Flight levels turned into real heights above the sea, using the day's altimeter setting — plus the temperature error that moves the answer again, in the direction nobody wants.
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The altimeter setting is required, with no default. A flight level is a pressure reading, and it is only a height once you say what the pressure at sea level actually is that day.
FL100 with QNH 983 hPa (29.03 inHg)
9,217 ft
above mean sea level · 2,809 m · 783 ft LOWER than the level number suggests
Against the rule of thumb
Nominal, from the level number10,000 ftwhat FL100 would be at 1013.25
Computed from the ISA relation9,217 ftthe figure this tool reports
By the 27 ft per hPa rule9,183 ft-34 ft against the computed figure
Feet per hectopascal at this level37.0against 27 near the ground
The 27-feet rule is a sea-level figure. It is close enough in the circuit and steadily less so with height, because a hectopascal is worth more feet where the air is thinner. At this level one hectopascal is 37 feet, not 27.
What a hectopascal is worth, by height
Computed from the standard atmosphere, not assumed
Pressure altitude
Feet per hPa
Against the rule of thumb
Sea level
27.3
1% higher
FL50
31.7
17% higher
FL100
37.0
37% higher
FL150
43.4
61% higher
FL200
51.2
90% higher
FL250
60.9
126% higher
FL300
73.0
170% higher
FL350
88.1
226% higher
At FL350 a hectopascal is worth over three times what it is on the ground. This is why altimeter setting errors matter more at low level in absolute feet but why the correction itself has to be computed rather than scaled, and it is one reason everyone above the transition altitude uses the same standard datum instead of correcting individually.
Cold air, and the direction it errs
Standard temperature at FL100-4.8 °C15 °C at sea level, less 1.98 per thousand feet
Deviation from standard-20.2 °Ccolder than standard
Altimeter error at this level-808 ftyou are LOWER than indicated
An altimeter assumes a standard temperature profile. In colder air the column of air between the ground and a given pressure is shorter, so the aircraft is genuinely lower than the instrument says. Four feet per thousand feet per degree below standard is the usual rule, and it points the wrong way for terrain clearance — which is what the old line “from hot to cold, don’t be bold” is about.
Three settings, three different readings
What the altimeter shows, depending on what is in the window
Setting
Datum
Reads
Note
QNH
Sea level pressure for the area
Height above mean sea level
On the ground at an airfield, the altimeter reads the airfield’s elevation. The normal setting below the transition altitude.
QFE
Pressure at the airfield itself
Height above the airfield
Reads zero on the runway. Still used for circuit work at some fields, and a genuine hazard if confused with QNH.
Standard (1013.25)
The fixed standard datum
Pressure altitude — a flight level
Every aircraft above the transition altitude uses this, so they are separated correctly from each other whatever the weather is doing.
Where altitudes stop and flight levels start
Region
Transition altitude
Note
United States & Canada
18,000 ft
A single national figure, so flight levels start well above almost all terrain.
United Kingdom
3,000–6,000 ft, by area
Low, and varies. Above high ground the transition altitude is raised accordingly.
Much of continental Europe
3,000–10,000 ft, by airport
Set per airport or per terminal area. Harmonisation to a common figure has been a long project.
India
4,000 ft or higher, by airport
Published per aerodrome, with higher figures where the terrain requires it.
China
3,000 m
Levels are metric, so the numbers are metres rather than hundreds of feet.
Above the transition altitude everyone sets 1013.25, which is the point: aircraft are then separated from each other correctly whatever the weather is doing, at the cost of nobody’s altimeter reading a height above the sea. Below it, everyone uses the local QNH, so everyone is separated from the ground correctly instead. The transition is where the priority changes.
What this converter covers
Flight level and QNH to true altitude, the rule of thumb compared against the standard atmosphere, cold-weather error, and the three altimeter settings.
Flight level to true altitude for any altimeter setting, in feet and metres
The 27 ft per hPa rule compared against the ISA relation it approximates
What a hectopascal is worth from sea level to FL350
Cold-temperature altimeter error, and which way it points
QNH, QFE and standard settings, and transition altitudes by region
Setting required FL100 is not 10,000 ft Cold-weather error Feet, metres, hPa, inHg
Free, no signup — exact by definition, not an estimate.
Updated 8 September 2026
At a glance
Formula shown
Pressure altitude against 1013.25 hPa · roughly 27 ft per hPa near the surface
Scenario support
FL100 at QNH 983 is about 9,220 ft · 20 °C below standard costs 4 ft per 1,000 ft
Educational estimate
Planning support from the values you enter — not professional advice.
