# True Airspeed Calculator

Calculate true airspeed from indicated airspeed, altitude, and temperature. Essential for flight planning, fuel calculations, and navigation.

## What this calculates

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.

## Inputs

- **Indicated Airspeed (IAS)** (kts) — min 0 — Airspeed shown on the airspeed indicator.
- **Pressure Altitude** (ft) — min -1500, max 45000 — Altitude with altimeter set to 29.92 inHg (1013.25 hPa).
- **Outside Air Temperature** (°C) — min -70, max 60 — Outside air temperature at your altitude.

## Outputs

- **True Airspeed (TAS)** (kts) — True airspeed in knots.
- **True Airspeed** (mph) — True airspeed in miles per hour.
- **True Airspeed** (km/h) — True airspeed in kilometers per hour.
- **Density Altitude** (ft) — Density altitude at these conditions.
- **Mach Number** — Speed as a fraction of the speed of sound.

## Details

**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.

## Frequently Asked Questions

**Q: What is the difference between IAS, CAS, and TAS?**

A: IAS (Indicated Airspeed) is what the instrument reads. CAS (Calibrated Airspeed) corrects IAS for instrument and position errors. TAS (True Airspeed) corrects CAS for altitude and temperature. For most light aircraft, IAS and CAS are close enough that pilots go directly from IAS to TAS.

**Q: Why do pilots care about true airspeed?**

A: TAS is your actual speed through the air mass, which is needed for navigation. Ground speed (what matters for getting to your destination) is TAS adjusted for wind. Using IAS for flight planning would give incorrect time, distance, and fuel calculations.

**Q: Does TAS affect stall speed?**

A: The airplane stalls at approximately the same IAS regardless of altitude, because the wing needs a certain dynamic pressure to fly. But the TAS at stall increases with altitude. At 10,000 feet, your true stall speed is about 17% higher than the number on the airspeed indicator.

**Q: What is pressure altitude?**

A: Pressure altitude is the altitude indicated when the altimeter is set to 29.92 inHg (standard sea-level pressure). It is the field elevation corrected for non-standard pressure: PA = field elevation + (29.92 - altimeter setting) x 1000. At higher altitudes, pressure altitude is simply what the altimeter reads when set to 29.92.

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Source: https://vastcalc.com/calculators/physics/true-airspeed
Category: Physics
Last updated: 2026-04-08