Calculator
Estimate True Airspeed (TAS) from Indicated Airspeed (IAS), pressure altitude, and outside air temperature (OAT).
Note: This calculator assumes IAS is close to EAS/CAS and uses a standard atmosphere pressure model from pressure altitude. It provides a practical flight-planning estimate, not a certified performance computation.
What this IAS to TAS calculator does
At higher altitudes, the air gets less dense. Your pitot-static system senses dynamic pressure, so the same indicated speed can correspond to a much higher true speed as density drops. This calculator helps you convert indicated airspeed to a realistic true airspeed estimate by combining:
- Indicated Airspeed (IAS)
- Pressure Altitude
- Outside Air Temperature (OAT)
The result is useful for cross-country planning, fuel/time estimates, and understanding why your aircraft covers more ground aloft even when the airspeed indicator appears unchanged.
IAS vs TAS: quick explanation
Indicated Airspeed (IAS)
IAS is what your airspeed indicator shows. It is driven by pressure measurement and is the primary speed reference for handling, takeoff, approach, and structural limits.
True Airspeed (TAS)
TAS is your aircraft’s actual speed through the surrounding airmass. It matters for navigation and enroute timing. At sea level in standard conditions, IAS and TAS are close. As altitude increases, TAS rises for the same IAS.
Formula used in this calculator
The calculator uses the density-ratio approach:
Here, ρ is air density at your pressure altitude and OAT, and ρ₀ is standard sea-level density. Pressure is derived from pressure altitude using the ISA pressure model, then density is computed with the ideal gas law.
How to use it correctly
- Enter IAS from your panel or EFIS.
- Set the speed unit you want to use.
- Enter pressure altitude in feet (not indicated altitude).
- Enter outside air temperature in °C.
- Click Calculate TAS.
For best results, use stable cruise values. Turbulence and transient readings can make any instantaneous TAS estimate noisy.
Rule of thumb comparison
A common pilot shortcut says TAS increases by about 2% per 1,000 ft above sea level (in typical conditions). That rule is great for quick mental math, but this calculator is more precise because it includes actual temperature.
Common mistakes to avoid
- Using indicated altitude instead of pressure altitude — this can skew results.
- Ignoring OAT — temperature has a direct impact on density and TAS.
- Assuming IAS equals CAS/EAS exactly — instrument/position error may create small differences.
- Confusing TAS and groundspeed — wind still determines your actual speed over the ground.
FAQ
Is this accurate enough for flight planning?
Yes, for practical planning and education. POH/AFM performance tables and onboard avionics remain the authoritative source.
Why is my TAS much higher than IAS at altitude?
Because thinner air means you must move faster through that air to create the same dynamic pressure seen by the instrument.
Does this include wind?
No. TAS is speed through the air. Groundspeed requires wind correction.
Final note
Understanding IAS and TAS is a core pilot skill. Use this tool to build intuition, cross-check cruise numbers, and improve fuel/time estimates on every leg.