Mach Number Speed Altitude Calculator

Mach number from speed and altitude: Ma = v / v_sound(T)

The Mach number compares true airspeed to the local speed of sound: Ma = v / v_sound. How fast sound travels in air comes down to absolute temperature, and in the ISA standard atmosphere that temperature drops as you climb. At sea level (20 °C) you get v_sound ≈ 343 m/s, while at FL350 (~−50 °C) it's down to v_sound ≈ 297 m/s. The practical upshot is that the same true airspeed reads as a higher Mach number up high. Airliners cruise around Mach 0.78–0.85, which is the transonic regime. Push past Mach 1 and a shock cone forms, the source of the sonic boom that keeps supersonic flight off-limits over land. For perspective, the Concorde cruised at Mach 2.04, the SR-71 Blackbird reached Mach 3.2, and anything above Mach 5 counts as hypersonic.

Applications

You'll see it in aeronautical engineering when designing transonic and supersonic aircraft, in missile and rocket guidance, in the ballistics of supersonic projectiles, in atmospheric studies of how sound propagates, and in the aerodynamics of reentry vehicles.

FAQ

Why does Mach 1 depend on altitude? The speed of sound tracks temperature, and ISA temperature drops about 6.5 °C per km right up to the tropopause. Colder air carries sound more slowly, so the same true airspeed ends up reading as a higher Mach.

What is critical Mach number? It's the free-stream Mach at which airflow over some part of the wing first hits Mach 1. Go beyond it and shockwaves appear while drag climbs sharply, an effect known as drag divergence.

Why was the Concorde retired? Several things piled up at once: heavy fuel burn, the restrictions on overland sonic booms, the 2000 Gonesse crash, and the collapse in demand after 9/11. Together they left supersonic commercial flight running at a loss.