How fast is Mach 2 in mph, knots and km/h?

Mach 2 is approximately 1,522 mph, 1,323 knots or 2,450 km/h under International Standard Atmosphere conditions at sea level. It means twice the local speed of sound, rather than one fixed speed. In cold ISA air at −56.5°C, common around high-altitude supersonic cruise, Mach 2 is closer to 1,320 mph.

Mach 2 conversion table

For Aviation & Real-World Flying calculations, use the sea-level figure for a general conversion and the colder-air figure when discussing high-altitude supersonic aircraft. These values are rounded.

Why does Mach 2 change with altitude?

Mach 2 changes in mph, knots and km/h because Mach number is a ratio to the local speed of sound. That speed depends chiefly on absolute air temperature: colder air has a lower speed of sound, so travelling at Mach 2 requires a lower true airspeed.

A mistake we see constantly is doubling 761 mph—the approximate speed of sound at ISA sea level—and treating 1,522 mph as universal. It is valid only for those atmospheric conditions. Our guide to how Mach differs from indicated airspeed explains why aircraft switch to Mach references during high-altitude flight.

Are those knots true or indicated airspeed?

The knot figures are true airspeed, or KTAS, because Mach multiplied by the local speed of sound gives speed through the surrounding air mass. An aircraft flying at Mach 2 in cold ISA air is therefore travelling at roughly 1,147 KTAS, not 1,147 KIAS.

Indicated airspeed will be much lower at altitude and is affected by pressure, density and compressibility corrections. For the unit definitions and exact conversion factors, see our explanation of knots, nautical miles and aviation speed.

Does Mach 2 equal ground speed?

No. Mach 2 describes speed relative to the air mass, while ground speed depends on wind as well. In still air, true airspeed and ground speed match; a direct 100-knot tailwind added to 1,147 KTAS would produce approximately 1,247 knots over the ground.

Crosswinds require vector calculation rather than simple addition. Our account of Concorde’s speed and Mach 2 cruise provides a practical real-aircraft reference.

How do you calculate Mach 2 at another temperature?

You can calculate Mach 2 from static air temperature using the dry-air approximation below:

1. Convert Celsius to kelvin: T = °C + 273.15.
2. Calculate the local speed of sound: a = √(1.4 × 287.05 × T) metres per second.
3. Multiply by two: Mach 2 speed = 2 × a.
4. Convert the result: multiply metres per second by 2.23694 for mph, 1.94384 for knots or 3.6 for km/h.

Use static air temperature, not total air temperature. At Mach 2, ram heating makes total air temperature substantially warmer and would produce the wrong conversion.