The letters ft are printed right on the dial of a barometric altimeter, and the hands sweep over them: the long hundreds hand climbs and descends, the short thousands hand creeps after it, and the thin ten-thousands hand trembles at the rim. In the window on the right the pressure drum rolls round to 29.92, on the left the striped low-altitude flag shows and hides, a warning lamp blinks, and the marker on the flight-level tape slides up to FL350; the blue dashes on the tape are metric flight levels.
Quantity data sheet
length · aviation sheet 1/1 · rev. 2026-09Foot ft · ′
Unit outside the SI · quantity: length; exactly 0.3048 m since 1959
Sixteen left feet at the church door
In 1535 the surveyor Jakob Köbel printed in Frankfurt a simple recipe for getting a lawful foot without any standard: on a Sunday after the service, stop sixteen men at the church door, tall and short, as they come out, and set their left feet one behind the other. The length obtained is the lawful rod, and its sixteenth part is the lawful foot, so the accuracy of the measure rested on the law of large numbers long before mathematicians stated it.
Today the foot is defined far more dully, as exactly 0.3048 metre, but in one field it holds on more firmly than the metre: almost the whole sky over the planet is marked out in feet, and aircraft are separated by flight levels every thousand feet even where nobody on the ground uses feet at all.
This sheet shows what happens when an aircraft crosses into airspace with metric flight levels, why American surveyors measured with a different foot until 2022, and how many feet there were in Europe before they came down to two.
The symbol ft is written in lower case and without a full stop, and it does not change in the plural: 6 ft, not 6 fts. In everyday heights and on drawings the foot is marked with a single prime ′ and the inch with a double one, so six feet two inches is written 6′2″; a flight level is written solid with the letters FL and hundreds of feet, FL350, with no unit.
The international foot of 0.3048 m — with the US survey foot, which is two parts per million longer and lived on in maps until 2022, and with the old feet of Europe, where the Rhineland foot was 314 mm and the Paris foot 325. Nor confuse a flight level with a height: FL350 means 35.000 feet not above the ground nor above the sea, but on the standard pressure of 1013.25 hectopascals.
01 · Definition
yard · inch · flight levelThe foot is a unit of length equal to one third of the international yard and to twelve inches; since the yard was defined in 1959 as 0.9144 metre, the foot is exactly 0.3048 metre. Like the inch, it has no standard of its own, and all its accuracy is borrowed from the metre.
The foot lives most widely in the air: flight altitude almost everywhere in the world is set in feet, and flight levels are counted in hundreds of feet on standard pressure, so FL350 means 35.000 feet on an altimeter set to 1013.25 hectopascals. The International Civil Aviation Organization would prefer metres, but permits the foot as a temporary alternative and has never set a date for ending it.
A step of a thousand feet between flight levels is 304.8 metres, and where the sky is marked in metres the step was made 300 metres. The two ladders almost coincide, but at every rung they drift apart by sixteen feet, and an aircraft flying from one marking into the other has to climb or descend slightly.
Sixteen feet at every rung
On the left the sky is marked in feet, on the right in metres, and in the middle runs the boundary between two control areas. Amber lines are foot levels every thousand feet, blue dashes are metric levels every three hundred metres; the aircraft flies left to right on the chosen level and at the boundary moves to the nearest metric one, while the oncoming one flies a rung higher the other way. The bracket between them, as they drift apart, shows the real vertical separation in feet.
Everyone is wrong by the same amount, so no one collides
An altimeter measures pressure, not distance, and at a flight level every crew sets the same standard pressure, although the true sea-level pressure that day is different. The true height of aircraft at FL350 may differ from 35.000 feet by a few hundred, but by the same amount for all of them, so the separation between them is kept; the error is shared, and it is safe as long as everyone uses one foot and one pressure.
02 · Conversion
units · old feet · flight levelsFrom feet to metres and back
Enter a length or a height, and the sheet will show it in metres, inches, yards, miles and flight levels, convert it into the old feet of various cities and compare the foot ladder of flight levels with the metric one. The length is the same in every tab; only the yardstick changes.
Rule of thumb: to get metres, multiply feet by three and divide by ten, then add a correction of one and a half per cent.
