Mach Number
A dimensionless speed ratio equal to the speed of an object or fluid divided by local speed of sound in the surrounding medium.
Governing formula
M = v / v_sound
SI unit
Dimensionless
In depth
Subsonic flow ($M < 1$), transonic ($M ≈ 1$), supersonic ($M > 1$), and hypersonic ($M > 5$). Specifies compressible fluid dynamics behavior.
Examples in the real world
Supersonic fighter jets flying at Mach 2 ($~2400 km/h$ at sea level).