Convert m to km

The metre has been defined since 1983 as the distance light travels in 1/299,792,458 of a second. It is not a physical object any more; the platinum bar in Paris was retired.

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.

Going from metres to kilometres moves the decimal point 3 places to the left and changes nothing else. There is no factor to remember and no rounding to decide: the digits stay in the same order and only their position changes.

1,234 m is 1.234 km — same digits, moved along. That is worth knowing because it is the one kind of conversion you can check at a glance: if the digits of the answer are not the digits you started with, something other than the conversion has happened to them.

Move the decimal three places and the number changes name as well as unit. Athletics fixed its distances as whole metres when the oval track was standardised, so the events are the 800, the 1,500, the 5,000 and the 10,000. Road running inherited the kilometre from the map, so the same lengths appear as the 5 km, the 10 km and the half marathon.

The distinction is not pedantry, because the two are governed differently. A track race is run on a surface measured to the centimetre and unchanged from meeting to meeting, while a road course is certified once and shaped by whatever streets were available. Records are kept in separate lists, and a personal best over 10,000 m says something different from one over 10 km.

A standard outdoor track is 400 m in lane one, measured along a line 30 cm out from the inside kerb because that is where a runner's path actually falls. Every metric distance then resolves into laps: 5,000 m is twelve and a half, 10,000 m is twenty-five, 1,500 m is three and three quarters and starts on the back straight to make the finish line come out right.

The mile is the odd one out and shows the seam between the systems. At 1,609.344 m it is four laps plus a little over nine metres, which is why a mile race on a track starts from a line marked slightly behind the 1,500 m start. The awkward figure is the giveaway that the imperial distance came first and the metric track was built around it afterwards.

A marathon converts to 42,195 m and the untidiness has a cause: the distance is not metric in origin. It was fixed at 26 miles and 385 yards, the length run in London in 1908, and standardised in that form in 1921. Every metric statement of it since has been a translation, which is why it lands on three decimal places in kilometres and on a five-digit whole number in metres.

The half inherits the problem and makes it worse, since half of 42.195 is 21.0975 km, a figure with four decimals that every race rounds off in its own way. Converting to metres gives 21,097.5 m and a half metre that no course can be measured to. The right response is to accept the distance as a name rather than a number and to round when quoting it.

A watch reports metres and totals them into kilometres, and the total is nearly always longer than the course. The device samples a position at intervals and joins the samples with straight lines, which cuts corners on tight bends and adds length on gentle ones, and it drifts among tall buildings and under tree cover where the signal reflects before it arrives.

A certified road course, by contrast, is measured along the shortest route a runner could legally take, hugging every bend, and course measurers add a small deliberate margin so that a course is never accidentally short. The two methods disagree by design. A trace reading a couple of hundred metres over the advertised distance is a normal result, not evidence about the course.

Some figures lose meaning the moment they become kilometres. A swimming pool is 50 m or 25 m, a rowing course is 2,000 m, a sprint is 100 m, and writing any of those as a fraction of a kilometre produces a decimal nobody in the sport would recognise. Below about a kilometre the metre figure is the name of the thing.

The same applies to anything vertical. Summits, contour intervals and elevation gain stay in metres however large they get, partly because the numbers rarely exceed a few thousand and partly because a climb expressed in kilometres invites comparison with horizontal distance, which is misleading — a kilometre of ascent and a kilometre along a road are not remotely the same amount of work.

Fitness applications record in metres and display in kilometres, and the rounding happens at the display. Six separate outings of 8,400 m each total 50,400 m, which is 50.4 km, but six figures each rounded to 8.4 km on screen total 50.4 km only by luck — round each to 8 km and the week comes out at 48. Sum the raw metres and convert once at the end.

The same discipline applies to any total assembled from legs. Convert at the point where the number is read by a person, not at each intermediate step, because every rounding is a small bias and repeated rounding in the same direction accumulates. One conversion at the end carries all the precision the original measurements had and none of the noise.

Distance signage on a metric road network is given in kilometres and, close to a junction, in hundreds of metres, and it is never given as a plain metre figure. The reason is reading time: a driver has a second or two to take in a sign, and a four-digit number is slower to read and easier to misread than a one- or two-digit one with a decimal.

The figures are also rounded, which converting quietly conceals. A sign reading 5 km means somewhere close to five kilometres and is accurate to a few hundred metres at best, so restating it as 5,000 m turns a rounded approximation into a figure that looks surveyed. If a metre-level distance to a junction is genuinely needed, it comes from the route data rather than from the sign.

A route profile is the place both units appear on the same chart, and they are doing different jobs. The horizontal axis is kilometres because the distances run to tens; the vertical axis is metres because the climbs run to hundreds. Anybody reading the chart is converting between them continuously without noticing.

That mixture is also where gradients become legible. A climb of 300 m over 6 km is an average of five per cent, and the arithmetic only works if the ascent is in metres and the distance in kilometres is multiplied out to metres first. Getting the units aligned before dividing is the whole of the calculation; the rest is reading the shape of the line.