Calculators › Speed converter
IAS, TAS and Mach converter
Convert between IAS, TAS, Mach and groundspeed at any altitude and temperature.
Convert a speed
Type in any one of these three, and the others follow.
Crossover altitude
Airliners climb at a set IAS, then switch to a set Mach number. This is where the two meet.
Four speeds, one aircraft
| Speed | What it is | Who uses it |
|---|---|---|
| IAS | What the airspeed indicator reads. It reflects the air pressure on the aircraft, so it’s what the wings “feel”. | Pilots, and controllers giving speed instructions lower down |
| TAS | The real speed through the air. It’s higher than IAS as the air gets thinner. | Flight planning |
| Mach | The speed as a fraction of the speed of sound, which falls as the air gets colder. | Pilots and controllers at high level |
| Groundspeed | The speed over the ground: TAS plus or minus the wind. | Radar, and separation between aircraft |
Why the crossover altitude matters
Climbing at a steady IAS, the Mach number keeps rising. At the crossover altitude it reaches the planned figure, and the aircraft holds that Mach from then on, so its IAS starts to fall. In a descent, the same happens the other way round. A controller’s speed instruction has to make sense on the correct side of that line.
How accurate is it?
The calculator uses the standard compressible-flow equations and the International Standard Atmosphere. It treats IAS as calibrated airspeed, so it ignores the small instrument and position errors of a real aircraft. It only covers subsonic speeds. TAS depends on the temperature you give, while the link between IAS and Mach depends only on pressure altitude.
Quick answers
What is the difference between IAS and TAS?
IAS is what the airspeed indicator shows, and TAS is the real speed through the air. They match at sea level in standard air, and TAS climbs above IAS as the air thins, by roughly 2% per 1,000 ft.
How do you convert Mach to knots?
Multiply the Mach number by the speed of sound at your temperature. In standard air at FL350 that’s about 576 kt, so Mach 0.78 is about 450 kt TAS.
What is the crossover altitude?
The altitude where a planned IAS and a planned Mach number are the same speed. Climbing at 300 kt and Mach 0.78, it’s about 29,300 ft.
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For training and simulation only. These calculators are for learning and for games. Their results are automatic, and they don’t account for everything a real operation does. Never use them for real flight planning, navigation or air traffic control. Use your official charts, approved tools and procedures.
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