CAS to TAS Calculator
Convert Calibrated Airspeed (CAS) to True Airspeed (TAS) using pressure altitude and outside air temperature.
Model uses a compressibility-aware pitot/static equation (subsonic) and ISA pressure from pressure altitude.
What Is CAS and Why Convert It to TAS?
When you look at the airspeed indicator in most aircraft, the number shown is closely related to IAS (Indicated Airspeed). After correcting IAS for instrument and position errors, you get CAS (Calibrated Airspeed). CAS is useful for flying the airplane safely—think takeoff speeds, climb speeds, and stall margins.
TAS (True Airspeed), on the other hand, is your actual speed through the surrounding airmass. TAS becomes especially important for navigation, flight planning, fuel estimates, and ETA accuracy. At higher altitude, the air gets thinner, so for the same CAS, TAS is higher.
How This CAS to TAS Calculator Works
This page calculates TAS from three values:
- Calibrated Airspeed (knots)
- Pressure Altitude (feet)
- Outside Air Temperature in °C (optional)
If you leave OAT blank, the calculator uses ISA standard temperature at the selected pressure altitude.
Core Steps Used
- Find static pressure from pressure altitude (ISA atmosphere).
- Convert CAS into impact pressure using sea-level calibration.
- Solve for Mach number at altitude from impact/static pressure ratio.
- Convert Mach to TAS using local speed of sound from temperature.
This gives a more realistic result than a simple rule-of-thumb in many conditions, especially as speed increases.
Quick Practical Example
Suppose you are flying with:
- CAS = 140 knots
- Pressure altitude = 10,000 ft
- OAT = -5°C
After calculation, TAS will be significantly higher than 140 knots because lower air density requires more true velocity to produce the same dynamic pressure at the pitot/static system.
CAS vs TAS vs Groundspeed
These three speeds are easy to mix up:
| Speed Type | What It Means | Best Use |
|---|---|---|
| CAS | Corrected instrument airspeed | Aircraft handling and performance references |
| TAS | Actual speed through the airmass | Flight planning and leg timing |
| Groundspeed | Speed over the ground | ETA and navigation progress |
Remember: Groundspeed = TAS ± wind component.
Common Mistakes Pilots Make
1) Mixing pressure altitude and indicated altitude
This calculator needs pressure altitude, not raw indicated altitude unless your altimeter setting is 29.92.
2) Using unrealistic OAT values
If OAT is unavailable, ISA temperature is usually a better assumption than guessing.
3) Confusing CAS with IAS
At low speeds in many GA aircraft, IAS and CAS can be close, but they are not always identical—especially near flap extension ranges or unusual configurations.
When You Should Care Most About TAS
- Cross-country planning
- Fuel and reserve calculations
- Performance checks at altitude
- Comparing expected vs actual enroute timing
FAQ
Is this calculator valid for all aircraft?
It is suitable for typical subsonic flight planning and educational use. Extremely high-speed, highly specialized, or non-standard systems may require aircraft-specific performance data.
Does humidity matter?
Humidity has a minor influence on density and speed of sound, but dry-air assumptions are standard for quick operational calculations.
What if I only know IAS?
You can often use IAS as a practical proxy for CAS at lower speeds, but the best practice is to apply your aircraft’s IAS-to-CAS correction table first.
Bottom Line
CAS helps you fly the airplane. TAS helps you manage the trip. Use this CAS to TAS calculator anytime you want more accurate enroute speed estimates based on altitude and temperature.