Aerodynamics
Mach number
At a glance
Meaning of Mach number
The ratio of aircraft speed through the air to the local speed of sound.
More about Mach number
Mach number compares two speeds and has no unit. The local speed of sound depends chiefly on air temperature.
It is not a fixed conversion from kilometres per hour at every altitude. Aircraft use the appropriate Mach and indicated-airspeed limits for their operating conditions.