ias to tas calculator

Convert Indicated Airspeed (IAS) to True Airspeed (TAS) using pressure altitude and outside air temperature (OAT). If OAT is left blank, the calculator uses ISA standard temperature at your selected altitude.

Indicated Airspeed (IAS) in knots

Pressure Altitude (ft)

Outside Air Temperature (°C) (optional)

Tip: For quick planning, a common rule is TAS increases ~2% per 1,000 ft.

What does IAS to TAS mean?

IAS (Indicated Airspeed) is what your airspeed indicator shows. It reflects dynamic pressure at the pitot-static system and is critical for handling, climb, approach speed, and stall margins. TAS (True Airspeed) is your actual speed through the air mass.

As altitude increases and air density drops, the same IAS corresponds to a higher TAS. That means your aircraft covers more distance over time for the same indicated speed.

Why pilots convert IAS to TAS

Flight planning: TAS helps estimate enroute time and fuel burn.
Navigation: Groundspeed calculations require TAS plus wind.
Performance awareness: At altitude, your indicated speed can look low while true speed is much higher.
Cross-country efficiency: Better cruise planning and range forecasting.

How this calculator works

This tool uses ISA pressure at pressure altitude and combines it with the entered OAT to estimate local air density. Then it applies:

TAS ≈ IAS / √σ, where σ = ρ/ρ₀ (density ratio relative to sea level standard conditions).

Assumptions used:

IAS is treated as close to CAS/EAS for practical GA planning.
Subsonic operation where compressibility effects are modest.
Pressure altitude is correctly set from current altimeter setting.

Quick rule-of-thumb comparison

A common mental shortcut is:

TAS ≈ IAS × (1 + 0.02 × altitude in thousands of feet)

The calculator also shows this estimate so you can compare quick cockpit math with a density-based result.

Example

If you fly at 120 KIAS and 8,000 ft pressure altitude on a warm day, your TAS may be around the upper 130s in knots, depending on OAT. That is a significant difference when estimating time enroute over long legs.

Important limitations

This is an educational planning calculator, not a certified flight instrument.
At higher speeds/altitudes, compressibility and instrument error corrections become more important.
Use POH/AFM performance tables, approved avionics, and operational procedures for real-world decisions.

IAS, TAS, and groundspeed: do not mix them up

IAS

Used for aircraft control and structural limits (V-speeds are generally indicated/calibrated references).

TAS

Actual speed through the surrounding air.

Groundspeed (GS)

Speed over the ground after wind is applied. You can have high TAS but low GS in strong headwinds.

Frequently asked questions

Can I leave temperature blank?

Yes. The calculator will assume ISA standard temperature at your chosen pressure altitude.

Why does TAS increase with altitude if IAS stays the same?

Because thinner air produces less dynamic pressure for a given true speed. To show the same IAS, you must move faster through that thinner air.

Is this the same as E6B output?

It is conceptually similar and should be close for common GA cases, but exact values can differ depending on corrections and method.

Bottom line

Use this IAS to TAS calculator to quickly convert indicated speed into a realistic cruise airspeed for planning. It is especially helpful for cross-country pilots, student pilots learning performance concepts, and anyone comparing route options in variable altitude and temperature conditions.