Mach Number

Definition

How fast something moves compared to the speed of sound — Mach 2 means twice the speed of sound.

$M = \frac{v}{c}$

Symbol: $M$ · Unit: dimensionless

Technical definition

Dimensionless ratio M = v/c, where v is the object's speed and c is the local speed of sound. M < 1 is subsonic, M = 1 transonic, M > 1 supersonic, M > 5 hypersonic. The speed of sound varies with temperature and medium.

Example

A jet at Mach 2 at high altitude travels about 2,100 km/h — but the same Mach number near the ground (where sound is faster in warmer air) means a different absolute speed.

Common confusion

Mach number is not a fixed speed — it depends on the local speed of sound, which changes with temperature and altitude. Mach 1 at sea level (~343 m/s) differs from Mach 1 at 10 km altitude (~299 m/s).