Mach Number

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What is Mach Number?

A Mach number is the ratio of an object's speed to the local speed of sound in the surrounding medium. Mach 1 means the object is travelling at the speed of sound. Mach 0.5 is half the speed of sound; Mach 2 is twice the speed of sound.

The speed of sound varies with temperature and the medium it travels through. At sea level and 15°C in air, Mach 1 is approximately 340.3 m/s (1,225 km/h or 761 mph). At cruising altitude (~10,000 m), where temperatures reach around -50°C, Mach 1 drops to about 295 m/s (1,062 km/h). This is why airliner cruise speeds are quoted in Mach - the aircraft's aerodynamic behaviour depends on Mach number, not absolute speed.

When to use Mach Number

Use Mach numbers when describing aircraft performance, particularly for jet aircraft, supersonic vehicles, and anything where compressibility effects in air matter (above approximately Mach 0.3). For subsonic vehicles like cars, trains, and ships, use km/h, mph, or knots instead.

Worked examples for Mach Number

This table quickly gives you the overview you need to understand Mach Number and its most important comparisons.

Common pitfalls

Mach 1 is not a fixed speed - it changes with air temperature. At 10,000 m altitude, Mach 1 is about 295 m/s (1,062 km/h), significantly slower than the 340 m/s at sea level. Never convert a Mach number to km/h without specifying the altitude and temperature conditions.

Frequently asked questions about Mach Number

What is the difference between subsonic, transonic, supersonic, and hypersonic?

Subsonic: Mach below 0.8. Transonic: Mach 0.8–1.2 (shock waves begin forming). Supersonic: Mach 1.2–5. Hypersonic: Mach 5 and above. The Concorde cruised at Mach 2; the Space Shuttle re-entered at around Mach 25.

Who is Mach named after?

Ernst Mach, an Austrian physicist (1838–1916) who studied the shock waves formed by projectiles moving faster than sound. The unit was named in his honour by the aeronautical engineer Jakob Ackeret in 1929.

Test your knowledge

Quiz: how well do you know Mach number?

5 questions · ~2 min

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1. What does a Mach number represent?

A The absolute speed of an aircraft in km/h, measured at a standard reference altitude of 10,000 meters to enable consistent comparison between aircraft types B The ratio of an aircraft's ground speed to its cruising airspeed, used to measure efficiency relative to its design operating range C The ratio of an object's speed to the local speed of sound in the surrounding medium, so Mach 1 means travelling at exactly the speed of sound D The percentage of the speed of light at which an object travels, expressed as a decimal fraction used in both aeronautics and relativistic physics

ℹ The definition states a Mach number is the ratio of an object's speed to the local speed of sound in the surrounding medium. Mach 1 means the object is travelling at the speed of sound; Mach 0.5 is half the speed of sound; Mach 2 is twice.

2. Why do commercial airliners quote cruise speed in Mach rather than km/h?

A Because aerodynamic behaviour depends on Mach number, not absolute speed - the same true airspeed produces very different aerodynamic effects at high altitude (where Mach 1 is lower) versus at sea level B Because international air traffic control regulations require all flights to report speed in Mach numbers above 25,000 feet to maintain standard separation distances C Because km/h measurements require GPS correction for wind speed and direction, which introduces errors at high altitude that Mach number inherently avoids D Because Mach numbers are proportional to fuel consumption, making them the natural metric for flight planning and fuel efficiency monitoring

ℹ The definition explains that an airliner's aerodynamic behaviour depends on Mach number, not absolute speed - which is why cruise speeds are quoted in Mach. At cruising altitude (~10,000m), Mach 1 drops to about 1,062 km/h, significantly below the sea-level value of 1,225 km/h.

3. According to the examples table, at what Mach number do commercial airliners typically cruise, and what is the km/h equivalent at sea-level reference conditions?

A Mach 0.5 = 612.5 km/h - a subsonic speed balancing fuel efficiency with passenger comfort B Mach 1 = 1,225 km/h - transonic speed that minimises journey time while staying within engine design limits C Mach 2 = 2,450 km/h - the same speed the Concorde used for supersonic commercial operations D Mach 0.85 = 1,041 km/h - just below the transonic range where shock waves begin forming on control surfaces

ℹ The examples table shows commercial airliners cruise at Mach 0.85, equivalent to 1,041 km/h at sea-level reference conditions. This places them just below the transonic range (Mach 0.8-1.2) where shock waves begin to form.

4. What does the pitfalls section warn about converting Mach numbers to km/h?

A Mach numbers above 1.0 are only valid for military aircraft; commercial aviation regulations prohibit supersonic flight because of sonic boom noise regulations B Mach 1 is not a fixed speed - it changes with air temperature, so at 10,000m altitude Mach 1 is about 1,062 km/h, significantly slower than 1,225 km/h at sea level; never convert a Mach number to km/h without specifying altitude and temperature conditions C The Mach number on flight instruments is calibrated airspeed that already compensates for pressure differences at altitude and does not need temperature correction D Mach measurements become unreliable above Mach 3 because shock wave interactions alter the pressure readings that instruments rely on to compute the speed ratio

ℹ The pitfalls section states Mach 1 is not a fixed speed - it changes with air temperature. At 10,000m altitude with temperatures around -50°C, Mach 1 is about 295 m/s (1,062 km/h), significantly slower than 340 m/s at sea level. The altitude and temperature must always be specified.

5. What are the correct Mach number ranges for subsonic, transonic, supersonic, and hypersonic flight?

A Subsonic below Mach 1.0; Supersonic Mach 1.0-3.0; Hypersonic Mach 3.0+; transonic is not a recognised aviation category B Subsonic below Mach 0.8; Transonic Mach 0.8-1.5; Supersonic Mach 1.5-5.0; Hypersonic Mach 5+; Concorde cruised at Mach 2.0 C Subsonic below Mach 0.8; Transonic Mach 0.8-1.2 (shock waves begin forming); Supersonic Mach 1.2-5; Hypersonic Mach 5 and above - the Space Shuttle re-entered at around Mach 25 D Subsonic below Mach 0.5; Transonic Mach 0.5-1.0; Supersonic Mach 1.0-4.0; Hypersonic Mach 4.0+

ℹ The FAQ gives the exact ranges: Subsonic below Mach 0.8, Transonic Mach 0.8-1.2 (where shock waves begin forming), Supersonic Mach 1.2-5, and Hypersonic Mach 5+. It notes the Concorde cruised at Mach 2 and the Space Shuttle re-entered at around Mach 25.

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