Airspeed Tool

IAS / TAS / Mach Converter

Convert between indicated airspeed, true airspeed, and Mach number at any altitude and temperature using ICAO standard atmosphere formulas.

Input mode

Indicated Airspeed

kts

Altitude

OAT (optional)

degC

ISA: -54.3C at this altitude

Results

IAS280kts

TAS503kts

Mach0.873

Speed of sound576kts

About

Converts between Indicated Airspeed (IAS), True Airspeed (TAS), and Mach number at any altitude and temperature. Uses ICAO standard atmosphere equations, with optional OAT input to account for ISA deviations.

How to Use

1Select your input mode - IAS, TAS, or Mach - using the mode buttons.
2Enter the known airspeed value in the first field.
3Enter your pressure altitude in feet.
4Optionally enter OAT to override the ISA temperature at that altitude.
5Read all three airspeed representations and the speed of sound from the results panel.

Tips

//TAS equals IAS only at sea level under standard conditions (15C, 29.92 inHg). Above that, TAS is always higher.
//At FL350 a typical IAS of 280 kts equates to roughly 480 kts TAS - important for fuel burn and ETAs.
//Enter OAT from your weather briefing to get accurate results on hot or cold days when ISA deviations are large.
//Mach number depends on temperature, not just altitude - a warmer day means a higher speed of sound and lower Mach for the same TAS.
//For oceanic flight planning, airlines file TAS. Use this converter to cross-check your FMC entry.

Standard Atmosphere

TASIAS / sqrt(sigma)

MachTAS / speed of sound

sigmadensity ratio (rho / rho0)

a (kts)38.9675 x sqrt(T_K)

sigma and T_K follow the ICAO standard lapse rate below FL360, isothermal above.

FAQ

What is the difference between IAS and TAS?: IAS is what your airspeed indicator reads - it reflects dynamic pressure on the pitot tube. TAS is your actual speed through the air mass, corrected for air density. At high altitudes where air is thinner, TAS is significantly higher than IAS.
Why does Mach matter in cruise?: At high altitudes, airframe buffet onset and critical Mach limits are expressed in Mach rather than knots. Most jet aircraft have a Mmo (maximum operating Mach) around M0.82-0.92. Monitoring Mach prevents overspeed in the thin air of the flight levels.
When should I enter OAT instead of leaving it blank?: Use OAT when the day is significantly warmer or colder than ISA. A large ISA deviation (ISA+20 is common on hot days) meaningfully changes TAS and Mach, and the default ISA assumption would give inaccurate results.