Mach & Altitude Converter
Altitude
Set it once here, in feet, meters, or flight level. It drives the Mach conversion below and the local speed of sound and ISA temperature shown here.
Speed ↔ Mach
Mach is a ratio, not a fixed speed: it's how fast you're going relative to the speed of sound around you. Climb higher and the air gets colder, sound travels slower through cold air, so the same true airspeed reads as a higher Mach number up high than it does at sea level.
How this is calculated
This uses the International Standard Atmosphere: a fixed model where sea level starts at 15°C and temperature drops in a straight line, about 1.98°C per 1,000 ft, up to the tropopause at 36,089 ft (11,000 m). Above that, in the lower stratosphere, it holds steady at -56.5°C. The speed of sound is derived from that temperature:a = 38.97 × √T(K), from a = √(γRT) with γ = 1.4 and R = 287.05 J/(kg·K).
It's the model every performance chart and flight manual is built on, and it's a standard-day approximation, not the actual weather outside the window. Real air runs warmer or colder than ISA depending on the day and the latitude, and Mach depends on the actual local temperature, not the standard-day one. So the same true airspeed can read a slightly different Mach number on a hot day over the equator versus a cold one over Siberia, even at the identical altitude. Treat these numbers as the standard-day baseline, not a live reading from row 34.
Curious why airliners settle into the 35,000 to 38,000 ft band in the first place? We wrote about how high planes actually fly.
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