Mach 1 is the speed of sound in the air around you. On a standard day at sea level (15 °C), that’s about 340 m/s, 1,225 km/h or 761 mph. It isn’t a fixed speed, though. The speed of sound depends on air temperature, so at a typical airliner cruising altitude of 11 km, where the standard atmosphere is −56.5 °C, Mach 1 is only about 1,062 km/h, or 660 mph.
What is Mach 1 in different units? #
At sea level in the standard atmosphere:
| Unit | Mach 1 |
|---|---|
| Meters per second | 340.3 m/s |
| Kilometers per hour | 1,225 km/h |
| Miles per hour | 761 mph |
| Knots | 661 kn |
| Feet per second | 1,116 ft/s |
NASA’s Glenn Research Center gives the sea-level figure as “about 761 mph or 1100 feet/second” in its speed of sound explainer. The more precise 1,116 ft/s comes from the formula below.
Why does the speed of sound change? #
Sound is a pressure wave passed from molecule to molecule, and warmer molecules move faster, so they pass it on faster. NASA’s page puts it simply: the speed of sound in air “depends on the type of gas and the temperature of the gas.” The formula is:
a = √(γ × R × T)
- γ (gamma) is the ratio of specific heats, about 1.4 for air.
- R is the specific gas constant for air, about 287.05 J/(kg·K).
- T is the absolute temperature in kelvin.
At 15 °C (288.15 K): √(1.4 × 287.05 × 288.15) = 340.3 m/s. At 20 °C it’s 343.2 m/s. At −40 °C it drops to 306.1 m/s. Air pressure on its own doesn’t change the result. Temperature is what matters, and temperature changes with altitude.
Speed of sound by altitude #
The International Standard Atmosphere is a model that assumes air cools by 6.5 °C per kilometer up to 11 km, then stays at −56.5 °C up to 20 km. Real days vary, but this is the reference aviation uses.
| Altitude | Standard temperature | Mach 1 (m/s) | Mach 1 (km/h) | Mach 1 (mph) |
|---|---|---|---|---|
| Sea level | 15 °C | 340.3 | 1,225 | 761 |
| 1,000 m | 8.5 °C | 336.4 | 1,211 | 753 |
| 3,000 m | −4.5 °C | 328.6 | 1,183 | 735 |
| 5,000 m | −17.5 °C | 320.5 | 1,154 | 717 |
| 10,000 m | −50 °C | 299.5 | 1,078 | 670 |
| 11,000 to 20,000 m | −56.5 °C | 295.1 | 1,062 | 660 |
So Mach 1 is about 13% slower in the stratosphere than at sea level.
What does a Mach number mean? #
A Mach number is a ratio: the object’s speed divided by the local speed of sound. Mach 0.5 is half the speed of sound, Mach 2 is twice. Engineers group speeds roughly like this:
| Range | Mach number |
|---|---|
| Subsonic | below about 0.8 |
| Transonic | about 0.8 to 1.2 |
| Supersonic | above 1 and below about 5 |
| Hypersonic | above about 5 |
Because it’s a ratio, the same Mach number means different ground speeds at different altitudes. Mach 2 at sea level would be about 1,522 mph, while Mach 2 at 11 km is about 1,320 mph.
Worked examples #
An airliner cruises at Mach 0.85 at 11,000 m. 0.85 × 295.1 m/s = 250.8 m/s, which is 903 km/h or 561 mph through the air. Its speed over the ground also depends on the wind.
A jet flies Mach 1.5 at 10,000 m. 1.5 × 299.5 = 449 m/s, or 1,617 km/h (1,005 mph).
Counting the seconds after lightning. Sound travels about 343 m/s at 20 °C, so it takes roughly 3 seconds to cover a kilometer and about 5 seconds to cover a mile. Count the seconds between flash and thunder and divide by 3 for kilometers or by 5 for miles.
Why do unit converters use a fixed Mach value? #
Converting Mach to mph needs a temperature, and a unit converter doesn’t know where you are. So converters that include Mach use a stated reference, usually sea level in the standard atmosphere. That’s the right answer for sea-level conditions and roughly 13% too high for high-altitude flight. For altitude, multiply the Mach number by the speed of sound from the table above.
Convert Units Calculator handles it this way. Version 1.0.5 adds a Speed unit labeled “Mach (sea level)” that uses 340.29 m/s, next to m/s, km/h, mph, feet per second and knots, so the reference is in the name rather than hidden. Type 1 Mach and it lists 1,225 km/h and 761 mph together.
For everyday speed conversions, see mph to km/h. The kelvin temperatures in the formula come from the Celsius figures in the table, and we explain the scales in Celsius to Fahrenheit formula. Knots and nautical miles are covered in km to miles.
Frequently asked questions #
Is Mach 1 the speed of sound? #
Yes. Mach 1 means moving at the local speed of sound. Anything faster is supersonic and anything slower is subsonic.
How fast is Mach 1 in km/h? #
About 1,225 km/h at sea level on a standard 15 °C day. At the cold air of 11 km altitude, it’s about 1,062 km/h.
How fast is Mach 10? #
Using the sea-level value, Mach 10 is about 3,403 m/s, 12,250 km/h or 7,612 mph. Hypersonic vehicles fly high, where the air is colder, so their actual speed at Mach 10 is lower than that.
Does the speed of sound depend on altitude? #
Indirectly. It depends on air temperature, and temperature usually drops with altitude up to about 11 km. Pressure and density on their own don’t change it.
Why is Mach 1 slower at high altitude? #
Because the air is colder. In the standard atmosphere it’s −56.5 °C at 11 km, and sound travels about 295 m/s in air that cold, compared with 340 m/s at 15 °C.