True Airspeed Calculator
True airspeed is calibrated airspeed corrected for air density: 100 kt CAS at 8,000 ft is about 113 kt TAS at standard temperature, and the 2% per 1,000 ft rule of thumb gives 116 kt.
For general informational use. Verify critical aviation calculations and weather with an authoritative source.
True airspeed vs indicated airspeed in the standard atmosphere
The airspeed indicator measures dynamic pressure, so in thinner air it reads lower than the aircraft’s real speed through the air. The table compares exact ISA values (low-speed, ignoring compressibility) with the “add 2% per 1,000 ft” rule of thumb.
| Calibrated airspeed | 4,000 ft | 8,000 ft | 12,000 ft |
|---|---|---|---|
| 100 kt | 106 kt (rule: 108) | 113 kt (rule: 116) | 120 kt (rule: 124) |
| 120 kt | 127 kt (rule: 130) | 135 kt (rule: 139) | 144 kt (rule: 149) |
| 150 kt | 159 kt (rule: 162) | 169 kt (rule: 174) | 180 kt (rule: 186) |
The 2% rule slightly overstates TAS as altitude increases, which is why calculators and E6B computers use pressure altitude and temperature instead. A warmer-than-standard day raises TAS further, a colder day lowers it.
IAS, CAS, EAS, TAS: the chain
- IAS is what the instrument shows.
- CAS corrects IAS for instrument and position error, using the table in the flight manual. At cruise the difference is often only a few knots.
- EAS corrects for compressibility. It matters above roughly 200 kt and 10,000 ft and is negligible for most light aircraft.
- TAS corrects for air density, using pressure altitude and temperature.
Why it matters in flight planning
Groundspeed, time en route and fuel burn all start from TAS, not IAS. Plan a 3-hour leg at 10,000 ft with IAS instead of TAS and you will underestimate groundspeed by about 15–20%, which shows up as arrival times and fuel that do not match your log. At high levels, jets use Mach instead: see the Mach Number Calculator.
Common mistakes
- Entering indicated altitude instead of pressure altitude when the altimeter setting is far from standard.
- Using the temperature from the ground instead of the outside air temperature at cruise level.
- Applying the TAS correction twice: once here and again on a flight-planning app that already does it.
From calibrated to true airspeed
The airspeed indicator measures dynamic pressure, so in thinner air it reads less than the speed at which the aircraft actually moves through the air mass. True airspeed (TAS) is that real speed and is the number you need for navigation, fuel planning and flight plans.
How to use it
- Enter calibrated airspeed (or indicated airspeed if you have no calibration table).
- Enter pressure altitude in feet.
- Enter outside air temperature.
- Read true airspeed, density ratio and the rule-of-thumb estimate.
Method and calculation
Air density ratio σ = (P/P₀) ÷ (T/T₀), where P/P₀ = (1 − 6.8756 × 10⁻⁶ × PA)^5.2559 and T is the outside air temperature in kelvin (T₀ = 288.15 K). TAS = CAS ÷ √σ. Compressibility is ignored, which is appropriate for most piston and turboprop speeds below about 200 kt and 10,000 ft.
Worked example and practical notes
120 kt CAS at 8,000 ft pressure altitude with OAT 5 °C gives a density ratio of about 0.77 and a TAS of about 137 kt. The rule of thumb (add 2% per 1,000 ft) gives 139 kt, which is close.
Accuracy and practical tips
Indicated airspeed differs from calibrated airspeed by installation and instrument errors that are listed in the aircraft flight manual. At jet speeds and altitudes, use a calculator that includes compressibility or work from Mach number.
Method based on the standard atmosphere and FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25) rules of thumb. Use your aircraft flight manual (AFM/POH) and approved flight-planning tools for real operations.
Last reviewed: October 2026
Read the guide: Density Altitude Explained: Hot and High Days
Frequently asked questions
Why is TAS higher than indicated airspeed at altitude?
Thinner air produces less dynamic pressure for the same true speed, so the indicator reads lower than the real speed.
What is the 2% rule?
True airspeed increases by roughly 2% of indicated airspeed for every 1,000 ft of altitude. It is a quick cross-check, not an exact value.
Should I enter IAS or CAS?
CAS is better. If you only know IAS, the result includes your instrument and position error.
Does this work for jets?
Not accurately at high speeds. Compressibility becomes significant above about 200 kt and at high altitude.