Mach Number to Speed Converter
Convert Mach numbers to speed at various altitudes.
Results
Mach numbers express speed relative to the speed of sound but the actual speed in km/h or mph depends on altitude. Sound travels slower in thinner and colder air at high altitudes - Mach 1 is 1,235 km/h at sea level but only 1,062 km/h at 40,000 feet. This calculator accounts for altitude so you get accurate speed conversions whether you are reading about fighter jets or supersonic passenger flight.
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Frequently asked questions
At sea level, Mach 1 is about 767 mph (1,235 km/h). At cruising altitude of 35,000-40,000 feet, it drops to about 660 mph because the air is colder and thinner.
The speed of sound depends on air temperature. At higher altitudes the air is much colder (about -57C at 40,000 ft vs 15C at sea level), so sound waves travel slower.
Most commercial jets cruise at Mach 0.78 to 0.85, which is about 550-590 mph at cruising altitude. This is just below the transonic range where drag increases sharply.
At sea level, Mach 2 is about 1,534 mph (2,469 km/h). The Concorde cruised at Mach 2.04 at 60,000 feet, which was about 1,350 mph at that altitude.
The SR-71 Blackbird holds the record at Mach 3.3 (about 2,193 mph at altitude). The X-15 rocket plane reached Mach 6.7 but was an experimental aircraft, not operational.