True Airspeed Calculator (TAS from IAS)
True airspeed (TAS) is your aircraft's speed through the air mass, corrected for the lower air density at altitude. Enter your indicated airspeed, pressure altitude, and optional outside air temperature to get TAS, density altitude, and the density ratio.
Speed unit
Altitude unit
°C
Speed relative to the surrounding air mass
- 1
ISA temperature at altitude
288.15 − 6.5 K/km × 2.438 km = 272.3 KStandard atmosphere lapse rate: 6.5 K per 1 000 m. - 2
OAT in Kelvin
-0.8 + 273.15 = 272.3 K - 3
Pressure ratio (T_ISA ÷ T₀)^5.256
(272.3 ÷ 288.15)^5.256 = 0.74279 - 4
Density ratio σ = (P/P₀) × (T₀/T_OAT)
0.74279 × 288.15 ÷ 272.3 = 0.786 - 5
True airspeed TAS = IAS ÷ √σ
120 ÷ √0.786 = 135.4
How does this calculator work?
TAS = IAS / √σ, where σ = ρ/ρ₀ is the ISA density ratio at your pressure altitude corrected for outside air temperature. At FL100 on a standard day, σ ≈ 0.74, so TAS is about 16% above IAS. Enter IAS, altitude, and OAT to get exact TAS, density altitude, and density ratio.
Formula
How this is calculated
The airspeed indicator (ASI) measures dynamic pressure q = ½ρv². At sea level on a standard day, the instrument is calibrated to give the correct airspeed. But at altitude, air density ρ is lower, so the same dynamic pressure is produced by a higher true velocity — the ASI reads lower than the actual speed through the air.
This calculator uses the ICAO International Standard Atmosphere (ISA) model to find the density ratio σ = ρ/ρ₀ at the given pressure altitude. In the troposphere (below 36,089 ft / 11,000 m) ISA temperature falls at 6.5 K per 1,000 m and pressure follows T^5.256. If the actual OAT differs from ISA, the density ratio is corrected: σ = (P/P₀) × (T₀/T_OAT). True airspeed then follows TAS = IAS / √σ.
Density altitude — the ISA altitude equivalent to the actual density — is approximated as pressure altitude + 120 ft per °C above ISA. This formula is valid for the EAS approximation (IAS ≈ EAS), which is accurate below about 250 knots; above that, compressibility corrections apply. The stratosphere case (above 36,089 ft) uses an isothermal layer at 216.65 K.
Frequently asked questions
At altitude, air density is lower. The airspeed indicator measures aerodynamic (dynamic) pressure — lower density means lower pressure for the same true speed, so the indicator reads less than your actual speed. TAS = IAS / √σ, and σ < 1 above sea level, so dividing by a fraction less than 1 gives a larger TAS.
Pressure altitude is the altitude your altimeter reads when set to the standard pressure of 1013.25 hPa (29.92 inHg). It is used in the ISA model to compute air density. In practice, set the altimeter sub-scale to 1013 hPa and read the indicated altitude.
A common aviation approximation is: TAS ≈ IAS + 2% of IAS per 1,000 ft of altitude. At 10,000 ft, TAS ≈ IAS × 1.20 (about 20% faster). This is rougher than the ISA formula but useful for mental arithmetic; the true correction also depends on temperature.
Also known as
TG we-Calculate Editorial Team. (2026). True Airspeed Calculator (TAS from IAS) [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/true-airspeed-calculator
TG we-Calculate Editorial Team. "True Airspeed Calculator (TAS from IAS)." TG we-Calculate. 2026. https://we-calculate.com/calculator/true-airspeed-calculator.
TG we-Calculate Editorial Team, "True Airspeed Calculator (TAS from IAS)," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/true-airspeed-calculator
@misc{wecalculate_true_airspeed_calculator, title = {True Airspeed Calculator (TAS from IAS)}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/true-airspeed-calculator}}, year = {2026}, note = {TG we-Calculate} }
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