What Is Mach 1 in km/h and mph? Why the Number Changes

The short answer

Mach 1 is the speed of sound in the air around the object. At sea level on a standard day (15 °C), that is about 340 m/s, or roughly 1,225 km/h (761 mph). Because the speed of sound depends on air temperature, Mach 1 is slower high up where the air is cold, about 1,062 km/h at typical airliner cruising heights. The Speed Converter uses a fixed value of 343 m/s (1,234.8 km/h), so treat its Mach results as sea-level approximations.

What a Mach number is

A Mach number is a ratio, not a fixed speed:

Mach number = speed of the object ÷ speed of sound in the surrounding air

• Below Mach 1: subsonic
• About Mach 0.8 to 1.2: transonic (parts of the airflow over a wing can be supersonic even if the aircraft is not)
• Above Mach 1: supersonic
• Above about Mach 5: commonly called hypersonic

Because it is a ratio, "Mach 2" means twice the local speed of sound, whatever that happens to be at that height and temperature.

Why Mach 1 is not one fixed number

The speed of sound in air depends mainly on temperature: warmer air carries sound faster. NASA's educational material gives 340 m/s for sea level on a standard 15 °C day and notes that the speed of sound rises as air temperature increases.

For dry air, the speed of sound can be estimated with a = √(γ × R × T), where γ = 1.4, R = 287.05 J/(kg·K) and T is the temperature in kelvin. Using that formula:

Air temperature	Speed of sound	In km/h
-56.5 °C (standard air at about 11 km and above)	295.1 m/s	1,062
0 °C	331.3 m/s	1,193
15 °C (standard sea level)	340.3 m/s	1,225
20 °C	343.2 m/s	1,236
30 °C	349.0 m/s	1,257
40 °C	354.7 m/s	1,277

Pressure on its own does not change the speed of sound much; altitude matters because the air gets colder as you climb in the lower atmosphere.

Converting Mach to km/h and mph

Multiply the Mach number by the speed of sound you are assuming.

Using the standard sea-level value (340.3 m/s)

• km/h = Mach × 1,225
• mph = Mach × 761
• knots = Mach × 661

Example: Mach 2 at sea level

2 × 1,225 = 2,450 km/h (about 1,522 mph).

Example: Mach 0.85 at cruising altitude

At -56.5 °C the speed of sound is about 295.1 m/s (1,062 km/h). 0.85 × 1,062 = about 903 km/h. The same Mach 0.85 at sea level would be 0.85 × 1,225 = 1,041 km/h. This is why pilots and engineers quote Mach number rather than km/h at high altitude: the aircraft's behaviour depends on the ratio, not the ground speed.

Example: km/h to Mach

A car travelling at 1,225 km/h at sea level on a 15 °C day is at about Mach 1.0. At 900 km/h it would be 900 ÷ 1,225 ≈ Mach 0.73.

What the Speed Converter does

The Speed Converter lists Mach (sea level) alongside km/h, mph, m/s, knots and feet per second. Its Mach unit is fixed at 343 m/s = 1,234.8 km/h, which is the speed of sound in dry air at about 20 °C.

Mach	km/h	mph	m/s	knots
0.8	987.84	613.82	274.4	533.39
1	1,234.8	767.27	343	666.74
2	2,469.6	1,534.54	686	1,333.48
5	6,174	3,836.36	1,715	3,333.69

These are the values the converter shows. They are about 0.8% higher than the 15 °C standard-day figures above. For school problems that state "take the speed of sound as 343 m/s" the converter gives exactly the expected answer. If your textbook or question uses 340 m/s or 330 m/s, multiply by that value instead, or convert m/s with the converter and multiply yourself.

How to use it

Type the Mach number in Value to convert.

Set From to Mach (sea level).

Choose km/h, mph or another unit as the target, or read every unit from the table below the result.

To go from km/h to Mach, set From to Kilometers/hour and the target to Mach.

Common misunderstandings

• "Mach 1 is 1,235 km/h everywhere." Only at about 20 °C. It is lower in colder air.
• "A sonic boom happens only at the moment the aircraft passes Mach 1." A supersonic aircraft produces a shock wave continuously while it flies faster than sound; people on the ground hear the boom as that shock wave passes over them.
• "Mach number depends on wind." Mach number compares the aircraft's speed through the air with the speed of sound in that air. Ground speed, which wind does change, is a different measurement.
• Mixing up knots and km/h. Aviation speeds are often given in knots. For that conversion, see Knots to km/h and mph.

Quick reference

• Mach 1 at 15 °C sea level: 340.3 m/s, about 1,225 km/h, 761 mph
• Mach 1 in the Speed Converter: 343 m/s, 1,234.8 km/h, 767.27 mph
• Mach 1 at about 11 km altitude (standard atmosphere): about 295 m/s, 1,062 km/h

For converting between m/s and km/h by hand, the rule is to multiply by 3.6; see How to Convert km/h to m/s.