Aviation Calcs

True Airspeed

True Airspeed Calculator

Enter calibrated (or indicated) airspeed, pressure altitude, and either outside air temperature or ISA deviation. You get true airspeed, Mach number, and equivalent airspeed from the ICAO Standard Atmosphere and the subsonic compressible pitot-static formula. Results update as you type.

If you only have IAS, enter it here and apply the POH calibration if you need a certified CAS.

Set 29.92 inHg / 1013.25 hPa and read the altimeter, or compute it with the density altitude calculator.

Outside air temperature (OAT / SAT)
ISA deviation (ISA +/−)

Edit OAT or ISA deviation — the other follows. If you last edited ISA +/−, changing pressure altitude keeps the deviation and updates OAT.

True airspeed

— kt

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How to convert CAS or IAS to true airspeed

True airspeed is how fast the airplane is moving through the air. It is the number you need for a wind triangle, an ETA, and most performance comparisons at altitude. The airspeed indicator is closer to calibrated airspeed: it senses impact pressure, which depends on density. In thinner air the same CAS means a higher TAS.

This page uses the ICAO Standard Atmosphere (Doc 7488 / ISO 2533) for pressure at a given pressure altitude, including the tropopause at 11 km, and the standard subsonic pitot-static relations:

qc / p0 = (1 + 0.2 (CAS / a0)2)3.5 − 1
M = √( 5 [(qc/p + 1)2/7 − 1] )
TAS = M · √(γ R T)

Equivalent airspeed is TAS × √σ, where σ is the density ratio. At low speed and altitude, EAS, CAS, and IAS are close; the difference grows as you go faster and higher. The familiar “add 2% of CAS per 1,000 feet” rule is a useful mental check, not a substitute for the compressible formula.

Worked example

100 kt CAS, 8,000 ft pressure altitude, ISA temperature (about −0.8°C):

At sea-level ISA, TAS equals CAS exactly, by the definition of calibrated airspeed.

How to use it

  1. Enter CAS or IAS, pressure altitude, and OAT — or enter an ISA deviation and let OAT follow.
  2. Read TAS, Mach, and EAS. Use TAS in the E6B wind triangle, the time–speed–distance calculator, and the turn radius calculator (turn geometry also wants true airspeed).
  3. For a certified number, apply the airspeed calibration table in the POH so that you start from CAS, not raw IAS.

Frequently asked questions

What is the difference between IAS, CAS, EAS, and TAS?

Indicated airspeed (IAS) is what the airspeed indicator shows. Calibrated airspeed (CAS) is IAS corrected for instrument and position error using the POH airspeed calibration table. Equivalent airspeed (EAS) is CAS corrected for compressibility. True airspeed (TAS) is the actual speed through the air, which is what you use for navigation, wind triangles, and time-speed-distance.

How do I convert indicated airspeed to true airspeed?

Treat the indicated value as calibrated if you do not have a position-error correction, then combine it with pressure altitude and outside air temperature. At higher altitude the air is thinner, so TAS is higher than CAS — roughly 2% per 1,000 feet as a mental check. This page uses the full subsonic compressible formula, not just the rule of thumb.

Do I enter OAT or ISA deviation?

Either one. ISA temperature is 15°C at sea level, decreasing about 2°C per 1,000 feet of pressure altitude up to the tropopause. ISA deviation is OAT minus that standard temperature. Edit either field and the other follows.

Does this work above the tropopause or near Mach 1?

Pressure at a given pressure altitude follows the ICAO troposphere below 11 km (36,089 ft) and the isothermal lower stratosphere above that, so flight-level calculations are defined. The airspeed formula itself is the subsonic pitot relation and is not valid at Mach 1 or above.