Convert indicated or calibrated airspeed to true airspeed using the pressure altitude and outside air temperature. You also get the Mach number, density altitude and equivalent airspeed, with the exact compressible-flow formulas and a table of TAS at other altitudes.
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Airspeed
From the airspeed calibration table in your POH, e.g. +2 or −3; leave empty if you enter CAS
Altitude & temperature
Altimeter set to 29.92 inHg / 1013 hPa
Static air temperature; leave empty to use the ISA standard temperature
Enter your values to see the result.
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How to use the True Airspeed Calculator
Enter your airspeed (IAS or CAS) and pick knots, mph or km/h.
Add the IAS correction from the POH calibration table if you entered IAS (optional).
Enter the pressure altitude: the altimeter reading with 29.92 inHg (1013 hPa) set.
Enter the outside air temperature, or leave it empty for a standard day.
Read the true airspeed, Mach number and density altitude, and check the table for other altitudes.
True airspeed formulas
The calculator uses the standard subsonic pitot-static relations and the International Standard Atmosphere (ISA):
Static pressure from pressure altitude: p = 101,325 Pa × (1 − 6.8756×10⁻⁶ × h)5.2559 up to 36,089 ft, and p = 22,632 Pa × e−4.8063×10⁻⁵ × (h − 36,089) above it.
Impact pressure from CAS: qc = P₀ × [(1 + 0.2 × (CAS / a₀)²)3.5 − 1], with a₀ = 661.48 kt, the sea-level speed of sound.
Mach number: M = √(5 × [(qc / p + 1)2/7 − 1]).
True airspeed: TAS = M × a, where a = √(1.4 × 287.05 × T) m/s, or about 38.97 × √T knots with T in kelvin.
Density altitude: σ = (p / P₀) × (288.15 / T), DA = 145,442 × (1 − σ0.235) ft.
Worked example: 120 kt CAS at 8,000 ft pressure altitude and 5 °C. Static pressure is 75,262 Pa, the Mach number comes out at 0.210, the speed of sound at 278.15 K is 334.3 m/s (649.9 kt), so TAS ≈ 0.210 × 649.9 ≈ 137 kt. The rule of thumb gives 120 × 1.16 = 139 kt.
TAS for 120 knots CAS at standard temperature
Pressure altitude
ISA temperature
True airspeed
Rule of thumb (2% / 1,000 ft)
Sea level
15 °C
120 kt
120 kt
4,000 ft
7.1 °C
127 kt
130 kt
8,000 ft
−0.8 °C
135 kt
139 kt
12,000 ft
−8.8 °C
144 kt
149 kt
16,000 ft
−16.7 °C
153 kt
158 kt
20,000 ft
−24.6 °C
164 kt
168 kt
The rule of thumb runs a little high as altitude increases; the exact formula is what flight computers and air data computers use.
Tips for using true airspeed
Use pressure altitude, not indicated altitude. Set 29.92 inHg briefly or correct the indicated altitude by about 1,000 ft per inHg of altimeter setting difference.
Use static OAT. Most light aircraft OAT gauges are close enough. In fast aircraft the probe reads warmer because of ram rise; use the static air temperature (SAT) if available.
Plan with TAS, fly with IAS. Stall, rotation and approach speeds stay the same in indicated airspeed at any altitude, but your true airspeed and ground speed on landing are higher at high-elevation airports.
Next step: put the TAS and forecast winds into the E6B flight computer to get heading and ground speed.
Frequently asked questions
What is true airspeed?
True airspeed (TAS) is the actual speed of the aircraft through the surrounding air. The airspeed indicator shows indicated airspeed, which is based on dynamic pressure and reads lower than TAS as the air gets thinner. TAS is what you use for navigation, together with the wind, to find ground speed.
How do you calculate true airspeed?
First correct IAS to calibrated airspeed (CAS) using the POH table. Then use the pressure altitude and outside air temperature: the calculator turns CAS into impact pressure, divides by the static pressure at your altitude to get the Mach number, and multiplies Mach by the speed of sound at the OAT: TAS = M × √(1.4 × 287.05 × T).
What is the rule of thumb for true airspeed?
TAS is roughly CAS plus 2% for every 1,000 feet of altitude. At 10,000 ft, 120 knots CAS gives about 144 knots TAS. The rule works best at standard temperature and lower altitudes; warm air makes TAS higher and cold air makes it lower.
What is the difference between IAS, CAS, EAS and TAS?
IAS is what the instrument shows. CAS corrects IAS for installation and instrument error. EAS corrects CAS for air compressibility, which matters above about 200 knots and at high altitude. TAS corrects EAS for air density, so it is the real speed through the air.
Why is true airspeed higher than indicated airspeed at altitude?
The airspeed indicator measures the pressure of the air hitting the pitot tube. Thinner air at altitude produces less pressure at the same real speed, so the indicator reads lower. The airplane must move faster through thin air to get the same indicated airspeed, so TAS exceeds IAS.
Does temperature affect true airspeed?
Yes. Warmer air is less dense, so at the same indicated airspeed and pressure altitude your true airspeed is higher. Roughly, each 5 °C above standard adds about 1% to TAS. That is why you need the OAT, not just altitude.
Does this calculator work for jets and Mach numbers?
Yes, for subsonic flight. It uses the full compressible-flow relation between calibrated airspeed, impact pressure and Mach number, and the ISA model up to 65,000 ft including the stratosphere. It stops with a message if the inputs would mean supersonic flight.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.