EAS, CAS & TAS Converter (Compressibility)
The speed chain jets actually use: CAS → EAS (compressibility correction) → TAS, with the error of skipping EAS quantified at your speed and altitude.
Below ~200 KCAS and FL200 the correction hides inside a knot, which is why GA ignores EAS entirely. At 250 KCAS and FL250 it's several knots — and structural engineers define Vᴀ and flutter margins in EAS for exactly this reason.
Formula
⚠️ For flight planning and education only — verify with your POH/AFM, certified instruments and official sources. Not for primary navigation or airworthiness decisions.
The speed chain jets actually use: CAS → EAS (compressibility correction) → TAS, with the error of skipping EAS quantified at your speed and altitude.
About EAS, CAS & TAS Converter (Compressibility)
Between CAS and TAS lives a speed most pilots never compute: equivalent airspeed, the one the airframe's structure actually feels. At altitude and speed, ram compression makes the pitot system read high, so the honest chain is CAS → (compressibility correction) → EAS → (density correction) → TAS. This converter runs the exact subsonic pitot equations and—instructively—shows how many knots the naive CAS/√σ shortcut overstates at your numbers.
How to use EAS, CAS & TAS Converter (Compressibility)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula qc/p₀ = (1+0.2(CAS/a₀)²)^3.5 − 1; M = √(5((qc/p+1)^{2/7} −1)); TAS = M·a₀·√θ; EAS = TAS·√σ substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use EAS, CAS & TAS Converter (Compressibility)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula qc/p₀ = (1+0.2(CAS/a₀)²)^3.5 − 1; M = √(5((qc/p+1)^{2/7} −1)); TAS = M·a₀·√θ; EAS = TAS·√σ with sources cited on the page
- ✓Below ~200 KCAS and FL200 the correction hides inside a knot, which is why GA ignores EAS entirely. At 250 KCAS and FL250 it's several knots — and structural engineers define Vᴀ and flutter margins in EAS for exactly this reason.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
What exactly is equivalent airspeed?+
The sea-level-density speed producing the same dynamic pressure your airframe currently feels: EAS = TAS·√σ. Structural limits, gust loads and flutter boundaries scale with dynamic pressure, so engineers define them in EAS. CAS equals EAS at sea level and drifts above it as compressibility inflates the pitot reading.
When does compressibility start to matter?+
As a planning line: above 200 KCAS or FL200, and seriously above both. The error grows roughly with M² × altitude: 3–5 kt for turboprops in the mid-20s, 10+ kt for jets at 300 KCAS in the 30s — which is why jet ADCs compute Mach first and derive everything from it.
Why does the pitot over-read at high speed?+
Ram compression: air piling into the pitot tube compresses, adding pressure beyond the incompressible ½ρV². The ASI's calibration (built on sea-level supersonic-free assumptions) interprets that extra pressure as extra speed. The exact subsonic relations this tool uses (the 3.5-power law) account for it precisely up to Mach 1.
Does my light aircraft ever need EAS?+
Two niche cases: interpreting maneuvering speed correctly (Vᴀ certification is dynamic-pressure-based) and any flutter-adjacent discussion for fast homebuilts — Vne for many experimentals is specified in TAS or EAS, not IAS, precisely because flutter follows true dynamic conditions. Below 200 KCAS the numeric difference stays inside a knot; the conceptual difference can still matter.
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