✈️ Aviation

Knots to Mach

Convert knots, kilometres per hour, miles per hour or metres per second to Mach, using the speed of sound at your air temperature. Press Convert.

Written and reviewed by SM Arijul Islam · Last reviewed 28 October 2026 · Editorial standards · Report an error

About the Knots to Mach

Mach is a ratio, not a fixed speed. Mach 1 is the local speed of sound, which is about 340.3 m/s at 15 °C at sea level but falls to about 295 m/s at −56 °C in the lower stratosphere. That is why an airliner cruising at 480 knots is around Mach 0.85 while a fighter at the same indicated speed near sea level is closer to Mach 0.72.

This converter uses the standard dry-air relation a = 331.3 × √(1 + T ÷ 273.15) m/s, where T is in °C. It is accurate enough for flight-planning sanity checks, not for certification. Above about Mach 0.3, compressibility effects matter; above Mach 1, use the actual temperature profile and aircraft data.

How the calculation works

Speed of sound a = 331.3 × √(1 + T ÷ 273.15) m/s, with T in °C.
1 knot = 0.514444 m/s = 1.852 km/h.
1 mph = 0.44704 m/s; 1 km/h = 0.277778 m/s.
Mach number = speed ÷ a.

Reference values

Speed of sound at common temperatures
Air temperature	Speed of sound	Mach 0.8	Mach 1
−56 °C (FL350)	295.4 m/s	459 kt	574 kt
−20 °C	319.2 m/s	496 kt	621 kt
0 °C	331.3 m/s	515 kt	644 kt
15 °C (ISA sea level)	340.3 m/s	529 kt	661 kt
30 °C	349.5 m/s	543 kt	679 kt

Worked example: the same speed at two altitudes

At sea level on a 15 °C day, the speed of sound is 340.3 m/s, or 661.5 knots. A jet at 480 knots is Mach 0.726. At 35,000 ft and −56 °C, the speed of sound is 295.4 m/s (574 knots), so the same 480 knots is Mach 0.836.

That is why Mach number is not a fixed speed. It depends on the local temperature, and temperature falls with altitude in the troposphere. An aircraft can hold a constant true airspeed while its Mach number rises in colder air, which is one reason high-altitude cruise involves a Mach limit as well as a speed target.

Frequently asked questions

Is Mach 1 always 767 mph?

Only at about 15 °C at sea level. In cold air at altitude the speed of sound is lower, so Mach 1 is slower in metres per second.

What is indicated airspeed versus Mach?

Indicated airspeed is a pressure measurement that falls with altitude. Mach is the true airspeed divided by the local speed of sound. They are related but not interchangeable.

Does humidity matter?

Slightly. Humid air is less dense and the speed of sound rises a little. For most flight-planning checks the temperature effect dominates.

Why does Mach number change with altitude?

Because the speed of sound depends on temperature, and temperature falls with altitude in the troposphere. True airspeed can stay the same while the Mach number rises in colder air.

Sources and standards

A knot is exactly 1,852 metres per hour by international agreement. Mach is a ratio to the local speed of sound, which falls as temperature drops, so the Mach figures here assume the standard atmosphere rather than a fixed speed.