Aviation measures altitude in feet almost everywhere, including in countries that are metric in every other respect, because vertical separation between aircraft is a safety standard and a shared standard beats a local one. A foot is exactly 0.3048 metres, so 35,000 ft is 10,668 m — a figure nobody would choose to fly at if they were starting from metres.
The round numbers give the history away. Cruise levels sit at 31,000, 33,000, 35,000, 37,000 and 39,000 ft; in metres those are 9,449, 10,058, 10,668, 11,278 and 11,887. One of those lists was designed and the other is a translation.
What is a flight level?
A flight level is an altitude expressed in hundreds of feet against a standard pressure setting rather than the local one. FL350 means 35,000 ft on an altimeter set to 1013.25 hectopascals, which is 10,668 m.
The standard setting is the point. Two aircraft using the local pressure of two different airfields could be a hundred metres apart while both read the same height. Above the transition altitude everyone switches to the same reference, so the readings become comparable even though none of them is the true height above the ground.
Where are metres used instead?
A small number of countries run their airspace in metres, and aircraft convert at the boundary. China, Russia, Mongolia and several of their neighbours use metric flight levels; China moved its levels to metric equivalents of the standard foot-based ones in 2007, which is why its cruise altitudes are still spaced like feet.
The conversion is done by published table rather than mental arithmetic, precisely because the numbers are awkward. Crossing from FL350 into metric airspace means stepping to a level that reads round in metres, and that step is a procedure, not a calculation.
What does "exactly 0.3048" mean?
It means the foot has been a metric unit since 1959. The international yard and pound agreement defined the yard as exactly 0.9144 m, and the foot as a third of that, which lands on 0.3048 with no remainder. Feet are not an alternative to the metric system; they are a defined multiple of it.
That is why altitude conversions are exact rather than approximate. Every figure below is a definition worked through.
| Altitude | In metres | What it is |
|---|---|---|
| 1,000 ft | 304.8 m | Circuit height at many airfields |
| 10,000 ft | 3,048 m | Where the speed restriction usually lifts |
| 29,032 ft | 8,849 m | The summit of Everest |
| 35,000 ft | 10,668 m | A common long-haul cruise level |
| 38,000 ft | 11,582.4 m | A step up as fuel burns off |
| 41,000 ft | 12,496.8 m | The ceiling of many narrow-body jets |
Why does the aircraft climb during the flight?
Because it gets lighter. A long-haul aircraft burns tens of tonnes of fuel across a flight, and a lighter aircraft cruises more efficiently higher up where the air is thinner. Step climbs of 2,000 ft — 609.6 m — are planned into the route rather than improvised.
This is also why the cruise level on the seat-back map keeps shifting by a few hundred metres. Nothing has gone wrong; the display is converting a step change in feet and rounding it differently each time.
Why is the transition altitude different everywhere?
Because it depends on the terrain underneath. The transition altitude is the height at which everyone stops using the local pressure setting and switches to the standard one, and it has to sit comfortably above the highest ground and obstacles in the region — so a country with mountains sets it higher than a country without.
In much of Europe it has historically sat as low as 3,000 or 4,000 ft, which is 914 or 1,219 m, with a long-running effort to harmonise upward. In the United States it is 18,000 ft — 5,486 m — across the whole country. Below that height the altimeter reads a genuine height above sea level, so it can be compared with the terrain printed on the chart.
That is why two systems exist at all. Near the ground you want to know how far you are above the hill; in the cruise you want to know how far you are from the aircraft above you. Only one reference can answer each question.
Questions people ask
Are runways measured in feet too? Not necessarily. Many metric countries publish runway lengths in metres while still working altitude in feet. The two conventions are independent of each other.
What about vertical speed? Feet per minute wherever feet are used. A standard approach descent of 700 ft/min is 3.6 metres per second.
Why 1013.25 hectopascals? It is standard sea-level pressure in the International Standard Atmosphere, the model that turns a pressure reading into a height. In the United States the same setting is quoted as 29.92 inches of mercury.
Does the altimeter show my real height? Above the transition altitude, no, and deliberately so. It shows a pressure altitude every other aircraft is also showing, which is what keeps them apart.
Is cabin altitude the same thing? No. Cabin altitude is the pressure inside the aircraft expressed as the height it corresponds to, typically 6,000 to 8,000 ft — 1,829 to 2,438 m — while the aircraft itself is at 35,000 ft. Two altitudes, one aeroplane.
Why are approach charts in feet and metres both? Because the crew needs the aviation figure and the ground obstacle data may be surveyed in metres. Where a chart carries both, the smaller figure in brackets is usually the metric one.
The safety argument that kept aviation on feet is the one that keeps it from switching now: a mixed system is more dangerous than either pure one. Feet to metres converts the levels, kilometres to miles handles ground distance, and Celsius to Fahrenheit covers the outside air temperature the flight deck reads in a third unit again.