Convert ft to km

A foot is twelve inches, and it is the unit heights and room sizes are quoted in across the United States. Aviation quotes altitude in feet almost everywhere, including in countries that are metric for everything else; China is the exception, where controllers clear aircraft in 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 factor is 0.000305, and almost nobody carries that around. Rounded to 0.0003 it is off by 1.6 % — which stays invisible on small numbers and turns into a whole unit somewhere around 100 ft.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

The foot 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 0.0003048 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 two units are separated by more than three orders of magnitude, and the number to hold is 3,280.84 feet to the kilometre. The other landmark is more useful still: 5,280 feet is a mile, and a mile is 1.609344 km, so a figure in feet can be divided by 5,280 and multiplied by 1.6 as a check on whatever the calculator says.

Going the other way the factor is exact. A foot is 0.3048 m, so 0.0003048 km, with the zeros belonging to the definition rather than to a rounding. The asymmetry is worth noticing because it decides which direction is safe to round: a figure converted into kilometres and back into feet drifts, while one converted into feet and back into kilometres does not.

Cycling and hiking are where a figure in feet routinely has to become a metric one, because elevation gain is quoted in feet in the United States and in metres almost everywhere else. A ride with 8,000 feet of climbing has 2.438 km of ascent; a hundred-mile ride with 10,000 feet has 3.048 km. Those are the numbers a European rider will recognise, once they are written as 2,438 m and 3,048 m.

The complication is that neither figure is a measurement. Elevation gain is summed from thousands of small changes, and two devices covering the same route disagree by five to ten per cent depending on whether they use barometric or satellite altitude and how aggressively they filter noise. Converting to three decimal places of a kilometre implies a metre of accuracy on a number that is uncertain by hundreds.

Endurance sport has invented several targets that are pure accumulated vertical, and they sit on both sides of this conversion. The Vertical Kilometre is 1,000 m of ascent, which is 3,280.84 feet. Everesting is riding or running repeats of one hill until the total ascent reaches the height of Everest, 8,848 m or 29,029 feet. The Empire State Building Run-Up climbs 86 floors and about 320 m, or 1,050 feet.

Converting between them is the only way to compare an American total against a European one, and it is where this pair stops being an odd couple. Ten thousand feet of climbing is 3.048 km, so a little over three vertical kilometres; a 29,029-foot Everesting attempt is 8.848 km of vertical, which is more than the distance most people cycle in an hour, gained upward.

A conversion is only useful if the reader can hold the result, and a kilometre is too big a unit for most figures given in feet. Five hundred feet is 0.1524 km. A thousand is 0.3048. Two thousand is 0.6096. Every one of those answers begins with a zero and a decimal point, and every one of them is easier to read as metres: 152.4, 304.8, 609.6.

The threshold is 3,280.84 feet, the point where the kilometre answer passes one. Below it, convert to metres and say so; above it, kilometres start to carry meaning, and above about ten thousand feet they are clearly the better unit. This is a decision about the reader rather than about the arithmetic — the factor is the same either way, and only the placement of the decimal point changes.

The figures that arrive on this page are usually sums: an accumulated distance, a season of climbing, a dataset of segment lengths added together. A sum inherits the error of every term in it, and the errors do not cancel when they share a bias — a tracker that overreads every segment by half a per cent overreads the total by half a per cent, however many segments there are.

That makes the rounding advice unusual here. Convert the total, then round it to about three significant figures, because the fourth was never real. A season total of 284,617 feet becomes 86.7 km, not 86.7513. Where the total came from a device, quote the device and the total in the unit it produced alongside the converted figure, so the next person can see which number was measured and which was derived.

The United States kept two feet from 1959 until the end of 2022. The international foot is exactly 0.3048 m; the survey foot was 1200/3937 m, roughly 0.30480061 m, retained because a century of surveying, elevation benchmarks and state plane coordinate systems had been computed with it. The difference is two parts per million, which is nothing at room scale and is the whole reason this section is on this page rather than on one about small dimensions.

Over the distances converted here it becomes measurable. A hundred kilometres computed with the wrong foot is out by twenty centimetres; a state plane coordinate of a million feet is out by two feet, which is enough to move a boundary. The survey foot was retired on 31 December 2022, so any modern figure uses the international foot the calculator above assumes — but an older elevation, coordinate or legal description may not, and only its source can say.

A large share of the feet figures that need converting are rates rather than distances, and a rate carries a time unit that has to be converted too. Vertical speed is quoted in feet per minute across aviation, so a 1,000 ft/min climb is 304.8 m/min, which is 5.08 m/s or 18.3 km/h. A 500 ft/min descent is 2.54 m/s. The distance conversion alone gives the first number and stops one step short of the one anybody metric would use.

The step that gets skipped is the minute. Metric climb and descent rates are given in metres per second, not per minute, so dividing by sixty is part of the conversion rather than an optional tidy-up. The same trap appears in flow rates, feed rates and lift speeds: convert the length, convert the time, and check the result against something you already know the size of, because a factor-of-sixty error looks perfectly plausible on its own.