True airspeed calculator
Convert calibrated airspeed to true airspeed for a given pressure altitude and temperature.
How it works
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.
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.
Live: the coldest air being reported right now
Temperature drives the density ratio, and therefore the gap between calibrated and true airspeed. Feed one of these into the calculator with a cruising altitude to see the effect.
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For planning and study, not for operational use. Aircraft performance comes from your POH, not from a generic formula.