Indicated Airspeed Conversion Calculator

How to use this calculator

1. Enter Indicated airspeed (IAS) in kt, km/h or mph.
2. Enter Calibration correction (from your aircraft data), or leave it blank to treat CAS as equal to IAS.
3. Give the Pressure altitude at which the reading was taken, in feet or meters.
4. Add the Outside air temperature at that level and select °C, °F or K.

The correction is the one input this page cannot supply. It comes from the airspeed calibration table or chart in the aircraft's own flight manual and changes with speed and flap setting, so look it up for the condition you are working on. The sign convention is: positive when CAS is higher than IAS, negative when it is lower. The field accepts values from −50 kt to 50 kt and has its own unit selector, which may differ from the IAS unit.

Formula

What the variables mean

IAS

indicated airspeed read from the instrument

correction

the position and instrument error correction for that speed and configuration, taken from the aircraft's own airspeed calibration data; enter 0 if you have none

CAS

calibrated airspeed

EAS

equivalent airspeed, CAS corrected for compressibility

TAS

true airspeed

P, T

static pressure at the pressure altitude and outside air temperature in kelvin

Worked example

Reading the result

All three speeds are shown in the unit you chose for IAS. Calibrated airspeed (CAS) is simply IAS plus your correction. Equivalent airspeed (EAS) will be slightly below CAS; the gap is tiny at light-aircraft speeds and altitudes and becomes significant only when both are high. True airspeed (TAS) is normally the largest of the four above sea level and has knots or km/h beside it. Mach number is TAS divided by the local speed of sound. A note under the results says whether a correction was applied or CAS was taken as equal to IAS.

Assumptions and limitations

• The calibration correction is a single number you supply; the calculator has no aircraft data and cannot check it.
• A blank correction is treated as zero, which is a simplification and not a statement about any real aircraft.
• The conversion from CAS onward uses the subsonic compressible pitot relation with standard-atmosphere static pressure at the pressure altitude entered.
• Dry air is assumed, and the temperature entered is taken as the static temperature of the outside air.
• Instrument lag, a blocked pitot or static port, and errors that vary with sideslip or angle of attack are not modelled.
• Inputs that would give a calibrated airspeed of zero or less, or Mach 1 or more, are rejected.

Common mistakes

• Reversing the sign of the correction, subtracting it when the calibration data says to add it.
• Reading the correction for one flap setting and applying it to another.
• Leaving the correction blank and then quoting the CAS row as if it were a calibrated value.
• Assuming EAS and CAS are always interchangeable, which stops being true at high speed and altitude.
• Entering field elevation as pressure altitude when the altimeter setting is well away from standard.
• Expecting TAS to equal IAS at altitude because the reading on the instrument has not changed.