Cruise True Airspeed Calculator

True Airspeed: TAS = IAS · √(ρ₀/ρ)

True Airspeed (TAS) is how fast the aircraft is actually moving through the air mass. Indicated Airspeed (IAS) is just the number the pitot-static system puts on the panel. They drift apart because the pitot tube measures dynamic pressure, and that pressure depends on how dense the air is. Climb higher, the air thins out, and the instrument starts reading low. To recover the real figure you apply TAS = IAS · √(ρ₀/ρ), with ρ₀ being sea-level density (1.225 kg/m³). Up at FL350 air is roughly one-third as dense as it is at sea level, which pushes TAS about 50% higher than IAS. Take 250 KIAS at FL350: that works out to roughly 470 KTAS, the figure flight planning and ATC strips actually rely on.

Applications

Flight planning and fuel math, wind-triangle work, ground-speed solutions, reading ATIS and METAR, filling out a navlog, glass cockpits like the Garmin G1000 or Collins Pro Line that show IAS, TAS and GS side by side, performance computation on turboprops and jets, plus the classroom side of PPL/CPL training.

FAQ

Why does the airspeed indicator under-read at altitude? It's calibrated for sea-level density. Higher up, the thin air can't generate as much dynamic pressure for the same true speed, so the needle sits lower than it should.

What is the difference between TAS and Ground Speed? TAS is your speed through the air mass. Ground Speed (GS) is your speed over the dirt. What takes you from one to the other is the wind vector.

Is there a quick rule of thumb? There is. In the lower troposphere, tack on roughly 2% per 1,000 ft of altitude to your IAS and you've got a decent TAS estimate. Want it tighter? Fall back on the density-ratio formula or just read it off the flight management system.