Convert Airspeed
IAS is treated as CAS: no instrument or position error. Standard atmosphere (ISA), no temperature deviation.
Crossover Altitude
The crossover altitude is the height where a calibrated airspeed and a Mach number give the same true airspeed. Pilots climb at a fixed indicated speed until it equals the target Mach, then hold Mach.
Worked Examples
The same speeds run through the converter above. Pick any row to load it into the tool.
| Altitude | IAS / CAS | Mach | TAS |
|---|---|---|---|
| 0 ft | 250.0 kt | 0.378 | 250.0 kt |
| 10,000 ft | 250.0 kt | 0.452 | 288.7 kt |
| 30,000 ft | 300.0 kt | 0.791 | 465.9 kt |
| 35,000 ft | 280.0 kt | 0.821 | 473.4 kt |
| 39,000 ft | 250.0 kt | 0.806 | 462.3 kt |
| 35,000 ft | 264.4 kt | 0.780 | 449.6 kt |
| 41,000 ft | 253.4 kt | 0.850 | 487.5 kt |
How This Is Calculated
Static pressure comes from the standard atmosphere: it starts at 101,325 Pa at sea level and falls with height, using the same temperature model as our Mach converter, with the tropopause at 36,089 ft. Calibrated airspeed becomes impact pressure, the push of air into the pitot tube, and that push measured against the static pressure at your altitude gives the Mach number. True airspeed is Mach times the local speed of sound.
These are the same standard-atmosphere and airspeed equations EUROCONTROL uses in its BADA aircraft performance model. Real air runs warmer or colder than standard, so treat the results as the standard-day baseline. The formulas hold below Mach 1.
Questions People Ask
What Is the Difference Between IAS, CAS and TAS?
Indicated airspeed (IAS) is what the airspeed indicator shows, worked out from the pressure of air ramming into the pitot tube. Calibrated airspeed (CAS) is IAS corrected for small errors in the instrument and in where the sensors sit on the airframe. On an airliner the gap is usually a few knots, so this tool treats the two as the same. True airspeed (TAS) is how fast the plane actually moves through the air around it. On a standard day at sea level, CAS and TAS match. As the plane climbs they drift apart.
Why Does True Airspeed Go Up With Altitude at the Same Indicated Airspeed?
The airspeed indicator measures how hard the air pushes into the pitot tube, and thin air pushes less. At sea level, 250 kt CAS is 250 kt true. At 10,000 ft the air is about 26% less dense, so the plane has to move faster to get the same push: the same 250 kt on the dial is about 289 kt true.
What Is Crossover Altitude?
It is the altitude where a given calibrated airspeed and a given Mach number work out to the same true airspeed. Airliners climb at a fixed indicated speed, and as they climb that speed is worth a higher and higher Mach number. When it reaches the target Mach, the pilots switch over and hold Mach for the rest of the climb. For 300 kt CAS and Mach 0.78, the crossover in the standard atmosphere is about 29,300 ft, where both mean about 461 kt true.
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