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E6B Calc

How to calculate true airspeed from calibrated airspeed (and Mach)

True airspeed is calibrated airspeed corrected for air density. Thinner air, same needle reading, faster you're actually going. 120 kt CAS at 8,000 ft, 29.92 inHg, 5 °C comes out to 137 kt TAS, Mach 0.210 — 17 knots you don't see on the gauge but absolutely feel in the fuel burn. E6B Calc's free True Airspeed screen works it out from CAS, altitude, altimeter setting and OAT.

3 min read · Updated 2026-09-11

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Why you need true airspeed

The airspeed indicator measures dynamic pressure, and that falls as air thins out. So at altitude you're covering more ground than the needle lets on. Every navigation figure that matters — the wind triangle, ground speed, time, fuel on the navlog — starts from true airspeed, not from what you glance at on climb-out.

IAS, CAS, EAS and TAS in one line each

  • IAS: what the needle shows.
  • CAS: IAS corrected for instrument and position error, from the airspeed calibration table in your POH.
  • EAS: CAS corrected for compressibility, which matters above about 200 kt.
  • TAS: EAS corrected for density. Your real speed through the air mass.

How to calculate true airspeed from indicated airspeed

There's no IAS field here, on purpose: the IAS-to-CAS correction is different for every airframe and every flap setting, so a generic field would just be wrong for someone. Look your planned IAS up in the POH's airspeed calibration table, get CAS, enter that. In a lot of light singles at cruise the two sit within a couple of knots of each other — but check your own table before you assume that applies to you.

Step by step in E6B Calc

  1. On E6B, open True Airspeed under Flight.
  2. Enter Calibrated airspeed in your speed unit.
  3. Enter Indicated altitude: the altitude you will fly.
  4. Check the Altimeter setting (it starts at standard) and enter the Outside air temperature at that altitude.
  5. Read True airspeed, Mach number, Equivalent airspeed, Pressure altitude and Density altitude.

Worked examples

E6B Calc results; TAS and EAS in whole knots
CASAltitudeAltimeterOATTASMachEAS
120 kt8,000 ft29.92 inHg5 °C137 kt0.210120 kt
250 kt35,000 ft29.92 inHg−54 °C428 kt0.741238 kt

Row one is an ordinary light-aircraft cruise, density altitude 8,690 ft, TAS 17 kt above CAS. Row two is why the app bothers with compressible-flow equations at all: the old slide-rule method, CAS over the square root of the density ratio, would say 449 kt — about 21 kt too fast, and at jet speeds that error doesn't shrink.

The true airspeed formula in aviation

For slow aircraft, TAS ≈ CAS ÷ √(density ratio) — the shortcut the mechanical slide rule uses. The app runs the longer chain instead: impact pressure from CAS, Mach at the actual pressure, then TAS from Mach and OAT. At 120 kt the two methods agree within a knot; at jet speeds they diverge sharply.

Tips and mistakes

  • Use the temperature at cruise altitude, not on the ground. If you only have the ground value, subtract about 2 °C per 1,000 ft for a planning estimate.
  • A rough mental check: TAS is about 2 % more than CAS per 1,000 ft of altitude.
  • The app refuses supersonic inputs and shows Beyond the model.
  • Speed can be kt, km/h or mph in Settings > Units of measurement; altitude is always feet.

Questions and answers

Can I calculate true airspeed from ground speed?
Not directly. Ground speed already includes the wind. You would need the wind triangle in reverse, which E6B Calc does not do.
How do I calculate true airspeed in a climb?
Use an average: the CAS you climb at and the altitude and temperature about two thirds of the way up. Enter those on the True Airspeed screen.
What is Mach used for here?
Mach is the TAS divided by the local speed of sound. For light aircraft it is a curiosity; for fast aircraft it is what the cruise is flown by, which is why the app shows it.
Why does my E6B give a different answer at high speed?
A mechanical E6B ignores compressibility. Above about 200 kt that makes it read high; E6B Calc includes it.

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