A level is a pressure, not a height
FL100 is not ten thousand feet. It is the height at which the air pressure is whatever the standard atmosphere says ten thousand feet should be — and on most days the atmosphere is not standard.
An altimeter is a barometer with an altitude scale. Set it to 1013.25 hPa and it reads pressure altitude: the altitude the standard atmosphere would put that pressure at. Every flight level is that reading divided by a hundred.
FL100 on four different days
Sea-level pressure
True altitude
Difference
960 hPa — deep low
about 8,600 ft
1,400 ft low
983 hPa
about 9,220 ft
780 ft low
1013.25 hPa — standard
exactly 10,000 ft
none
1040 hPa — strong high
about 10,670 ft
670 ft high
Fourteen hundred feet is not a rounding. It is the difference between clearing a ridge and not, which is why the transition altitude exists and why terrain clearance below it is always worked in altitudes rather than levels.
Why everyone sets the same number
If flight levels are not real heights, the obvious question is why anyone uses them. The answer is that they solve a different problem, and solve it perfectly.
Two aircraft a thousand feet apart in the cruise need to stay a thousand feet apart. If each set its own local altimeter setting, and those settings came from different stations hundreds of miles apart, the vertical gap between them would depend on a weather map.
Set every altimeter to the same fixed datum and that problem disappears entirely. Nobody knows their height above the sea, and it does not matter, because everyone is wrong by the same amount and the separation between them is exactly what it says.
Below the transition altitude the priority reverses. Now the aircraft is near the ground and needs to know its height above it, so everyone uses the local QNH and accepts that separation between distant aircraft is slightly less exact.
The transition altitude is simply where that trade changes hands — and the third setting, QFE, goes one step further by reading zero on the runway, which is convenient for circuit work and hazardous anywhere else.
The 27-foot rule is a ground rule
Every student learns that one hectopascal is worth about 27 feet, or that an inch of mercury is about a thousand. Both are true near the surface, and both quietly stop being true with height.
Feet per hectopascal, from the standard atmosphere
Pressure altitude
Feet per hPa
Sea level
27.3
FL100
37.0
FL200
51.2
FL300
73.0
FL350
about 88
The reason is that pressure falls roughly exponentially with height while feet do not. Higher up there is less air above you, so removing one hectopascal of it takes a longer climb.
For altimeter setting purposes the rule of thumb is nevertheless fine, because the correction is applied where it matters — near the ground, at the transition, and on approach. Where it goes wrong is when the same 27 is used to reason about the cruise, and the calculator above shows both figures side by side for exactly that reason.
Cold air errs the wrong way
There is a second error stacked on top of the pressure one, it is not corrected by any altimeter setting, and it points towards the ground.
An altimeter converts pressure to height using the standard temperature profile. In colder-than-standard air the column between the ground and any given pressure is shorter, so the aircraft is genuinely lower than the instrument reads.
The working rule is four feet per thousand feet per degree below standard. At 5,000 ft in air 20 °C below standard that is 400 feet — the aircraft is at 4,600 while the altimeter says 5,000.
Two things make this worse than the arithmetic suggests. It scales with height above the setting source, so a 3,000 ft approach in a Scandinavian winter loses more than a circuit does. And it is invisible: nothing on the instrument panel indicates it, because the instrument is working exactly as designed.
Which is what the mnemonic from hot to cold, don’t be bold is for, and why cold-temperature correction tables are published, and why states designate airports where applying them is mandatory below a stated temperature. The rule here is a rule of thumb; the tables are the authority.
Where the transition sits
The altitude at which everyone stops using QNH and switches to the standard datum varies enormously, and the variation is a genuine trap for anyone flying across borders.
Transition altitude by region
Region
Transition altitude
United States & Canada
18,000 ft, nationally
United Kingdom
3,000–6,000 ft, by area
Continental Europe
3,000–10,000 ft, by airport
India
4,000 ft or higher, by aerodrome
China
3,000 m, with metric levels
The American figure is high and uniform, so flight levels there sit above almost all terrain and the pressure error never threatens the ground. The European approach keeps it low and local, which means the transition happens in busy airspace and often at a different height at each airport.
