How do you convert mph to Mach?

Mach number is defined as the ratio of the object's speed to the local speed of sound. So Mach = speed_mph / speed_of_sound_mph. Because the speed of sound changes with altitude and temperature, you should pick the correct reference condition.

What speed of sound does this tool use?

By default it uses standard atmosphere at sea level, about 761.2 mph. You can change to typical flight levels (10,000–36,000 ft) or enter a custom speed of sound.

Why does Mach change with altitude?

The speed of sound depends on the temperature of the air. At higher altitudes, standard atmosphere is colder, so the speed of sound is lower and the same mph corresponds to a higher Mach number.

MPH to Mach Converter | Convert Miles per Hour to Mach

Convert speed

Speed (mph) input

Atmosphere / condition

Result

Mach number: –

Speed of sound used: –

Note: Mach is unitless. Here we assume dry air, standard atmosphere at the chosen altitude.

Need the reverse? Try Mach to MPH.

Formula used

Mach number is defined as

Mach = V / a

where:

V = object speed (mph, in this tool)
a = local speed of sound (mph)

Why does the speed of sound change?

The speed of sound in air depends mainly on temperature. Colder air → lower speed of sound → same mph corresponds to a higher Mach.

Typical standard-atmosphere values (mph)

Sea level: ~761.2 mph
10,000 ft: ~734 mph
20,000 ft: ~707 mph
30,000 ft: ~679 mph
36,000 ft (≈11 km): ~661 mph

FAQ

Is Mach always based on mph?

No. Mach is a ratio, so the units cancel out. We use mph here because your input is in mph, but you could do the same with km/h or m/s as long as both speeds use the same unit.

How accurate is this?

For general educational, hobbyist and quick aviation-style calculations, using ISA (International Standard Atmosphere) values like these is perfectly fine. For performance-critical calculations, use real-time temperature.

Can Mach be less than 1?

Yes. Mach < 1 is subsonic, Mach ≈ 1 is transonic, Mach 1–5 is supersonic, Mach > 5 is hypersonic.