Mach number is a dimensionless number representing the ratio of inertia force and elastic force.

It can be expressed with the following equation:

where;
v : Velocity [m/s]
a : Speed of sound [m/s]

Mach number indicates the degree of fluid compression.
In general, when Mach number > approx. 0.3, the effect of compression must be taken into account in calculations (Mach number 0.3 corresponds to density change of approx. 5%).

Mach number is named after an Austrian physicist Ernst Mach.