Mach number and the speed of sound
Mach number compares an aircraft's true airspeed with the local speed of sound. Mach 1 means the airplane is traveling at the local sound speed. Mach 0.80 means its true airspeed is 80 percent of the local speed of sound.
The speed of sound is not one fixed speed
In air, the speed of sound depends primarily on temperature. Warmer air has a higher local sound speed and colder air has a lower one. That is why there is no single knots value that always equals Mach 1 throughout the atmosphere.
Mach-number formula
Mach number = true airspeed ÷ local speed of sound, using the same units for both. If true airspeed is 450 knots and the local speed of sound is 600 knots, the airplane is traveling at Mach 0.75.
Why altitude appears to matter
Altitude influences Mach calculations indirectly because atmospheric temperature changes with altitude. In the standard troposphere, temperature generally decreases as altitude increases, so local sound speed also decreases. The relationship changes in higher atmospheric layers where the temperature profile is different.
True airspeed versus indicated airspeed
Mach is based on the ratio between aircraft speed through the air and local sound speed, not simply the cockpit indicated airspeed number. High-speed aircraft use air-data systems that account for compressibility and provide Mach directly.
Why Mach limits exist
As airflow over parts of the aircraft approaches sonic speed, shock waves and compressibility effects can appear even when the airplane itself is below Mach 1. Aircraft may therefore have a maximum operating Mach number, or MMO, that protects handling qualities and structural limits.
Temperature example
If two aircraft fly at the same true airspeed in different air temperatures, they can have different Mach numbers. The aircraft in colder air is operating at a higher fraction of the local sound speed because sound travels more slowly there.
Use the aircraft's actual instruments
PilotDesk can demonstrate the relationship between temperature, sound speed, and Mach. Operational high-speed limits must come from the aircraft's approved instruments, AFM, and limitations—not a general calculator.
Related PilotDesk tools
ISA temperature · True airspeed estimate · Temperature conversion