# Mach number

The **Mach number** (**Ma**) is a dimensionless quantity representing the ratio of velocity to to the local speed of sound

where

- is the Mach number
- is the local flow velocity
- is the speed of sound in the medium.

By definition, at Mach 1, the local flow velocity is equal to the speed of sound. If the Mach number is less than 1, the flow is classified as **subsonic**. If the Mach number is greater than 1, the flow is classified as **supersonic**.

The local speed of sound, and thereby the Mach number, depends on the properties of the surrounding medium, in particular the temperature. The Mach number is primarily used to determine the approximation with which a flow can be treated as an incompressible flow. The medium can be a gas or a liquid. If < 0.2–0.3 and the flow is steady and isothermal, compressibility effects will be small and simplified incompressible flow equations can be used.