Mach Number

What is Mach Number?

The coefficient term used for motion in the air is known as the Mach number. It denotes the ratio of flow of velocity after a certain limit of the sound's speed. In simplistic terms, it is the ratio of the object's speed to the surrounding space's sound speed. The Mach number determines the speed of the article by comparing it with the speed of sound. For many calculations and comparisons, considering the various fast speed aircraft and airplanes Mach Number is useful. The Mach number formula with examples will be learned in this article.

Concept of Mach Number

For explaining the concept in a simple manner; Mach number 1 speed is equated to the speed of sound. If Mach 0.75 denoted, it would be considered 75% of the sound speed, which is also known as subsonic. If Mach 1.65 is indicated, it will be regarded as 65% faster than the sound speed, also known as supersonic speed.   

The sound depends on the surrounding space at a specific pressure and temperature; also, we can determine whether the flow is incompressible with the help of the Mach Number.

The surrounding space can either be gas or liquid. 

The boundary will be stable, but the medium can be flowing, or the boundary can travel in a medium, meaning it is at a rest state. There can be a situation where both the medium and boundary are traveling with some speed.

The medium can be channeled through various devices like the wind tunnels or sometimes may immerse in the medium. As the ratio of two speeds expresses the number, Mach Number is considered as a dimensionless number.

Since it was worked upon by an Austrian Physicists and Philosopher by Ernst Mach, it has been named as Mach number. Since it is a dimensionless quantity and not a measurable unit, the word Mach is put before the number, like Mach 1 and not 1 Mach.

What is Mach Number in Physics?

Mach Number Definition - The ratio of the flow velocity after a certain limit of the sound speed is known as the Mach number. In simple terms, it is the ratio of the speed of the body to the sound's speed in the surrounding medium.

Mach Number Formula

The Mach Number formula is:

M = v/c


  • M = Mach number 

  • v= the local flow velocity limit

  • c= speed of sound in that medium

\[M_{a}\] = \[\frac{v}{c}\]

\[M_{a}\] → Mach Number

V → speed or velocity of an object.

c → speed or velocity of sound.

In simple terms, the speed of sound can be equated to Mach 1. Thus Mach 0.75 will be 75% speed of the sound, also known as subsonic speed, and Mach 1.65 will be 65% faster than the speed of sound, known as the supersonic speed.

The local speed of sound is dependent on the pressure and temperature of the surrounding medium. Mach number can also determine if the flow is incompressible. The medium can either be gar or liquid. There are many scenarios where the medium can be flowing, but the boundary is stable, or the medium and boundary are both traveling, meaning that medium is at rest. Even though the boundary and medium both are traveling at a certain speed, but their velocities concerning each other are what matters. The medium can be channeled through various devices like the wind tunnel or can be immersed in the medium. Since Mach number is the ratio of two speeds, it termed as a dimensionless quantity.  

Mach Number Classification

Below based on the ratio values, the Mach number is termed in various regimes.


The aerodynamic features like the rounded noses or the leading edges found in the commercial aircraft. They have a mach speed below 0.8.


The aircraft with swept wings built have a mach speed value of 0.8 – 2.1.


The aircraft has a definite design and a complete movement of thin aerofoil sections, canards, and sharp edges that can fly at supersonic speed with mach speed value between 1.2 and 5.0.


The aircraft that fly at Mach value of between 5.0 - 10.0 has many distinctive features like the small wings and nickel-titanium skin that is cooled. A world record was created by the U.S. plane X-15, which flew at a speed of Mach 6.72.


When flying at mach levels of 10.0 – 25.0, thermal controls become an important factor while designing as the hotness of the surface needs to be considered beforehand.

Re-Entry Speeds:

When the mach speed is above 25.0, its craft doesn't require wings but a blunt design.

Classification of Mech Regime








Re-entry speeds







10.0 - 25.0

6,615 - 16,537

7,680 - 19,031

12,251 - 30,626

3,403 - 8,508


5.0 - 10.0

3,308 - 6,615

3,806 - 7,680

6,126 - 12,251

1,702 - 3,403


1.3 - 5.0

794 - 3,308

915 - 3,806

1,470 - 6,126

410 - 1,702


0.8 - 1.3

530 - 794

609 - 914

980 - 1,470

273 - 409







FAQ (Frequently Asked Questions)

1. What is the Significance of Mach Number?

Ans Mach Number Significance – Mach number is required to describe the speed of an airplane in terms of speed of sound. It can be derived by dividing the airplane speed by the speed of sound. If a plane is traveling at the sound speed, it is described as a plane traveling at Mach 1 speed.

2. What is the Maximum Mach Number Possible?

Ans  The speed of light is the only theoretical limit to speed. But there are the second set of parameters to determine the speed like the temperature and the atmospheric conditions, since Mach number is the ratio of an object speed to the speed of sound, and sound speed varies a lot (as does the speed of light, but comparatively less over the range of conditions one might face for a moving vehicle).

If you want the theoretical maximum over the Mach number, you need as fast as possible the speed of light, which is a little smaller, and the medium which has a lower speed of sound. This is like a super high molecular density like radon gas, where the speed of light is mach 2.2 million. It stays at room temperature and pressure and as you cool down the speed of sound, the medium changes to liquid or solid. I did not even begin to identify non-primary gases - something with a molecular weight higher than radon (like Ognesen that was just named element 118).

Therefore, it's safe to say that the theoretical maximum is at least a Mach few million. Even if you get the right combo of temperature, pressure, and gas, there may be tens of millions.