Calculate true airspeed from calibrated airspeed, pressure altitude, and outside air temperature. Essential for flight planning and navigation.
ISA temp at this altitude: 5.1°C
ISA deviation: -0.1°C
True Airspeed (TAS)
129.3 kt
Mach Number
M 0.199
% Increase from CAS
+7.7%
TAS in mph
148.7 mph
TAS in km/h
239.4 km/h
Rule of thumb: TAS increases ~2% per 1,000 ft of altitude above sea level.
At 5,000 ft, your true airspeed is 8% faster than your indicated airspeed.
See how true airspeed increases with altitude for the same calibrated airspeed.
| Altitude | ISA Temp | TAS (kt) | Mach | % Increase |
|---|---|---|---|---|
| Sea Level | 15°C | 120 kt | M 0.181 | +0.0% |
| 3,000 ft | 9°C | 125 kt | M 0.192 | +4.5% |
| 5,000 ft | 5°C | 129 kt | M 0.199 | +7.7% |
| 8,000 ft | -1°C | 135 kt | M 0.210 | +12.8% |
| 10,000 ft | -5°C | 140 kt | M 0.219 | +16.4% |
| 15,000 ft | -15°C | 151 kt | M 0.241 | +26.1% |
| FL180 | -21°C | 159 kt | M 0.257 | +32.5% |
| FL250 | -35°C | 179 kt | M 0.298 | +49.4% |
The raw reading on your airspeed indicator. It is affected by instrument and position errors but is what you see in the cockpit. Most V-speeds (Vne, Vs, Vfe) are referenced to IAS.
IAS corrected for instrument and position error. Found in the POH calibration chart. At typical cruise in most GA aircraft, IAS and CAS are within 1-2 knots.
CAS corrected for non-standard pressure and temperature (air density). TAS is your actual speed through the air mass and is used for flight planning, fuel calculations, and navigation. TAS always increases with altitude because the air becomes less dense.
TAS adjusted for wind. Groundspeed = TAS + tailwind (or - headwind). This is your actual speed over the ground, determining how long a trip takes and your fuel burn for the route.
Memory aid: IAS → CAS (correct for instrument error) → TAS (correct for density) → GS (correct for wind). On the FAA exam, you can generally assume IAS = CAS unless told otherwise.
True airspeed is derived from calibrated airspeed using the air density ratio:
Where the density ratio (ρ / ρ0) is calculated from:
For the quick E6B estimate: add ~2% to CAS for every 1,000 ft of altitude. This rule of thumb is surprisingly accurate up to about 10,000 ft in ISA conditions but underestimates TAS at higher altitudes.
Mach number is TAS divided by the local speed of sound. The speed of sound depends only on temperature: a = 38.967 × √T (in knots, where T is in Kelvin). At sea level on a standard day (15°C), the speed of sound is about 661 knots.
Airspeed conversions are tested on every FAA knowledge exam. Here is a sample question — one of 47+ question types you need to master.
An aircraft is flying at a CAS of 120 knots at 10,000 feet pressure altitude with an OAT of -10°C. What is the approximate TAS?
This is just one of 47+ question types on your FAA written exam. Are you ready?
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True airspeed (TAS) is the actual speed of an aircraft relative to the undisturbed air mass. It differs from indicated airspeed because the pitot-static system reads lower as air density decreases with altitude. TAS is critical for flight planning, fuel burn calculations, and navigation.
As altitude increases, air density decreases. Thinner air creates less dynamic pressure on the pitot tube at the same true speed, so the airspeed indicator reads lower. To maintain the same indicated airspeed, you must fly faster through the air — meaning your TAS is higher even though the IAS is the same.
Yes, for most general aviation aircraft in normal cruise, IAS and CAS are within 1-2 knots of each other. The difference is only significant at very low speeds (near stall) or very high speeds. For exam purposes, you can assume IAS equals CAS unless the question provides a correction chart.
A common pilot rule of thumb: TAS increases approximately 2% above CAS for every 1,000 feet of altitude. For example, at 10,000 ft, TAS is roughly 20% higher than CAS. This estimate works well for quick mental math below FL200 but becomes less accurate at higher altitudes and non-standard temperatures.
Warmer-than-standard temperatures reduce air density further, increasing TAS for a given CAS. Colder-than-standard temperatures do the opposite. This is why the calculator asks for the actual outside air temperature — it can significantly affect the result, especially at higher altitudes.
More free tools: Crosswind Calculator · Density Altitude · Weight & Balance · Pressure Altitude · METAR Decoder