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True airspeed calculator

Your airspeed indicator reads lower than your real speed through the air as you climb. Enter calibrated airspeed, pressure altitude and temperature to get true airspeed.

Calculator

Training aid only. Not for flight planning or operational use. Use official weather, your aircraft's POH or AFM, and approved flight-planning tools for real flights.

Enter values to see the result.

How it's calculated

The calculator uses the standard compressible-flow relations, the same ones behind the E6B's airspeed window. From the calibrated airspeed it works out the pressure the pitot tube senses, then the Mach number at your pressure altitude, then multiplies by the local speed of sound at your temperature.

The ground-school rule of thumb adds about 2% to calibrated airspeed for every 1,000 feet. At 8,000 feet on a standard day, that turns 120 knots calibrated into about 139 knots; the exact result is about 135 knots, so the rule runs a little high.

Indicated airspeed differs from calibrated by the instrument and position error in your POH's airspeed calibration table. At cruise in most trainers that's only a knot or two.