Mach to True Airspeed Calculator

Why jets cruise at a constant Mach number, not constant speed

Mach number is directly tied to the local speed of sound, which itself depends on air temperature and therefore altitude - this is why high-altitude jets are flown at a constant Mach number for most of cruise, letting true airspeed vary naturally with changing atmospheric conditions.

Worked example

For Mach 0.78 at an altitude where the local speed of sound is 573 knots:

TAS = 0.78 x 573 = 446.9 kt

The same Mach 0.78 setting would correspond to a different true airspeed in knots at a different altitude or temperature, since the local speed of sound itself changes with those conditions - this is exactly why Mach number, not true airspeed, is the constant reference pilots and air traffic control use for high-altitude jet cruise.