True Airspeed Calculator
Convert calibrated airspeed to TAS using the ISA density at your altitude.
Inputs
Result
True airspeed
174kt
+24 kt over CAS 150 kt
Density ratio
ρ / ρ₀ at this altitude & temp
0.739
ISA deviation
-0.2°C
TAS 174 kt (CAS 150 kt at 10,000 ft, ISA -0.2°C)
Formula
Ref- Density ratio ρ/ρ₀ from PA + OAT (ISA model)
- TAS = CAS ÷ √(ρ/ρ₀)
- Rule of thumb: TAS ≈ CAS + 2% per 1000 ft
Worked example
RefCAS 150 kt at 10,000 ft pressure altitude, OAT −5 °C.
- 1.Density ratio ρ/ρ₀ ≈ 0.74 at 10,000 ft near ISA.
- 2.TAS = CAS ÷ √(ρ/ρ₀) = 150 ÷ 0.86 ≈ 174 kt.
- 3.Cross-check: ~2% per 1,000 ft → +20% ≈ 180 kt.
TAS ≈ 174 kt (about +24 kt over CAS).
How it works
RefAs altitude increases the air thins, so the same dynamic pressure (indicated/calibrated airspeed) corresponds to a faster true speed through the air. Dividing CAS by the square root of the density ratio gives TAS — the speed you use in the wind triangle and for ground speed.
DGCA student tip
ExamThe quick rule is about 2% of CAS added per 1,000 ft, warmer air adding a little more. TAS always exceeds CAS at altitude, and it's TAS — never IAS — that goes into navigation calculations.
Common mistakes
Watch- Using indicated airspeed in the wind triangle instead of TAS.
- Ignoring temperature — hot air raises TAS for the same altitude.
- Applying the 2%/1000 ft rule with density altitude instead of pressure altitude.