True Airspeed (TAS) Calculator
True airspeed calculator. Converts calibrated airspeed to true airspeed using pressure altitude and outside air temperature, the E6B way. Educational only.
True Airspeed Calculator
How to use the true airspeed calculator
Enter your airspeed
Type the calibrated airspeed (CAS) in knots. For most light aircraft in the cruise, the indicated airspeed is close enough to CAS to use directly.
Enter pressure altitude
Set the altimeter to 29.92 and read the altitude, or use your cruise pressure altitude. TAS grows with altitude as the air thins.
Enter outside air temperature
Type the OAT in Celsius from the aircraft gauge. Warmer-than-standard air is thinner, so it raises TAS for the same indicated speed.
Read the result
You get the true airspeed and the density ratio. Combine TAS with the wind to work out ground speed for navigation — but cross-check with your POH and E6B.
True airspeed — how fast you really move through the air
Why indicated and true airspeed differ
An airspeed indicator doesn't measure speed directly — it measures the dynamic pressure of the air hitting the pitot tube, and converts that to a speed assuming sea-level standard density. That reading is the indicated airspeed (IAS), and after small instrument and position corrections it becomes calibrated airspeed (CAS). The trouble is that air thins with altitude, so at height the same dynamic pressure corresponds to a faster actual speed through the air. True airspeed (TAS) is that real speed of the aircraft relative to the surrounding air mass. The relationship is simple in principle: TAS equals CAS divided by the square root of the density ratio — the ratio of the actual air density to sea-level standard density. As you climb and the density ratio falls, TAS rises above CAS by roughly 2% per thousand feet as a rule of thumb, so an aircraft showing 100 knots at 8,000 feet is actually moving through the air at about 113 knots.
The density ratio itself depends on both pressure and temperature. Pressure altitude sets the pressure part — higher means lower pressure — while the outside air temperature sets the temperature part, since warm air is less dense than cold air at the same pressure. This calculator computes the density ratio from the standard-atmosphere pressure relationship at your pressure altitude, corrected by the actual temperature, then divides CAS by its square root. The reason TAS matters is navigation: the aircraft flies through a moving air mass, so your speed over the ground is TAS combined with the wind. Flight planning, fuel burn, and time en route all hinge on TAS, not on the indicated reading, which is why every cross-country pilot learns to convert.
"Your airspeed indicator quietly assumes sea-level air. Climb into thinner air and you're really going faster than it shows — about 2% more per thousand feet. True airspeed is what the map cares about."
An estimate to cross-check, not to trust blindly
The method here is the standard one taught for the E6B flight computer and is accurate enough for planning, but it carries assumptions. It treats indicated airspeed as calibrated airspeed, which is fine in the cruise for most light aircraft but can be off at low speeds or high angles of attack where position error grows, so for precise work you'd apply the CAS correction from the POH first. At high speeds and altitudes, compressibility effects make CAS differ from equivalent airspeed, and a simple conversion slightly overstates TAS — a refinement that matters for fast jets but not for typical general-aviation speeds. The result also depends on accurate pressure altitude and a correctly read outside-air-temperature gauge, both of which have their own errors. None of this makes the calculation unreliable for everyday navigation, but it does mean the output is an educational estimate to be cross-checked against your aircraft's documentation and an approved E6B, and used alongside current winds for planning. The pilot remains responsible for navigation and fuel decisions, made from official data rather than a web tool.
10 Facts About True Airspeed
TAS = CAS ÷ √(density ratio).
Rule of thumb: TAS rises ~2% per 1,000 ft.
The indicator assumes sea-level density.
IAS → CAS (instrument/position) → TAS (density).
Warmer air is thinner, raising TAS.
Ground speed = TAS combined with wind.
TAS drives flight planning and fuel burn.
At sea level on a standard day, CAS ≈ TAS.
At high speed, compressibility refines the conversion.
The E6B does this conversion mechanically.
Frequently asked questions
True airspeed is the actual speed of the aircraft relative to the air mass it's flying through. It differs from the indicated airspeed because the airspeed indicator assumes sea-level air density; as you climb into thinner air, the same indication corresponds to a higher true speed. TAS is what you combine with the wind to get ground speed, so it's the airspeed that matters for navigation, time en route, and fuel planning.
Divide the calibrated airspeed by the square root of the density ratio (the actual air density divided by sea-level standard density). The density ratio is found from the pressure altitude and the outside air temperature. As a quick estimate, TAS increases about 2% over CAS for every 1,000 feet of altitude, but the temperature-corrected calculation this tool uses is more accurate, especially on non-standard-temperature days.
Indicated airspeed (IAS) is the raw reading. Calibrated airspeed (CAS) is IAS corrected for instrument and position error. Equivalent airspeed (EAS) is CAS corrected for compressibility, significant only at high speed and altitude. True airspeed (TAS) is EAS corrected for density (altitude and temperature). For typical light-aircraft cruise speeds, EAS ≈ CAS, so this tool goes straight from CAS to TAS using the density correction, which is the conversion the E6B teaches.
Because air density depends on temperature as well as pressure. Warm air is less dense than cold air at the same pressure, so on a warmer-than-standard day the air is thinner and the true airspeed is higher for the same calibrated airspeed. Ignoring temperature (the simple 2%-per-thousand-feet rule) is fine for a rough estimate, but entering the actual outside air temperature gives a more accurate TAS, which is why the calculator asks for it.
For controlling the aircraft, yes — indicated airspeed is what matters for stall margin and structural limits, because those depend on dynamic pressure, not true speed. But for navigation you need true airspeed, since the aircraft moves through the air at TAS, not IAS. The calculator treats your entered CAS as effectively equal to IAS for cruise; for precise speeds, apply the CAS correction from your POH airspeed-calibration table first.
Combine TAS with the wind vector. A direct headwind subtracts from TAS and a direct tailwind adds to it; a crosswind requires solving the wind triangle, which also gives the heading correction (drift). The E6B or a flight-planning app does this for you. The key point is that you start from TAS, not indicated airspeed — so computing TAS correctly is the first step to an accurate ground speed and time en route.
Slightly, at high speeds and altitudes, because of compressibility. Above roughly 200 knots and high up, calibrated airspeed exceeds equivalent airspeed, and a density-only conversion gives a TAS a little high. For fast aircraft this compressibility correction matters; for typical general-aviation speeds it's negligible, which is why the standard CAS-to-TAS density method used here is perfectly adequate for light-aircraft planning.
Use it to learn and to estimate. For actual flight planning, cross-check against your aircraft's POH cruise-performance data and an approved E6B or flight-planning system, apply the CAS correction, and use current forecast winds. The pilot is responsible for navigation and fuel calculations using official tools and data. This calculator is an educational aid, not an approved performance source.
Because the density relationship is defined against the standard atmosphere, which is referenced to standard pressure. Pressure altitude — the altitude shown with the altimeter set to 29.92 — removes the effect of the day's pressure setting and ties the calculation to that standard. You then correct for the actual temperature separately. Using indicated altitude with a non-standard altimeter setting would introduce an error, so the conversion uses pressure altitude.
No. The values you enter are processed entirely in your browser. Nothing is sent to a server, stored, or shared, and no account is required. The calculation runs on your device only.
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