Why is true airspeed higher than indicated airspeed?

As you climb, the air thins, so the airspeed indicator under-reads because it senses dynamic pressure from fewer air molecules. True airspeed corrects for this lower density, so it grows roughly two percent for every thousand feet of density altitude.

What is the difference between pressure and density altitude?

Pressure altitude is what your altimeter shows with the setting at 29.92. Density altitude further corrects pressure altitude for temperature, and it is density altitude that actually drives the airspeed correction and aircraft performance.

Should I enter indicated or calibrated airspeed?

Enter calibrated airspeed for best accuracy. Calibrated airspeed is indicated airspeed corrected for the instrument and position error listed in your aircraft flight manual, which can be a few knots at the speeds where it matters.

How accurate is this compared to an E6B?

It uses the same density-ratio physics as a mechanical or electronic flight computer and matches it within a knot or two across normal general-aviation altitudes and temperatures. The rule-of-thumb figure is also shown for a quick mental cross-check.

Does this account for compressibility at high speed?

No. The simple density correction is accurate for typical piston and turboprop speeds. Above roughly 200 knots and high altitude a small compressibility correction applies, which jet flight computers add separately.

True Airspeed (TAS) Calculator

True airspeed is the speed of your aircraft through the air mass, which is what flight planning and wind calculations need, rather than the lower number your airspeed indicator shows in thin air. This calculator converts calibrated airspeed to true airspeed using your altitude and temperature.

How it works

The conversion is driven by air density, expressed as density altitude:

ISA temp     = 15 − 1.98 × (pressure altitude / 1000)   °C
density alt  = pressure altitude + 118.8 × (OAT − ISA temp)   ft
density ratio σ = (1 − 6.875e-6 × density altitude)^4.2561
TAS          = CAS / √σ

As density altitude climbs, the density ratio falls below one, so dividing by its square root scales calibrated airspeed up to true airspeed. The familiar rule of thumb, adding two percent per thousand feet of density altitude, is shown alongside for a quick check.

Example and notes

Cruising at 120 knots calibrated, 8,000 ft pressure altitude, with an outside air temperature of 0 degrees Celsius gives a density altitude near 9,200 ft and a true airspeed of about 138 knots. Always correct indicated airspeed to calibrated airspeed using your flight manual before entering it, and remember that on a hot day the density altitude, and therefore the airspeed correction, grows even at the same pressure altitude.