True Airspeed Calculator
Your airspeed indicator reads lower than your actual speed through the air at altitude because the air is thinner. At 8,000 feet, an indicated 120 knots is actually about 132 knots true airspeed. Pilots need true airspeed (TAS) for flight planning, fuel burn calculations, and accurate navigation. This calculator also shows density altitude and Mach number.
Why IAS and TAS differ:
The airspeed indicator measures dynamic pressure (the ram air pressure in the pitot tube). At higher altitudes, the air is less dense, so the same true speed produces less dynamic pressure. The indicator reads low by a predictable amount.
The correction:
TAS = IAS / sqrt(density ratio)
The density ratio depends on both pressure altitude (how high you are) and temperature (warmer air is less dense). At sea level in standard conditions, TAS equals IAS. At 10,000 feet, TAS is about 17% higher than IAS.
Pilot rules of thumb:
- Add about 2% to IAS for every 1,000 feet of altitude
- At 10,000 feet, TAS is roughly IAS × 1.17
- At 20,000 feet, TAS is roughly IAS × 1.39
When it matters:
- Flight planning: Ground speed = TAS plus or minus wind. If you plan with IAS instead of TAS, your ETA and fuel calculations will be wrong.
- Performance charts: Most POH performance data uses TAS for cruise performance.
- Stall speed: The stall IAS stays roughly constant with altitude, but the stall TAS increases. This means you are covering more ground at stall speed at altitude.
Mach number: At higher altitudes and speeds, pilots switch from IAS to Mach number. The speed of sound decreases with temperature, so Mach 0.78 at 35,000 feet is about 460 knots TAS.