Convert km to ft

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.

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.

The factor is 3280.839895, and almost nobody carries that around. Rounded to 3300 it is off by 0.58 % — which stays invisible on small numbers and turns into a whole unit somewhere around 1,000 km.

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 single pairing worth memorising for this conversion is 3 km to 10,000 ft. The true figure is 9,842.5 ft, so the approximation is 1.6 per cent low, and it scales: 6 km is about 20,000 ft, 9 km about 30,000, 12 km about 40,000. Every one of those is 1.6 per cent light, which at cruising altitude is roughly 600 ft — meaningful to a controller and irrelevant to a passenger.

For a better estimate, multiply the kilometres by three and add ten per cent. 11 km gives 33 plus 3.3, so 36,300 ft against a true 36,089. That correction takes the error from 1.6 per cent down to about 0.6, and it is still one multiplication and one shift of the decimal point.

Vertical separation between aircraft is the one measurement where every country has to agree, because the aircraft are in each other's airspace. Feet won that agreement and never lost it, so a flight level is a pressure altitude in hundreds of feet — FL350 means 35,000 ft — regardless of whose airspace it is in. The ground below is surveyed and mapped in metres almost everywhere, and the aircraft above it is not.

The exceptions are narrow and worth knowing. China assigns cruising levels in metres, so an aircraft crossing into Chinese airspace changes the units it reports in as well as the numbers. Russia used metric levels until 2011 and now uses feet above the transition. Everywhere else, an altitude quoted in kilometres has been converted by somebody for a lay audience, and the original was in feet.

Mountains are surveyed in metres and described in feet by roughly half their readership. Mont Blanc at 4,808 m is 15,774 ft; Kilimanjaro at 5,895 m is 19,341 ft; Denali at 6,190 m is 20,310 ft — and the last of those is the interesting one, because 20,310 ft is the primary figure and 6,190 m is the conversion, rounded. The direction of translation changes with the country that did the survey.

The thresholds climbers use are set in whichever system the culture used first, which is why they never line up. The Colorado "fourteeners" start at 14,000 ft, which is 4,267 m and nothing in particular. The Alpine four-thousanders start at 4,000 m, which is 13,123 ft. The death zone begins at 8,000 m, or 26,247 ft. Each threshold is round in exactly one system and arbitrary in the other.

European resorts publish base and summit altitude in metres and North American ones in feet, so comparing them means converting one side. A resort with a 1.5 km vertical drop has 4,921 ft of it, which puts it among the largest in North America; a 900 m drop is 2,953 ft, which is respectable in the Alps and unremarkable in the Rockies. The conversion is where the comparison actually happens.

Altitude sickness thresholds are the part of this that is not a marketing number. Symptoms become common somewhere above 2.5 km, which is 8,202 ft, and most people notice something by 3 km — the 10,000 ft that North American resorts print on their trail maps. A European figure in metres and an American one in feet are describing the same physiology, and the conversion is the only way to see that.

Everest is the standard demonstration. The 2020 joint Nepali and Chinese survey gives 8,848.86 m, which converts to 29,031.7 ft; the previous 8,848 m gives 29,029 ft. The two differ by three feet, and that difference is a genuine change in the measurement rather than a change in the arithmetic. Quoting 29,031.69 ft implies a centimetre of confidence in a summit made of snow.

Aviation numbers have the opposite problem: they are exact as levels and approximate as heights. FL350 is defined as 35,000 ft of pressure altitude, not 35,000 ft above the ground or the sea, and on a cold day the aircraft is genuinely lower than the number. Converting 35,000 ft to 10.668 km is precise arithmetic on a figure that was never a distance in the first place.

A foot is exactly 0.3048 metres. That number was agreed in 1959 by six English-speaking countries, in the same document that fixed the yard at exactly 0.9144 m and the inch at exactly 25.4 mm, and it replaced two feet that differed in the seventh decimal place. So a kilometre is exactly 1,000 divided by 0.3048 feet, and the decimal expansion 3,280.8398950131... repeats forever without ever being uncertain.

That is worth separating from the other kind of untidy number. 3,280.84 looks like a measurement and is not; it is a division of two exact quantities, and any converter that disagrees with it has rounded. The digits stop where the display stops, and a page quoting six of them and a page quoting ten are both correct.

The in-flight map is the commonest place a passenger meets both units within one screen. Altitude sits at 11,278 m or 37,000 ft depending on the setting, ground speed is in km/h or mph, and outside temperature is in whichever system the airline chose. Only the altitude is genuinely a feet-native number; the rest was metric first and converted for display.

Meteorology goes the other way and is worth a note because it produces kilometre figures that look like aviation ones and are not. Radiosonde and cloud-height data are metric almost everywhere, and a cloud base reported at 2 km is 6,562 ft — which an aviation forecast for the same airfield would have published as 6,500 ft, rounded to the nearest hundred, because that is the resolution an approach chart works in.

The other vertical measurement that resists metrication is depth. Recreational diving in the United States is planned and taught in feet while most of the rest of the world uses metres, and the limits were set independently in each system rather than converted: the recreational ceiling is 40 m in one and 130 ft in the other, and 130 ft is 39.6 m. Close enough to look like the same rule, and 40 cm apart.

The pattern repeats down the training ladder. An entry-level limit of 18 m is 59 ft against a taught 60; a deep speciality at 30 m is 98.4 ft against a taught 100. Each pair is a round number in its own system and a fraction in the other, which is the same shape as the mountain thresholds above and for the same reason — the number was chosen for memorability, not derived by converting anything.