Work out mach number at altitude instantly with clear inputs, formula shown and shareable results.
Mach number is true airspeed divided by the local speed of sound, and the speed of sound depends only on absolute temperature: a = sqrt(gamma x R x T). Because air gets colder with altitude, the same true airspeed gives a higher Mach number the higher you fly, which is why high-level cruise is Mach limited rather than airspeed limited.
Mach number
M = TAS / a, a = sqrt(1.4 x 287.053 x T) with T in kelvin
Aviation results are planning estimates only. Always verify take-off, landing, fuel, weight and balance figures against the certified Aircraft Flight Manual, current performance charts and applicable aviation regulations before flight.
The speed of sound falls with temperature, and temperature falls about 2 degrees per 1,000 ft in the troposphere, so the same TAS becomes a larger fraction of the local speed of sound.
The level where the climb speed schedule changes from a constant indicated airspeed to a constant Mach number, because Mach becomes the limiting factor above it.