What is the difference between IAS and TAS?

Indicated Airspeed (IAS) is what your airspeed indicator shows and is based on dynamic air pressure. True Airspeed (TAS) is your actual speed through the air mass and accounts for lower air density at higher altitudes and temperatures. TAS is always equal to or greater than IAS above sea level under ISA conditions.

Why does TAS increase with altitude?

At higher altitudes air density decreases, so the same indicated airspeed corresponds to a higher true airspeed. A rule of thumb is that TAS increases roughly 2% per 1,000 ft above sea level compared to IAS.

What is pressure altitude?

Pressure altitude is the altitude indicated when your altimeter is set to 29.92 inHg (1013.25 hPa). It differs from field elevation and is used for performance calculations because it reflects the actual pressure of the air mass regardless of local weather.

Does this calculator account for compressibility?

This calculator uses the subsonic density-ratio formula which is accurate for speeds below approximately 200 knots (Mach 0.3). At higher speeds, compressibility effects become significant and a more complex formula involving calibrated airspeed (CAS) and equivalent airspeed (EAS) should be used.

What is a standard day (ISA)?

The International Standard Atmosphere (ISA) defines sea-level conditions as 15°C (59°F) and 1013.25 hPa. ISA temperature decreases at 1.98°C per 1,000 ft. The deviation from ISA (ISA+/- °C) shows whether conditions are warmer or colder than standard.

Can I use Fahrenheit for temperature input?

Yes. Select °F in the temperature unit toggle and the calculator converts internally before computing TAS.

True Airspeed Calculator

Enter indicated airspeed, pressure altitude, and outside air temperature to instantly calculate true airspeed.

Flight Parameters

Indicated Airspeed (IAS)

kts

Pressure Altitude

Set altimeter to 29.92 inHg / 1013.25 hPa

Outside Air Temperature (OAT)

True Airspeed Result

Enter values and click Calculate TAS

Atmosphere Breakdown

Results will appear here.

Summary

True Airspeed (TAS) is the actual speed of an aircraft relative to undisturbed air. Unlike Indicated Airspeed (IAS), which is affected by air density changes with altitude and temperature, TAS accounts for these variations and gives the real aircraft velocity. This calculator uses standard ISA formulas to convert IAS to TAS given pressure altitude and outside air temperature (OAT).

How it works

Enter your indicated airspeed (IAS) in knots.
Enter the pressure altitude in feet (from your altimeter set to 29.92 inHg / 1013.25 hPa).
Enter the outside air temperature (OAT) in Celsius or Fahrenheit.
The calculator derives air density using the ISA model and temperature correction.
TAS is computed as IAS divided by the square root of the density ratio.
Results update instantly as you type.

Use cases

Flight planning — compute ground speed when combined with wind data.
Performance calculations for cruise, climb, and descent profiles.
Filing IFR flight plans that require TAS entry.
Cross-checking E6B or flight computer results.
Training and ground school exercises on density altitude concepts.
Comparing aircraft performance at different altitudes and temperatures.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu