Speed of Sound by Altitude Calculator
Mach 1 in knots, km/h, mph and m/s at any altitude — troposphere and stratosphere handled correctly, with the −2.4 kt-per-1,000-ft rule shown.
From 661 kt at sea level, sound slows ~2.4 kt per 1,000 ft until the tropopause, then holds at 573 kt through the isothermal stratosphere. Concorde's M2.0 was 'only' 1,147 kt for exactly this reason.
Formula
⚠️ For flight planning and education only — verify with your POH/AFM, certified instruments and official sources. Not for primary navigation or airworthiness decisions.
Mach 1 in knots, km/h, mph and m/s at any altitude — troposphere and stratosphere handled correctly, with the −2.4 kt-per-1,000-ft rule shown.
About Speed of Sound by Altitude Calculator
“The speed of sound” is a moving target: 661 knots at a standard beach, 573 in the stratosphere, and temperature — nothing else — decides it. This calculator returns Mach 1 at any altitude in four unit systems, models the tropopause correctly (the slowing stops at 36,089 ft where ISA goes isothermal), and accepts a temperature deviation for real days. A reference tool for students, simmer flight planning and settling aviation arguments.
How to use Speed of Sound by Altitude Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula a = √(γRT) = 38.97√(T K) kt; ISA: T = 15 − 1.98·(h/1000) °C, floored at −56.5 °C (tropopause) substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Speed of Sound by Altitude Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula a = √(γRT) = 38.97√(T K) kt; ISA: T = 15 − 1.98·(h/1000) °C, floored at −56.5 °C (tropopause) with sources cited on the page
- ✓From 661 kt at sea level, sound slows ~2.4 kt per 1,000 ft until the tropopause, then holds at 573 kt through the isothermal stratosphere. Concorde's M2.0 was 'only' 1,147 kt for exactly this reason.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
How fast is the speed of sound at 35,000 ft?+
About 576 knots (1,067 km/h, 296 m/s) on a standard day, where the temperature is −54 °C. That's 85 knots slower than at sea level — entirely because of the 69-degree temperature drop, not the thinner air. An airliner's M0.78 there is 449 kt of true airspeed.
Why doesn't air pressure affect the speed of sound?+
Higher pressure packs more molecules (more collisions) but each carries proportionally more inertia; the effects cancel exactly in a = √(γRT). Only temperature — the molecular speed itself — survives. Hence a hot 40 °C runway has faster sound (685 kt) than a −55 °C flight level, despite far higher pressure.
What happens above the tropopause?+
ISA holds −56.5 °C constant from 36,089 ft to 65,617 ft (20 km), so Mach 1 freezes at 573 kt through the entire band where jets and Concorde cruised. Above 20 km the stratosphere warms (ozone absorption) and sound speeds up again — a wrinkle relevant to balloons and the U-2, not airliners.
What was Concorde's Mach 2 in ground terms?+
About 1,147 kt TAS (2,124 km/h) in the −56.5 °C stratosphere — versus the 1,323 kt that 'twice sea-level Mach 1' naively suggests. The cold air gave supersonic transports a 13% discount on the true speed needed for their Mach number, one of the few favors physics did that program.
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