True Airspeed Calculator - TAS from IAS and Altitude

Indicated vs True Airspeed

Indicated airspeed (IAS) is what your instrument reads. True airspeed (TAS) is the actual speed through the air mass. At altitude, lower air density makes the pitot system under-report true speed, so TAS exceeds IAS.

TAS Formula

$$TAS = IAS \sqrt{\frac{\rho_0}{\rho}}$$

Sea-level standard density is $\rho_0$. At pressure altitude $h$ in feet, the ISA density ratio is:

$$\frac{\rho}{\rho_0} = \left(1 - 0.0000068756 \times h\right)^{4.2561}$$

When outside air temperature (OAT) is provided, density is corrected for non-standard temperature using the ratio of ISA temperature to actual temperature at the same altitude.

Aviation Applications

Pilots use TAS for flight planning, fuel estimates, and true ground speed when combined with wind. At 10,000 ft, TAS is typically 15 to 20 percent higher than IAS under ISA conditions.

Frequently Asked Questions

Why does TAS increase with altitude?

Air density decreases with altitude. The pitot tube measures dynamic pressure; at lower density, the same dynamic pressure corresponds to a higher true speed.

What is ISA?

The International Standard Atmosphere defines temperature, pressure, and density versus altitude. It provides a reference for performance charts and avionics calculations.

When should I enter OAT?

Enter outside air temperature when flying in non-standard conditions. Leave it blank to use ISA density at your pressure altitude.

Is this the same as ground speed?

No. Ground speed equals TAS plus or minus the wind component along your track. This calculator does not include wind correction.

What altitude should I use?

Use pressure altitude from your altimeter set to 29.92 inHg (1013 hPa). That matches the ISA density model used here.

Report

True Airspeed

Indicated vs True Airspeed

TAS Formula

Aviation Applications

Frequently Asked Questions

Related Tools

Recent Visits