True Airspeed Calculator

What Is a True Airspeed Calculator?

True airspeed (TAS) is the speed of an aircraft relative to the air mass it flies through. As altitude increases the air becomes less dense, so the same indicated airspeed (IAS) corresponds to a higher true speed. The True Airspeed Calculator converts IAS to TAS from pressure altitude, with an optional outside air temperature correction.

How the Calculation Works

The tool uses the International Standard Atmosphere (ISA) density ratio. For a pressure altitude h in feet:

$$\sigma = \frac{\rho}{\rho_0} = \left(1 - 6.8756 \times 10^{-6}\, h\right)^{4.2561}$$

If an outside air temperature is supplied, the ISA ratio is scaled by the temperature correction, and true airspeed follows from the density ratio:

$$\sigma = \sigma_{ISA} \times \frac{T_{ISA}}{T_{actual}}, \qquad TAS = \frac{IAS}{\sqrt{\sigma}}$$

Where:

• IAS is indicated airspeed in knots
• h is pressure altitude in feet
• σ is the air density ratio, rho divided by sea-level density rho₀
• T_ISA is the standard temperature 15 − 1.98 × (h/1000) in degrees Celsius
• T_actual is the entered temperature in kelvin (add 273.15)

Worked Example

An aircraft indicates 120 knots at a pressure altitude of 10,000 ft. The ISA density ratio is (1 − 0.068756)^4.2561 = 0.7385, so TAS = 120 / sqrt(0.7385) = 139.6 knots. The standard temperature at that altitude is about −4.8 °C. On a warmer day the density ratio falls, and the true airspeed rises further.

For related aviation tools, see the Wind Correction Angle Calculator, Wing Loading Calculator, and Weight on Other Planets Calculator.