E6B True Airspeed Calculator (PA + OAT Window)
The exact computation behind the E6B's TAS window — line up pressure altitude against temperature, read true airspeed — with every intermediate value shown.
When you rotate the E6B so pressure altitude lines up with OAT in the little window, you are mechanically dividing δ by θ. This tool shows the gears: the three atmosphere ratios that the aluminum hides.
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 exact computation behind the E6B's TAS window — line up pressure altitude against temperature, read true airspeed — with every intermediate value shown.
About E6B True Airspeed Calculator (PA + OAT Window)
Every pilot who has twisted a metal E6B to align pressure altitude with temperature has computed δ/θ without knowing it. This calculator performs the identical computation digitally — but unlike the wheel, it exposes the intermediate atmosphere ratios (δ, θ, σ) so instrument-rating students and checkride candidates can explain what the window actually does, not just that it works.
How to use E6B True Airspeed Calculator (PA + OAT Window)
- 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 δ = f(PA); θ = T/288.15; σ = δ/θ; TAS = CAS/√σ — exactly what the whiz wheel's TAS window encodes substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use E6B True Airspeed Calculator (PA + OAT Window)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula δ = f(PA); θ = T/288.15; σ = δ/θ; TAS = CAS/√σ — exactly what the whiz wheel's TAS window encodes with sources cited on the page
- ✓When you rotate the E6B so pressure altitude lines up with OAT in the little window, you are mechanically dividing δ by θ. This tool shows the gears: the three atmosphere ratios that the aluminum hides.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
What is the E6B's TAS window really doing?+
Mechanically dividing the pressure ratio by the temperature ratio. The altitude scale is logarithmic in δ and the temperature scale in θ; aligning them subtracts the logs, yielding σ, and the outer wheels then divide CAS by √σ. A 1940s slide rule computing 1970s ISA equations — this tool just removes the aluminum.
Why does the E6B want pressure altitude rather than indicated?+
Because δ — the pressure ratio doing the work — is defined against the standard datum. Set 29.92 momentarily, read the altimeter, and twist that number in. Using indicated altitude on a non-standard day mis-positions δ by up to several hundred feet of equivalent error.
My electronic E6B differs from the metal one by a knot — why?+
The metal wheel reads to the width of a pencil line (±1–2 kt at cruise speeds) and most users interpolate the temperature scale coarsely. Electronic versions and this page carry full precision. A one-knot disagreement is the wheel's resolution, not a formula difference — they encode the same atmosphere.
Is the E6B method exact or an approximation?+
The σ method is exact for incompressible flow — perfect for GA speeds. Above roughly 200 KCAS or FL250, compressibility makes CAS read high relative to the equivalent airspeed the formula assumes, so true TAS is slightly less than the window suggests. Jet crews use Mach-based computations instead (see our Mach tools).
Related Field tools
Sunrise & Sunset Calculator
Exact rise, set, solar noon and day length for any place and date — the NOAA solar equations with the refraction fine print included.
● LiveGolden Hour & Blue Hour Calculator
Tonight's golden hour and blue hour, computed from sun elevation — the photographer's light windows with the angles that define them.
● LiveDay Length Calculator
Hours of daylight for any date and latitude, how fast it's changing, and the swing between your solstices.
● Live