Convert ft to m

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 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.

The factor is 0.3048, and almost nobody carries that around. Rounded to 0.3 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 0.3048 m 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.

An aircraft crossing France, Japan or Brazil reports its altitude in feet, and every altimeter in the cabin reads in feet. This is not an American holdover that the rest of the world tolerates; it is the international standard, because vertical separation only works if every aircraft in the same airspace is using the same scale, and the instruments were already in feet when the rules were written. China is the significant exception and still assigns levels in metres. Russia used metres until November 2011 and now uses feet.

That is why a conversion in this direction is nearly always a translation rather than a calculation. 35,000 feet is 10,668 m exactly, 39,000 is 11,887.2, 10,000 is 3,048. The metric figures are untidy and the imperial ones are round, which is the reliable sign of which unit the number was born in — and a good check when a source quotes an altitude in metres and you need to know whether it was originally a level.

FL350 does not mean the aircraft is 10,668 metres above the sea. A flight level is a pressure altitude, read with the altimeter set to the standard 1013.25 hPa rather than to the local pressure, so it describes a surface in the atmosphere that rises and falls with the weather. On a cold day with low pressure, the true height of FL350 can be several hundred metres below the converted figure.

The conversion is still worth doing precisely, because the separation between levels is what the system depends on. A thousand feet of vertical separation is 304.8 m, and that is the figure the reduced-separation rules between FL290 and FL410 are built on. If you are reconciling aviation data against a metric model, convert the level to metres for the arithmetic and keep the level itself as the label — the level is an identifier, and rounding it to a metric value destroys that.

Aeronautical charts give obstacle and terrain elevations in feet, and every topographic map of the same ground gives them in metres. Anybody comparing the two is doing this conversion, usually under a minimum safe altitude that is itself in feet. Everest at 8,848.86 m is 29,032 feet; a 2,000 m ridge is 6,562 feet; an 800 m mast is 2,625. None of those land anywhere near a round figure on the other side.

The safe habit is to convert the terrain up and the clearance down, so that any rounding error moves the answer in the conservative direction. A 2,000 m obstacle rounded to 6,500 feet has quietly lost 62 feet of height, and a minimum altitude rounded generously has given some of it back. Where the two systems have to be mixed at all, do the arithmetic in one of them and convert the single final figure rather than converting each term.

Planning and building consent across Europe is written in metres, and often to two decimal places, because a limit is a legal threshold rather than a description. An American drawing describing a forty-foot building converts to 12.192 m, and against a twelve-metre limit that building is over by 192 mm — a figure that no amount of correct conversion will remove, and that is invisible if the number is rounded to "about twelve metres" early in the process.

The rule is to round away from the limit rather than to the nearest value. A dimension being tested against a maximum is rounded up; one being tested against a minimum, such as a headroom or a separation distance, is rounded down. Floor-to-floor heights show how quickly this accumulates: ten feet is 3.048 m, so a four-storey American section is 12.192 m of structure before anything on the roof, and each storey has contributed 48 mm of surplus that nobody chose.

The shipping container is the most widely converted foot dimension in the world, because it defines the size of doors, yards, chassis and warehouse bays in countries that measure everything else in metres. A forty-foot container is 12.192 m long, 2.438 m wide and 2.591 m high at standard height, or 2.896 m as a high cube. Those are exact conversions of 40 ft, 8 ft, 8 ft 6 in and 9 ft 6 in.

The twenty-foot container is the one that catches people out. It is 6.058 m rather than 6.096, because two of them plus the gap and couplings between them have to occupy exactly one forty-foot slot on a ship or a chassis. Converting "20 feet" gives the wrong answer by 38 mm, and the number that matters — the one a yard is set out to — is the 6.058. It is a good reminder that a name in feet is not always a dimension in feet.

Two definitions of the foot were in use in the United States until very recently. The international foot, exactly 0.3048 m, was agreed in 1959; American land surveying kept the older US survey foot of 1200/3937 m, which is longer by two parts in a million. On any building that difference is meaningless. On a state plane coordinate with a false origin hundreds of thousands of feet away it is not: the discrepancy is roughly a foot per hundred miles of coordinate value, and it lands as a systematic shift rather than as noise.

The United States retired the survey foot for new work at the end of 2022, which ends the ambiguity going forward and does nothing about a century of existing data. If you are converting a coordinate rather than a dimension, the metadata has to say which foot it is in before the conversion means anything — and a dataset that does not say is a dataset whose two-part-per-million error will surface later, in the field, as a wall in the wrong place.

The commonest failure with this conversion is not arithmetic, it is a unit setting. A model authored in feet and opened in a metric environment arrives either 3.2808 times too small or, if the file declared inches, 12 times out from that again. The tell is always the same: a door that is 0.9 units wide is metres, one that is 3 units wide is feet, and one that is 36 is inches. Check a known object before checking anything else.

Once the model is in metres, keep it there. Re-exporting to feet for one consultant and back again introduces rounding at every crossing, and dimensions that were exact — 12.192, 3.048, 2.591 — start appearing as 12.19 and 3.05 and never return to the values the drawing intended. Convert at the boundary, once, and record which unit the authoritative version is in, because on this pair the exactness is free and only the bookkeeping can lose it.