True Airspeed Calculator

How it works

Below about 200 kt, TAS = CAS ÷ √σ, where σ is the density ratio — the air's density here divided by its density at sea level on a standard day. Thin air means the same dynamic pressure on the pitot needs a higher true speed to produce it.

Higher up that stops being true, because air compresses ahead of the pitot and the instrument over-reads. This page works TAS the compressible way at every altitude — impact pressure from CAS, Mach from that impact pressure at the actual static pressure, then TAS = Mach × the local speed of sound — which gives the same answer as ÷ √σ down low and diverges where it should. At 250 KCAS and FL350 the difference is about 22 kt.

Indicated, calibrated, true

Indicated is what the instrument shows. Calibrated is indicated corrected for position and instrument error — your POH has the table, and the correction is usually largest at low speed and high angle of attack. True is calibrated corrected for density. Groundspeed is true airspeed corrected for wind, which is the wind triangle.

The rule of thumb

Add 2% per 1,000 ft to CAS. It is close on a standard day and drifts when the temperature does not cooperate — which is exactly when you would most like it to be right.