About this tool
True Airspeed (TAS) Calculator is a free, in-browser tool that converts calibrated (or indicated) airspeed into true airspeed using the air-density ratio at your altitude. Enter the CAS in knots, the pressure altitude, and the outside air temperature in °C or °F; the conversion runs locally with nothing uploaded.
It first finds the density altitude from pressure altitude and temperature (ISA = 15 − 1.98 × PA/1000 °C, then DA = PA + 118.8 × (OAT − ISA)). From that it computes the standard-atmosphere density ratio σ = (1 − 6.8756×10⁻⁶ × DA)^4.2559 and applies TAS = CAS ÷ √σ. For a sanity check it also shows the classic 2%-per-thousand-feet rule of thumb, TAS ≈ CAS × (1 + 0.02 × PA/1000), alongside the density ratio and density altitude used.
Because air thins with altitude and heat, your true speed through the air is faster than the indicated value — typically about 2% more per thousand feet. Use TAS for flight-planning groundspeed and wind-triangle work, but remember this is an estimate, not for real-world navigation, and should be verified with your aircraft's data and official sources.
Frequently asked questions
What is the difference between CAS, TAS and groundspeed?
Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position error. True airspeed (TAS) is CAS corrected for air density — your real speed through the air mass. Groundspeed is TAS adjusted for wind, which the wind-correction calculator handles.
How is TAS computed here?
The tool finds density altitude from pressure altitude and OAT, then the density ratio σ = (1 − 6.8756e-6 × DA)^4.2559, and applies TAS = CAS ÷ √σ. It also lists the 2%-per-1,000-ft rule of thumb for comparison.
Why is TAS higher than my indicated airspeed?
Thinner air at altitude produces less dynamic pressure for the same true speed, so the airspeed indicator under-reads. Correcting for the lower density ratio scales the speed up — roughly 2% for every 1,000 feet of altitude.
Is this suitable for actual navigation?
No — treat it as an estimate for planning and study, not for real-world navigation. It uses standard-atmosphere assumptions and ignores compressibility at high speed. Verify against your aircraft's performance data and official sources.
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