Airspeed Conversion Calculator

Convert between IAS, CAS, EAS, and TAS with real-time ISA atmosphere modeling. Essential for flight planning, performance calculations, and aviation education.

Key relationships (low-speed approximation, compressibility ignored):

  • TAS = EAS / √σ   (σ = density ratio ρ/ρ₀)
  • EAS = CAS (calibrated airspeed ≈ equivalent airspeed for speeds < 200 kt)
  • Mach = TAS / speed of sound
Takeoff (90 kt CAS, SL) Climb (250 kt CAS @10kft) Cruise TAS 450 kt @FL350 EAS 300 kt @20kft
Computing atmosphere & conversions...
Converted Airspeeds & Atmospheric Data
ISA conditions at altitude: 5000 ft · Temp: 5.1 °C · Pressure: 843 hPa · Density ratio σ: 0.862 · Sound speed: 340 m/s
Airspeed type Knots (kt) km/h mph m/s
IAS (indicated)* 120.0 222.2 138.1 61.7
CAS (calibrated)* 120.0 222.2 138.1 61.7
EAS (equivalent)* 120.0 222.2 138.1 61.7
TAS (true) 129.2 239.3 148.7 66.5
Mach number 0.195

* Note: IAS ≈ CAS ≈ EAS assumed (low speed, no compressibility/installation errors). For precise calibration, refer to aircraft POH.

Density ratio σ vs. altitude (current altitude marked)

Understanding Airspeed Types

Airspeed conversion is fundamental for flight operations, performance calculations, and aviation meteorology. This calculator uses the International Standard Atmosphere (ISA) model to relate different airspeed definitions.

IAS / CAS / EAS
  • IAS – Indicated Airspeed: raw instrument reading.
  • CAS – Calibrated Airspeed: IAS corrected for position/installation errors.
  • EAS – Equivalent Airspeed: CAS corrected for compressibility (ignored here, so CAS ≈ EAS).
TAS & Mach
  • TAS – True Airspeed: actual speed through air. TAS = EAS / √σ.
  • Mach – ratio of TAS to local speed of sound.

International Standard Atmosphere (ISA) Model

The calculator implements the ISA model up to 20,000 m (≈65,000 ft):

  • Sea level: 15°C, 1013.25 hPa, density 1.225 kg/m³
  • Lapse rate: -0.0065 K/m up to 11 km (troposphere)
  • Temperature offset allows non-standard days (Δ°C).

Formulas used:
T = T0 - 0.0065·h + ΔT (h in meters)
P = P0 · (T / T0)5.2561
ρ = P / (R·T) , R = 287.05 J/kg/K
σ = ρ / ρ0
a = √(γ·R·T) with γ=1.4

Applications in Aviation

1
Flight planning: Convert CAS from aircraft manual to TAS for wind/ground speed calculations.
2
Performance: Determine true airspeed for fuel consumption and range.
3
Climb/descent: Manage speeds in different atmospheric conditions.

Frequently Asked Questions

True airspeed rises with altitude because air density decreases. To maintain the same dynamic pressure (indicated by CAS/EAS), the aircraft must move faster through the thinner air. The relation is TAS = EAS / √σ.

Calibrated airspeed (CAS) is indicated airspeed corrected for instrument/position errors. Equivalent airspeed (EAS) is CAS corrected for compressibility effects. At speeds below 200 kt and altitudes below 10,000 ft, the difference is negligible. This calculator assumes CAS ≈ EAS for simplicity.

The ISA model is a standard reference for average atmospheric conditions. Real-world temperatures may differ. Use the temperature offset to adjust for non-standard days. This provides a good estimate for performance calculations.