Indicated Airspeed Calculator

What is Indicated Airspeed?

Indicated airspeed (IAS) is the airspeed shown directly on the cockpit airspeed indicator — a pitot-static instrument that measures the dynamic pressure of air entering the pitot tube. It requires no correction for altitude or temperature; the needle moves in direct response to the pressure difference between pitot (ram air) and static (ambient) pressure.

Because IAS is based on dynamic pressure rather than actual velocity, it reflects the aerodynamic forces acting on the aircraft at any given moment. This makes it the primary reference speed for all aircraft performance and handling: stall speeds, manoeuvring speed (Va), never-exceed speed (Vne), flap extension speeds (Vfe), and best-rate and best-angle-of-climb speeds (Vy and Vx) are all published in IAS.

IAS is distinct from — but related to — several other airspeed definitions:

• Calibrated Airspeed (CAS): IAS corrected for pitot-static instrument error and position error (the effect of airflow disturbance around the fuselage on the static port). At cruise speeds the difference is typically 1–3 knots.
• Equivalent Airspeed (EAS): CAS further corrected for air compressibility. Relevant above approximately 200 KIAS or FL200; negligible for most GA operations.
• True Airspeed (TAS): EAS (or CAS in GA) corrected for air density. TAS is your speed through the air mass and increases with altitude.

How to Calculate IAS from TAS

The reverse calculation — converting TAS back to IAS — is useful when you have a target groundspeed or TAS from a performance chart and need to know what to set on your airspeed indicator.

The density-based formula:

IAS = TAS × √(ρ / ρ₀)

Where ρ is the actual air density at your altitude and temperature, and ρ₀ is standard sea-level density (1.225 kg/m³). Because ρ < ρ₀ at any altitude above sea level, IAS will always be less than TAS.

The practical rule-of-thumb approximation:

IAS ≈ TAS / (1 + 0.02 × Pressure Altitude in thousands of feet)

This is simply the TAS rule of thumb run in reverse. The IAS calculator at the top of this page uses the full density formula, accepting TAS, pressure altitude, and OAT for a precise result.

Step-by-Step Example

Your POH cruise performance table shows a TAS of 155 knots at 10,000 ft pressure altitude on a standard day. You are flying at an indicated altitude of 9,800 ft with an altimeter setting of 30.12 inHg and OAT of −5°C. What IAS should you expect?

1. Pressure Altitude: PA = 9,800 + (29.92 − 30.12) × 1,000 = 9,800 − 200 = 9,600 ft
2. ISA Temperature at 9,600 ft: 15 − (9.6 × 2) = 15 − 19.2 = −4.2°C
3. Temperature deviation: −5 − (−4.2) = −0.8°C — very close to standard, so density is near the book value.
4. Rule-of-thumb IAS: 155 / (1 + 0.02 × 9.6) ≈ 155 / 1.192 ≈ 130 KIAS
5. With full density calculation: approximately 131 KIAS — confirming the POH performance figure is realistic for your conditions.

If your airspeed indicator is reading significantly higher or lower than this, it may indicate an instrument error, position error, or that actual atmospheric conditions differ from the forecast.

Why Aircraft Speed Limits Are Published in IAS

Every speed limitation in your aircraft flight manual — Vne, Vno, Va, Vfe, Vs — is expressed in indicated airspeed, not true airspeed. This is deliberate, and understanding why makes these limits easier to apply correctly.

Aerodynamic forces scale with dynamic pressure, not velocity. Structural loads, stall behaviour, and control effectiveness all depend on the pressure the airstream exerts on the aircraft surfaces — which is exactly what the pitot-static system measures. A wing stalls at the same IAS regardless of altitude, because the stall is a function of the angle at which the airflow separates, which is determined by dynamic pressure and angle of attack, not by the aircraft's actual speed through the air.

The practical consequence: if Vne is 163 KIAS, you must never exceed 163 on your airspeed indicator — period — regardless of altitude. At FL100, 163 KIAS corresponds to roughly 195 KTAS; at sea level they are nearly equal. The structural limit is the same in both cases because the aerodynamic load at 163 KIAS is the same.

One important exception: turbine aircraft and high-performance piston types operating above FL200 sometimes publish Mmo (maximum operating Mach number) alongside Vmo (maximum operating IAS). At very high altitudes, the speed of sound decreases and compressibility effects become significant — Mmo then becomes the more restrictive limit. For GA piston operations below FL200, IAS limits apply exclusively.

Frequently Asked Questions

What is an indicated airspeed calculator?

An indicated airspeed calculator converts a known true airspeed (TAS) back into the IAS you would read on your cockpit airspeed indicator, given your current altitude and temperature. It is the reverse of the TAS calculation and is useful when cross-checking POH performance figures against actual instrument readings or when planning a target cruise IAS for a given TAS.

Why does IAS decrease as altitude increases?

At altitude, air density is lower. Your pitot tube collects fewer air molecules per unit time, so the dynamic pressure at any given true speed is reduced. The airspeed indicator responds to dynamic pressure, so it reads a lower value even though the aircraft is moving at the same or greater speed through the air. This is why IAS is always less than TAS above sea level on a standard day.

What is the difference between IAS and CAS?

Calibrated airspeed (CAS) is IAS corrected for pitot-static system errors — primarily position error caused by disturbed airflow around the static port at various airspeeds and configurations. Your POH includes a calibration table or graph. At normal cruise speeds in most GA aircraft, the difference is 1–3 knots. At very slow speeds (approach and landing) or with flaps extended, position error can be larger and CAS may differ from IAS by 5–10 knots.

Do stall speeds change with altitude?

In IAS, no — stall speed remains essentially constant with altitude. The wing stalls at the same angle of attack and therefore the same dynamic pressure regardless of how high you are. Because IAS is a direct measure of dynamic pressure, the stall IAS is constant. In TAS, however, stall speed increases with altitude — you are moving faster through the thin air when you stall at altitude than you would be at sea level.

Should I use IAS or TAS when filing a flight plan?

Flight plans use TAS. ICAO flight plan Item 15 (cruise speed) requires true airspeed expressed in knots (preceded by "N") or Mach number (preceded by "M"). ATC uses your filed TAS to calculate estimated times for separation purposes. For your own fuel and time planning, you also work in TAS — convert to groundspeed by applying the wind, then use groundspeed to calculate block fuel and ETA.