True Airspeed Calculator
Enter the calibrated airspeed, pressure altitude and temperature to get the true airspeed, Mach number and equivalent airspeed, or run it backward from a TAS.
Free · runs in your browser · nothing is uploaded
The true airspeed calculator converts calibrated airspeed (CAS) to true airspeed (TAS) for the altitude and outside air temperature you enter, and it runs the reverse direction too. Choose the direction, type the speed in knots, mph or km/h, then add the pressure altitude and the temperature. The headline figures are True airspeed and Calibrated airspeed, with Mach number, equivalent airspeed and density ratio below them.
The page also sets the exact result beside the FAA rule of thumb, 2 percent per 1,000 ft, so you can see how far the shortcut drifts at your conditions. If you start from an indicated reading, the optional instrument correction under More options handles the difference.
How to use the true airspeed calculator
- Pick the Direction: Calibrated airspeed to true airspeed when you start from a CAS, or True airspeed to calibrated airspeed when your target is a TAS.
- Enter the Calibrated airspeed (CAS), or the True airspeed (TAS) in the reverse direction, and choose the Speed unit: knots, mph or km/h.
- Enter the Pressure altitude and set the Altitude unit to feet or meters. Use pressure altitude, not indicated altitude. The pressure altitude calculator converts one to the other.
- Enter the Outside air temperature and set the Temperature unit to °C or °F. Use the static outside air temperature, not a probe reading that includes ram rise.
- Open More options if the flight manual’s airspeed calibration gives a correction. Enter +3 when the manual says CAS is 3 above IAS. The default of 0 treats IAS as CAS, and the page shows a note saying so.
- Read the True airspeed and Calibrated airspeed headlines, then compare Rule of thumb minus exact value and Rule of thumb error (%).
- Open How it was calculated to see each step with your own numbers.
The presets load 120 kt at 8,000 ft, 250 kt at 25,000 ft, 100 kt on a hot day and a TAS to CAS example. Print / save result makes a sheet, Copy link copies a link to your inputs, and Reset restores the example.
How do you calculate true airspeed?
Convert the calibrated airspeed to an impact pressure, then use that with the static pressure at your pressure altitude to get the Mach number. Multiply Mach by the speed of sound at the outside air temperature, and the product is the true airspeed.
- Impact pressure: qc = P0 x [(1 + 0.2 x (CAS/a0)^2)^3.5 - 1], with P0 = 1013.25 hPa and a0 = 661.48 kt.
- Mach number: M = sqrt(5 x [(qc/p + 1)^(2/7) - 1]), where p is the standard static pressure at the pressure altitude.
- True airspeed: TAS = M x a, with the speed of sound a = sqrt(1.4 x 287.053 x T) and T in kelvin.
A worked case is 120 kt CAS at 8,000 ft pressure altitude and 5 °C. It gives a TAS of 136.6 kt and a Mach number of 0.210.
What is the IAS to TAS rule of thumb?
The FAA rule of thumb adds 2 percent to the calibrated airspeed for each 1,000 ft of altitude to estimate true airspeed. The rule ignores temperature and treats compressibility as zero.
At 120 kt CAS and 8,000 ft, the rule gives 139.2 kt, which is 2.6 kt or 1.9 percent above the exact 136.6 kt. In hot air it can read low instead: at 100 kt CAS, 5,000 ft and 30 °C, the exact TAS is 112.4 kt and the rule gives 110.0 kt.
What is the difference between IAS, CAS, TAS and EAS?
IAS is the reading on the airspeed indicator. CAS is IAS corrected for installation and instrument error. TAS is CAS corrected for altitude and nonstandard temperature, and EAS is CAS corrected for compressibility. Flight planning uses TAS.
The IAS to CAS error can total several knots and is generally greatest at low airspeeds. At 250 kt CAS, 25,000 ft and -35 °C, the TAS is 363.0 kt and the EAS is 243.3 kt, with Mach 0.604.
How accurate is the 2 percent rule?
The rule stays within about 4 kt of the exact TAS up to 20,000 ft for 120 kt CAS at 5.8 °C above standard, then falls behind higher up. Its error grows with altitude, with speed because of compressibility, and with temperature away from standard.
| Pressure altitude | Exact TAS (kt) | Rule TAS (kt) |
|---|---|---|
| 0 ft | 121.2 | 120.0 |
| 5,000 ft | 130.5 | 132.0 |
| 10,000 ft | 140.9 | 144.0 |
| 20,000 ft | 165.5 | 168.0 |
| 30,000 ft | 196.8 | 192.0 |
| 35,000 ft | 215.6 | 204.0 |
Sources
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge (2016), chapter 8 (airspeed definitions and the 2 percent rule); U.S. Standard Atmosphere 1976 (NASA-TM-X-74335) constants; isentropic pitot-static relations. Checked October 2026.
Questions from readers
Is IAS the same as CAS?
No. IAS is the instrument reading, and CAS is IAS corrected for installation error and instrument error. The error can total several knots, and the flight manual’s airspeed calibration chart gives the correction. In cruise the two are approximately the same.
Which temperature do I enter?
Enter the static outside air temperature. A probe reading that includes ram rise is higher than the true static value, so it will shift the result.
Do I need pressure altitude or indicated altitude?
Pressure altitude. It is the altitude read with the altimeter set to standard pressure, 1013.25 hPa. The pressure altitude calculator converts an indicated altitude to it.
Why is TAS higher than IAS at altitude?
For a given CAS, TAS increases with altitude. In the table above, 120 kt CAS at 35,000 ft gives 215.6 kt TAS at 5.8 °C above standard, while sea level gives 121.2 kt.
Can I use it above Mach 1?
No. The formulas are for subsonic flight, and the calculator refuses speeds at or above the speed of sound. It also warns when Mach is above 0.6.
How do I get groundspeed from TAS?
Groundspeed is TAS adjusted for wind, and this page gives TAS only. The E6B flight computer page solves the wind triangle from your TAS, wind direction and wind speed.
What is equivalent airspeed?
EAS is CAS corrected for compressibility. At low speed it is close to CAS: 120 kt CAS at 8,000 ft gives EAS 119.8 kt.
Why do pilots use indicated airspeed for takeoff and landing instead of true airspeed?
Pilots use indicated (calibrated) airspeed for takeoff and landing because lift and stall depend on dynamic pressure, which the airspeed indicator senses. Stall and approach speeds in the flight manual are listed as indicated speeds, so they stay about the same at any altitude, while TAS changes with air density.
Related tools
- Pressure altitude calculator: use it to get the pressure altitude to enter here.
- Density altitude calculator: use it for density altitude from temperature and pressure altitude.
- Mach number calculator: use it to convert Mach to TAS and CAS at any altitude.
- E6B flight computer: use it for the wind triangle and groundspeed from your TAS.
- Altitude and performance tools: use it to see every altitude and performance tool in one list.
- Crosswind calculator: use it to split a runway wind into crosswind and headwind parts.
- METAR decoder: use it to read temperature and altimeter from a weather report.
- Altimeter setting converter: use it to convert between inHg and hPa.