Calculator Inputs
Formula Used
The calculator uses the ideal density-ratio relation between true airspeed and indicated airspeed:
IAS ≈ TAS × √(ρ / ρ₀)
Here, ρ is local air density. The sea level standard density ρ₀ is 1.225 kg/m³. Dynamic pressure is also estimated:
q = 0.5 × ρ × TAS²
Temperature and pressure estimate air density. Humidity is included with a moist-air density correction. The final displayed speed can also include your entered instrument correction.
How to Use This Calculator
- Enter the true airspeed and choose the correct speed unit.
- Enter the flight altitude and choose feet or meters.
- Select a temperature method. Use standard atmosphere, ISA deviation, or actual outside air temperature.
- Select standard pressure or enter actual static pressure.
- Add humidity if known. Leave it at zero for dry standard air.
- Add an indicator correction if your aircraft data provides one.
- Press calculate. The result appears above the form and below the header.
- Use the CSV or PDF buttons to save the result.
Example Data Table
| True Airspeed | Altitude | Atmosphere | Density Ratio | Estimated IAS |
|---|---|---|---|---|
| 120 kt | 0 ft | ISA | 1.000 | 120.0 kt |
| 120 kt | 5,000 ft | ISA | 0.862 | 111.5 kt |
| 160 kt | 10,000 ft | ISA | 0.738 | 137.4 kt |
| 200 kt | 20,000 ft | ISA | 0.533 | 146.0 kt |
Article: True Airspeed and Indicated Airspeed
Understanding the Conversion
True airspeed describes how fast the aircraft moves through the air mass. Indicated airspeed describes what the cockpit indicator would show. The two values separate when air density changes. Dense air produces more pitot pressure. Thin air produces less pitot pressure for the same true speed. That is why an aircraft at altitude can travel fast, while the indicated value stays lower.
Why Density Matters
The calculator uses air density as the main bridge. Standard sea level density is compared with local density. Local density is estimated from altitude, pressure, temperature, and humidity. Warm air is less dense. High altitude air is less dense. Moist air is also slightly less dense than dry air. Each change lowers the indicated value for a fixed true airspeed.
Flight Planning Value
This tool is useful for study, training, and quick planning checks. It helps explain why climb performance, stall margins, and approach references depend on indicated speed. A pilot flies many limits by indicated speed because the wings respond to dynamic pressure. True airspeed is better for navigation and time planning. Both values are important, but they answer different questions.
Practical Notes
The result is an ideal estimate. Real aircraft may include position error, instrument error, and calibration tables. High speed aircraft may also need compressibility corrections. For most low speed examples, the density ratio method gives a clear educational answer. Use the correction field when you know an instrument offset. Use actual weather data when available. Use standard atmosphere data for quick comparisons.
Reading the Chart
The Plotly chart shows how indicated speed changes with altitude. It keeps true airspeed fixed. As altitude rises, density usually drops. The curve therefore moves downward. This visual pattern makes the physics easier to see. It also helps compare several classroom or aviation examples without repeating manual calculations.
FAQs
1. What is true airspeed?
True airspeed is the aircraft speed through the surrounding air mass. It is useful for navigation, fuel planning, timing, and wind correction.
2. What is indicated airspeed?
Indicated airspeed is the speed shown by the airspeed indicator. It is based on pitot-static pressure and is closely tied to aerodynamic force.
3. Why is indicated airspeed lower at altitude?
Air density decreases with altitude. For the same true airspeed, thinner air creates less dynamic pressure. The indicator therefore shows a lower value.
4. Is this the same as calibrated airspeed?
No. Calibrated airspeed includes corrections for position and instrument error. This calculator gives an ideal indicated estimate using density and optional correction.
5. Does humidity matter?
Humidity has a small effect. Moist air is slightly less dense than dry air. The effect is usually minor but helpful for advanced comparisons.
6. Can I use actual weather data?
Yes. Choose actual pressure and outside air temperature when available. This usually improves the estimate compared with standard atmosphere assumptions.
7. Does the formula work for jets?
It is best for educational and lower speed estimates. High speed aircraft may need compressibility, calibrated speed, and aircraft-specific correction tables.
8. Why fly by indicated airspeed?
Many aircraft limits relate to aerodynamic pressure. Stall speed, approach speed, and maneuvering speed are commonly managed with indicated airspeed.