The Chinese row is the one that surprises people most: levels there are metric, so the numbers are metres rather than hundreds of feet, and an aircraft cleared to 8,900 m is at what elsewhere would be near FL292. The conversion is exact — a foot is 0.3048 m — but the mental arithmetic is not, and the calculator above gives both.
Related calculators
Other pressure, distance and navigation tools:
Pressurepsi, bar, kPa, atm, torr and mmHg — plus why gauge and absolute differ by an offset that no factor can bridge.
LengthMillimetres to miles on the exact 1959 factors, with the mil kept clearly apart from the millimetre — they differ 25-fold.
TemperatureCelsius, Fahrenheit, kelvin and Rankine — and temperature CHANGES, which convert differently from readings.
Speedmph, km/h, m/s, ft/s and knots on exact factors, with the nautical mile behind the knot explained rather than assumed.
Wind Speed and BeaufortBeaufort force and what it looks like, in mph, km/h, knots and m/s — with the pressure it exerts and the averaging period behind the figure.
CoordinatesLatitude and longitude between degrees-minutes-seconds and decimal degrees, showing how far the common misreading would put you.
Altimetry is one of the better-documented corners of aviation, and these three cover it from the American, European and British sides respectively — which matters here, because the transition altitude is one of the places the three genuinely differ.
Pilot’s Handbook of Aeronautical KnowledgeFederal Aviation Administration · verified 2026-09-08 · The altimeter setting procedures, the definition of pressure altitude against the 29.92 inHg standard datum, the 18,000 ft transition altitude used in the United States, and the approximate one-inch-per-thousand-feet relationship near the surface
Altimeter setting procedures and altimeter temperature errorSKYbrary (EUROCONTROL) · verified 2026-09-08 · The QNH, QFE and standard settings and what each reads, the European practice of setting transition altitudes by aerodrome, and the cold-weather altimeter error that makes an aircraft lower than indicated
UK Civil Aviation AuthorityUK CAA · verified 2026-09-08 · The UK’s comparatively low transition altitudes, set by area and raised over high ground, and the regulatory basis for altimeter setting regions
Conversion note
Educational only. Nothing on this page is for use in flight, for flight planning, or for any operational decision, and it is not a substitute for an aircraft's own instruments, its flight manual, the applicable state's procedures, or current meteorological information. The calculations use the International Standard Atmosphere, which is a model: the real atmosphere departs from it constantly, and the departure is exactly what makes altimetry interesting. Cold-temperature corrections here use the common four-feet rule and are approximate; published cold-temperature correction tables exist for precisely this reason and are what an operator must use. Terrain clearance additionally depends on the accuracy of the altimeter setting received, on how far the aircraft is from the station that issued it, on instrument error, and on procedures that vary by state and by approach. Aircraft altimeters are calibrated instruments with their own tolerances and inspection requirements. If you are flying, use the procedures and figures your authority and your operator publish.
Published the Flight Level to Feet Converter: flight levels into true altitude above mean sea level for any altimeter setting, in feet, metres, hectopascals and inches of mercury.
Requires the altimeter setting and offers no default, because a flight level is a PRESSURE altitude read against the fixed 1013.25 datum and only becomes a height once the day's sea-level pressure is known.
Prices the difference: FL100 is about 8,600 ft in a 960 hPa low and about 10,670 ft in a 1040 hPa high -- a spread of over two thousand feet from the same level number.
Computes the correction from the ISA relation and shows the 27-feet-per-hectopascal rule of thumb beside it, since that rule is a sea-level figure: the real value is 37 at FL100, 51 at FL200 and 73 at FL300.
Adds the cold-temperature altimeter error, which no altimeter setting corrects and which points the wrong way for terrain: at four feet per thousand feet per degree below standard, an aircraft at 5,000 ft in air 20 degrees cold is genuinely at 4,600.
Explains why flight levels exist at all -- above the transition everyone sets the same datum so aircraft are separated correctly from each other whatever the weather, at the cost of nobody's altimeter reading a height above the sea.
Tabulates the three altimeter settings, QNH, QFE and standard, with what each reads, and the transition altitudes by region: 18,000 ft in North America, three to six thousand in the UK, and metric levels in China.
Carries an explicit disclaimer that the page is educational only and not for flight, flight planning or any operational decision.
Verified by 75 automated cases, asserting that a flight level and a true altitude coincide exactly at the standard datum, that low pressure puts the aircraft lower and high pressure higher across a sweep of settings, that feet per hectopascal rises monotonically and more than doubles between sea level and FL200, and that the cold error has the unsafe sign.
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