True Airspeed Calculator (TAS from Indicated or Calibrated Airspeed)
True airspeed from calibrated airspeed, pressure altitude and outside air temperature — how fast the aircraft really moves through the air.
Results
The dashed line marks the value you entered.
What this tool does
The airspeed indicator measures dynamic pressure, which depends on air density, so at altitude it reads low: the aircraft is faster through the air than the dial says. Dividing the calibrated speed by the square root of the density ratio gives true airspeed, and in the standard atmosphere that is a gain of roughly 2 % per 1,000 ft. Warm air is thinner and raises TAS further. This version ignores compressibility, which is small for light aircraft at ordinary speeds and altitudes; use the figures in the flight manual for real flight planning.
Formula
TAS = CAS ÷ √σ · σ = δ ÷ θ · δ = (1 − 6.8756×10⁻⁶ × h)^5.2559 · θ = (OAT + 273.15) ÷ 288.15
Variables
| Symbol | Meaning | Unit |
|---|---|---|
cs | Calibrated airspeed (CAS) — use IAS if you have no correction | kt |
pa | Pressure altitude | ft |
oa | Outside air temperature | °C |
TA | True airspeed (TAS) | kt |
TK | True airspeed in km/h | km/h |
SG | Air density ratio (σ, 1 = sea-level standard) | — |
TG | TAS above CAS | % |
DV | Temperature relative to the standard atmosphere (ISA) | °C |
Worked example
- Calibrated airspeed (CAS) — use IAS if you have no correction110 kt
- Pressure altitude8000 ft
- Outside air temperature5 °C
- True airspeed (TAS)125.4 kt
- True airspeed in km/h232 km/h
- Air density ratio (σ, 1 = sea-level standard)0.769
- TAS above CAS14.0 %
- Temperature relative to the standard atmosphere (ISA)5.8 °C
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.