Mach number is the ratio of an object's speed to the local speed of sound. Mach 1 equals the speed of sound at that altitude and temperature. At sea level on a standard day (15°C), the speed of sound is approximately 661 knots (1,225 km/h).

Why does Mach number depend on altitude?

The speed of sound depends only on air temperature, which decreases with altitude in the troposphere (up to ~36,089 ft). Lower temperature means a lower speed of sound, so the same true airspeed corresponds to a higher Mach number at higher altitudes.

What is the difference between subsonic, transonic, and supersonic?

Subsonic flight is below Mach 0.8, transonic is roughly Mach 0.8–1.2 (where shock waves begin forming), and supersonic is above Mach 1.2. Hypersonic flight typically refers to speeds above Mach 5.

Does this calculate indicated or true airspeed?

The speed input is treated as true airspeed (TAS). Mach number is computed as TAS divided by the local speed of sound. If you have indicated airspeed, use the True Airspeed Calculator first to convert it.

What temperature model does this use?

The ISA (International Standard Atmosphere) model: sea-level temperature 15°C, lapse rate of 1.9812°C per 1,000 ft up to the tropopause at 36,089 ft, then constant -56.5°C in the stratosphere. You can override OAT for non-standard conditions.

Mmo (Maximum Operating Mach number) is a speed limit found on many jet aircraft. It is typically around Mach 0.82–0.92 for commercial airliners and prevents transonic shock-induced control problems. This calculator can convert Mmo to an equivalent true airspeed for your planning altitude.

Mach Number Calculator

Enter your speed and altitude to calculate Mach number, or convert Mach number back to knots, km/h, and mph.

Mach Number Calculator

Conversion Mode

True Airspeed

Pressure Altitude

Set altimeter to 29.92 inHg / 1013.25 hPa

Outside Air Temperature (OAT) Override ISA

Using ISA standard temperature at entered altitude.

Result

Enter values and click Calculate

Atmosphere & Speed of Sound

Results will appear here.

Summary

Mach number is the ratio of an object's speed to the local speed of sound, which varies with air temperature (and therefore altitude). This calculator uses the International Standard Atmosphere (ISA) model to determine the speed of sound at your chosen altitude and temperature, then computes the Mach number precisely. It also supports reverse conversion — enter a Mach number and get the equivalent speed in knots, km/h, mph, or m/s.

How it works

Select your input mode: Speed to Mach, or Mach to Speed.
Enter your airspeed in knots, km/h, mph, or m/s (Speed to Mach mode).
Enter pressure altitude in feet or meters.
Optionally override the outside air temperature (OAT) for non-standard conditions.
The calculator derives the local speed of sound using the ISA temperature model.
Mach number (or equivalent speed) is computed and displayed with regime classification.

Use cases

Determining Mach number for high-speed aircraft performance planning.
Converting cruise Mach number to indicated or true airspeed equivalents.
Checking whether a flight regime is subsonic, transonic, or supersonic.
Flight simulation and academic aerodynamics exercises.
Cross-checking avionics or flight-management-system speed readouts.
Studying how the speed of sound changes with altitude and temperature.
Converting published aircraft Vmo/Mmo limits between speed units.
Training for pilot written exams covering airspeed and Mach concepts.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu

Mach Number Calculator

Use the Mach Number Calculator

Mach Number Calculator

Result

Atmosphere & Speed of Sound

Summary

How it works

Use cases

Frequently Asked Questions