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A foot is exactly 0.3048 metres, or 30.48 centimetres. It is named after a body part and it matches nobody's body part: the foot of an adult who is 5 ft 9 in tall measures about 26.8 centimetres. For your foot to be a foot long, you would have to be nearly 6 ft 6 in tall.
The circle on the map has a radius of 304.8 metres — one thousand feet exactly — centred on Charing Cross, the point from which every road distance to London has been measured for centuries. This article is part of the distance units and where they come from; its opposite number is the metre, which was built by measuring the planet instead of measuring a person. If you only want the number, the distance converter turns feet into metres instantly.
Start with the strange part. An adult's foot is on average 15.3% of their height: a person of 175 cm (5 ft 9 in) has a foot about 268 millimetres long. The unit is 304.8. The official foot is therefore about 14% longer than the average human foot, and that is not an oversight: it is what happens when a unit is inherited for two thousand years and everyone who pins it down pins it down slightly in their own favour.
There are sensible explanations — it was measured with a shoe on, it was measured on somebody important, it was nudged to fit convenient multiples — and none of them is proven. What is written down is the ideal. Vitruvius, in book III of De architectura, says the foot is one sixth of a person's height, and that a well-proportioned body is therefore exactly six feet tall. Six feet is 1.8288 metres, which would give the Vitruvian man a 30.5 cm foot. The real ratio is closer to one in six and a half: the classical canon is, quite literally, a little long in the foot.
While the unit came from the body, no two places could agree. The historical foot ranged from 250 to 335 millimetres depending on the country, and in many regions two or three coexisted depending on whether you were measuring cloth, timber or land.
| Foot | Length | Where |
|---|---|---|
| International foot | 304.8 mm | Today's, since 1959 |
| Roman pes | ~296 mm | Five feet made a pace; a thousand paces, a mile |
| Greek foot | ~302 mm | Different in every city (270-350 mm) |
| Belgic or North German foot | 335 mm | Brought to England by the Belgic Celts or the Anglo-Saxons |
| Pied du roi | 324.8 mm | France, until the Revolution |
| Castilian foot | 278.6 mm | A third of a vara; 20,000 made a league |
| US survey foot | 304.80061 mm | US land surveying until 2023 |
The spread is the point. An English plan and a French plan of the same building gave different numbers even though both said "feet", and the difference between the pied du roi and the English foot — two whole centimetres — was enough to start arguments about the size of a cargo, a field or a cathedral.
The best surviving recipe for manufacturing a legal foot dates from 1535 and was written by the German mathematician Jacob Köbel. It goes like this:
Stand at the door of a church on a Sunday and bid 16 men to stop, tall ones and small ones, as they happen to pass out when the service is finished; then make them put their left feet one behind the other, and the length thus obtained shall be a right and lawful rood to measure and survey the land with, and the 16th part of it shall be the right and lawful foot.
It is a beautiful piece of nonsense and it is also statistics: averaging sixteen arbitrary feet cancels out the giant and the dwarf, and the next village repeating the procedure will land on a similar number. With no standards, no platinum bars and no laboratories, Köbel had found the only honest way to standardise anything using people — use a lot of people.
The other classical method works from the bottom up, through the inch. An English text of around 1324, attributed to Edward II, defines the inch as three grains of barley, "dry and round", laid end to end lengthwise; twelve inches make a foot and three feet a yard. The same idea turns up in Castile, where the dedo was defined as four barleycorns.
And here is the good bit: the barleycorn is still in use. A third of an inch is 8.47 millimetres, and that is exactly the step between two consecutive UK or US shoe sizes. Every time somebody tries a size 9 after a size 8, they are moving up by one medieval grain of barley.
The words give the origin away too. Inch comes from the Latin uncia, "a twelfth part" — the same word that gives us the ounce. The Romance languages went the other way and named it after the thumb: French pouce, Spanish pulgada, Italian pollice. One language kept the fraction, the others kept the finger.
Greeks and Romans divided the foot into 16 digits. It was the Romans who later added the division into 12 unciae, and that is the one that won. It is not an accident or stubbornness: 12 divides by 2, 3, 4 and 6, while 10 only divides by 2 and 5. For a carpenter who needs a third of a board or a quarter of a beam and has no calculator, twelve is simply a better tool than ten.
That is the one real argument the body units have, and it is the trade the metric system refused: the ease of splitting things into thirds in exchange for the ease of multiplying by ten. The metre won because measuring stopped being a craftsman's problem and became a scientist's, an engineer's and a customs officer's. The other half of that story is in the origin of the metre.
Here is the final irony. The foot is no longer defined by anybody's foot: it is defined in metres. The International Yard and Pound Agreement, signed on 1 July 1959 by the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa, set the yard at exactly 0.9144 metres, and everything else falls out of that by arithmetic: the foot is a third of a yard, the inch a thirty-sixth.
Industry had in fact got there almost thirty years earlier. The 25.4 mm inch was adopted by the British Standards Institution in 1930 and by the American Standards Association in 1933, because the gauge blocks that parts were machined against came in millimetres and nobody wanted two sets. The 1959 treaty did not invent the number; it legalised the one the workshops were already using.
Before that, the British foot (0.3047997 m) and the American survey foot (0.30480061 m) were not the same length. The difference is two parts per million: two millimetres per kilometre, twenty centimetres over a hundred. Irrelevant for a table, expensive for a land registry — so the United States kept its old US survey foot for another six decades and only retired it on 1 January 2023. That was the last pre-scientific unit of length left in the Western world.
The conversion table, with the circle already drawn on the map for the distances you can actually see:
| Feet | In metres | What it is |
|---|---|---|
| 1 foot | 0.3048 m | 30.48 cm; 12 inches |
| 3 feet | 0.9144 m | Exactly one yard |
| 6 feet | 1.8288 m | Vitruvius's ideal height |
| 10 feet | 3.048 m | The height of a basketball hoop |
| 20 feet | 6.1 m | A container: the unit of world shipping |
| 1,000 feet | 304.8 m | The circle on the map |
| 5,280 feet | 1,609.344 m | One mile |
| 29,032 feet | 8,848.86 m | Mount Everest |
| 35,000 feet | 10,668 m | An airliner's cruising altitude |
Even in countries that buried the foot a century and a half ago, it is used every single day in one very specific place: the air. ICAO sets altitudes and flight levels in feet, so a pilot taking off from Madrid or Frankfurt climbs to 35,000 feet, not 10,668 metres. Only China, Mongolia and North Korea have stayed on metres; Russia and several ex-Soviet states switched to feet above the transition altitude on 17 November 2011, because running two altitude systems in the same sky is, quite literally, dangerous.
Feet and inches survive in other corners too, almost always for the same reason — the standard was written in English and changing it would cost more than it would fix:
The two units sum up the two ways of measuring anything. The foot came from the body and was stabilised by custom, decrees, barleycorns and sixteen neighbours leaving church; it was never exact until somebody defined it in metres. The metre came from the planet, was exact from day one by decree, and had the opposite problem: measuring the Earth properly took seven years and came out 0.2 millimetres short.
One was built from the bottom up and the other from the top down, which is why they still sit badly together: a foot has 12 inches, a mile has 5,280 feet, and a kilometre has 1,000 metres. Only one of those three numbers was chosen by somebody who wanted it to be easy.
Just want the number? The distance converter turns feet into metres, kilometres, yards or miles. Want to see it? Type a distance into the distances tool and it is drawn to scale over your own city. And if you want the whole story — the mile, the metre, the league and why we still cannot agree — it is in the distance units.