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.

What is the True Airspeed (TAS) Calculator?

Convert calibrated or indicated airspeed to true airspeed using pressure altitude and outside air temperature, with density altitude shown. Pilots use it for flight planning, nav-log construction, and wind-correction-angle work. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

True Airspeed (TAS) Calculator

Name: True Airspeed (TAS) Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

Groundspeed = TAS ± wind component. Your airspeed indicator always shows indicated (or calibrated) airspeed, which is what aerodynamically matters for stall speed and performance tables. But to calculate how long a leg takes or what ground track to expect, you need the actual speed through the air mass, and then you add or subtract the wind. At sea level on a standard day, indicated airspeed equals TAS. At 10,000 ft, TAS is roughly 20% higher — a Cessna indicating 100 knots is actually moving through the air at around 120 knots, and planning with the lower number would produce meaningful navigation errors on longer legs.

Fuel and endurance planning also depends on TAS. Fuel burn is driven by engine power, not ground speed, and the engine is burning fuel while it is running — so flight time is endurance in the air, which is distance divided by TAS (ground speed gives you time to a point accounting for wind, TAS gives you how long the engine runs). Both numbers matter and this calculator gives you the TAS component cleanly.

The rule of two percent

The simple rule of thumb — add 2% of indicated airspeed for each 1,000 feet of density altitude — is printed in many flight training texts and is close enough for mental arithmetic during planning. For example, at 8,000 ft density altitude: 100 knots indicated × (1 + 0.02 × 8) = 116 knots TAS. The formula-based result will be slightly different because the density correction is not perfectly linear with altitude, but the rule is accurate to within a knot or two at typical general-aviation altitudes. This tool shows both so you can cross-check which to trust at your specific altitude and temperature.

Hot weather and high elevation airports

Temperature above the ISA standard raises density altitude significantly above pressure altitude. On a hot summer day at a high-elevation airport, density altitude can be 3,000–5,000 ft above the field elevation. This inflates TAS relative to what a pressure-altitude-only correction would suggest, and it also reduces aircraft performance (longer takeoff roll, lower climb rate). Always enter the actual OAT from your OAT probe or the ATIS, not the standard ISA temperature, to get a realistic TAS and density altitude.