Altitude Converter

Altitude is measured in different units depending on the context.

Basic conversions:

• 1 foot = 0.3048 meters (exact)
• 1 meter = 3.28084 feet
• 1 kilometer = 3,280.84 feet

Flight Levels (FL): Flight levels are used in aviation above the transition altitude.

• Flight Level = altitude in feet / 100
• FL350 = 35,000 feet = 10,668 meters
• FL100 = 10,000 feet = 3,048 meters

Approximate pressure at altitude (standard atmosphere):

Those are the figures the calculator returns, and the same ones the air density converter works from.

Standard atmosphere pressure formula:

• Pressure (hPa) = 1013.25 × (1 − 0.0000225577 × altitude_m) ^ 5.25588

Pressure altitude, which is the one pilots actually need

Pressure altitude is what your altimeter reads with the subscale wound to the standard 1013.25 hPa (29.92 inHg). It is the figure that feeds density altitude, and therefore takeoff distance and climb rate, so it matters most on exactly the days you least want a surprise.

• Pressure altitude (ft) = field elevation (ft) + (1013.25 − QNH in hPa) × 27

Near sea level one hectopascal is worth about 27 feet, which falls straight out of the air’s density there: 1 hPa ÷ (1.225 kg/m³ × 9.807 m/s²) is 8.32 metres. You will also see “1,000 ft per inch of mercury” quoted, and it is a rougher rule, about 8% out, because an inch of mercury is 33.86 hPa and therefore closer to 920 feet.

On a low-pressure day the QNH drops, so pressure altitude climbs above your field elevation and the aeroplane performs as though the runway were higher than it is. A field at 1,000 ft with a QNH of 995 hPa gives 1,000 + 18.25 × 27, so it flies like 1,493 ft.

Aviation worldwide uses feet for altitude, except in China and some CIS countries which use meters.

The flight-level system is the clever bit worth understanding. Below a transition altitude every aircraft sets its altimeter to the local sea-level pressure to read true height, but above it everyone switches to the same standard setting of 1013.25 hPa and reports “flight levels” instead. The altitudes are no longer strictly true, but because every plane uses the identical reference, they all read consistently relative to each other, which is what keeps them safely stacked even as real atmospheric pressure rolls across the country.

That’s also why a barometric altimeter and your phone’s GPS altitude rarely agree. The altimeter infers height from air pressure, which shifts with the weather, so a passing low-pressure system makes it read high unless you reset it; GPS measures geometric height directly and ignores pressure entirely. The thinning of the air is the other thing the numbers hide: pressure roughly halves by 18,000 feet, which is why climbers gasp on high peaks and why airliner cabins are pressurized to feel like a comfortable 6,000 to 8,000 feet rather than the 35,000 outside.