A flight level is not a length but a conventional pressure converted into feet by the standard atmosphere, so the metres in the “flight levels” row mean a height in that same standard atmosphere, not above the ground. The old feet are rounded: one city often had two feet at once, one for builders and one for surveyors, and their length changed from standard to standard.
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04 · Measuring instruments
altimeter · radio altimeter · chain · rodWhat measures land and sky in feet
Feet that are really pascals
Inside the altimeter is a stack of evacuated metal capsules that swell as the outside pressure falls and turn the hands through levers. The scale is marked in feet by the standard atmosphere, so the instrument really shows pressure converted into height by a formula, and that is why the pilot sets the local pressure on the drum before landing.
Real feet above the ground
A radio altimeter sends a signal downwards and measures the time it takes to come back from the ground, so it shows the honest distance to the surface rather than pressure. It works only low down, usually up to 2500 feet, and so it starts to speak only on the approach, when a synthetic voice in the cockpit counts off the last hundreds and tens of feet to touchdown.
Sixty-six feet in a hundred links
Edmund Gunter's surveying chain, devised in 1620, consisted of a hundred links with a total length of 66 feet, and the entire land survey of the English-speaking world rested on it: ten square chains make an acre, eighty chains a mile. The chain joined the decimal count of links to the old feet, and that is why plot boundaries are still often multiples of sixty-six.
A standard made of parishioners
Sixteen random men give a rod whose spread is four times smaller than that of a single foot — the square root of sixteen, if the errors are taken as independent — so Köbel's recipe was not naive but statistically sound. He proposed getting a single foot by dividing the rod, not by laying down one foot, and it is the same idea by which repeated measurements are averaged today.
05 · Writing rules
symbol · primes · flight levelFL350 is not “350 feet”
In text the foot is written with the symbol ft after a space, in heights and on drawings with a single prime and no space, and a flight level with the letters FL and the number of hundreds of feet, with no unit after it.
The first line on the right adds a plural and a full stop to the symbol, the second takes two inches for two tenths of a foot, although 6.2 feet is six feet and 2.4 inches, and the third gives the flight level a unit, so that it is read as 350 feet instead of 35.000. The fourth turns the symbol into an abbreviation of the word, the fifth rounds an exact value and is out by one and a half per cent, and the sixth writes the square foot in words with full stops instead of a power.
06 · Neighbouring units
inch · yard · mileThe foot stands in the middle of the imperial ladder: below it are twelve inches, above it three feet in a yard and 5280 feet in a statute mile, and in the sky next to it lives the nautical mile, by which the path of an aircraft is counted, while its height is counted in feet.
The twelfth part of a foot, the Roman uncia, exactly 25.4 millimetres.
The foot is defined through the yard: 0.9144 metre divided by three.
Eight furlongs, eighty Gunter's chains, 5280 feet.
07 · Historical section
archive · from legion to flight levelHow the feet became many, then two, then one
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How the metric level catches up with the foot level
The height of each bar is how far the metric level lies above the nearest foot level below it, in feet, for thirteen rungs from 8900 to 12.500 metres. At each rung the difference shrinks by sixteen feet; the frame visits the rungs in turn.
Two hundred feet at the bottom and ten at the top — that is all that is left of the difference between 304.8 and 300 metres, but at the boundary those feet turn into a mandatory manoeuvre for every flight.
A temporary unit with no end date
In the annex on units of measurement to the Chicago Convention, the foot for height and the knot for speed are listed as non-SI units permitted temporarily, but the date for ending them has been left unset, and so it has gone on for decades. The reason is not the habits of pilots but that the sky is shared: switching to metres requires every country along an aircraft's route to switch on the same day, and as long as even one stays on feet, it pays to keep feet for everyone.
Where metric flight levels have survived, their table has been fitted to the foot one: every metric rung is written together with the nearest flight level in hundreds of feet, so that a crew with a foot altimeter can hold it, and the boundary between the two systems is crossed with one short manoeuvre instead of reshuffling the whole flow.
The accuracy of the foot has long ceased to be a question: its length follows from the metre without remainder, and the whole error of a flight level lies in the pressure and temperature of the air, which the altimeter converts into feet by the standard-atmosphere formula.