibration rather than converting the speed and pretending the test was the same.

The factor is 1.609344, and 1.6 is the version everybody carries. It is 0.58 per cent low, so 70 mph comes out as 112 km/h against a true 112.65 and 100 mph as 160 against 160.9. For judging whether a speed is fast, that is finished work.

For anything written down it is not. A specification that says a device works up to 45 mph and gets published as 72 km/h has lost half a kilometre an hour to the shortcut, and the reader has no way of telling whether the original was rounded too. Where the number is going into a document rather than a conversation, take the factor from the field above and round once, at the end.

Consecutive Fibonacci numbers approximate the mile-to-kilometre ratio closely enough to be useful, because the golden ratio is 1.618 and the factor is 1.609. So 5 mph is 8 km/h, 8 is 13, 13 is 21, 21 is 34, 34 is 55 and 55 mph is 89 km/h. Every one of those is right to within a kilometre an hour.

It holds because the ratio between successive terms converges to 1.618, which is 0.5 per cent above the true factor — an error in the opposite direction to the flat 1.6 shortcut and of about the same size. Between the two of them you can bracket any figure without a calculator: 1.6 is slightly low, Fibonacci slightly high, and the answer sits between.

Speed in miles per hour turns up on far more than road signs. American treadmills are calibrated in miles per hour, so a 6.0 setting is 9.66 km/h and a 7.5 is 12.07 — neither of which matches the round metric speeds a European runner trains at, and the machine will not always offer a unit switch. Consumer drones sold in the United States quote maximum speed and wind resistance in mph, boat manufacturers quote small powerboats the same way, and American mower and e-bike specifications follow.

The practical answer is to convert once and write the metric figure on the machine rather than reconverting every session. A treadmill programme built around 10, 12 and 14 km/h corresponds to 6.2, 7.5 and 8.7 mph, and setting those to the nearest tenth is closer than the calibration of the belt itself.

The United Kingdom is the case that makes this conversion feel arbitrary, because it is arbitrary. Metrication began in 1965 and reached fuel, food, building materials and medicine, but road signs were left in miles and yards and have stayed there. A British car is designed in millimetres, filled in litres, and driven against a speedometer in miles per hour.

That leaves two large mph countries rather than one, and they do not share the same numbers. British limits are 20, 30, 40, 50, 60 and 70 mph — 32, 48, 64, 80, 97 and 113 km/h — with 70 mph as the motorway maximum, which is lower than the 120 or 130 km/h a continental driver is used to. Converting the unit tells you the number; it does not tell you that the ceiling is genuinely lower.

Almost every headline speed figure of the last century was set in miles per hour and is repeated in that unit regardless of where it is being read. The land speed record of 763 mph is 1,228 km/h. A production car advertised at 200 mph is doing 322. An American drag strip quotes trap speed in miles per hour and always has.

Converting them is the only way to compare against anything metric, and the round-number effect is worth noticing when you do. A car marketed on "200 mph" was engineered to a target in that unit, and 322 km/h is a number nobody set out to reach; a European car marketed on "300 km/h" is 186 mph, which is why the two lists never line up. The roundness follows the market the car was aimed at rather than anything about the machine.

Going into kilometres per hour the figure always gets bigger, by roughly three fifths again. 60 becomes 97, 100 becomes 161. An answer that came out smaller than the number you typed is a direction error rather than a rounding one.

The other check worth doing is on the source. A speed described as "miles per hour" from a marine or aviation document may be knots, which are a different mile entirely and 15 per cent faster; a figure from a US highway sign never is. Where the origin is a boat, an aircraft or a wind forecast, confirm the unit before converting it.