True Airspeed Calculator

Use the true airspeed calculator to calculate the true airspeed (referred to as TAS) for all aircraft.

True airspeed136.65 knots16.65 knots above the indicated figure — an airspeed indicator measures dynamic pressure, so it under-reads in thin air

Pressure at altitude794.952 hPa78.46% of sea level, 23.475 inHg

Standard temperature at that altitude2 °CISA lapse rate of 6.5 K per km

Actual air density0.94469 kg/m³77.12% of sea-level standard density

Standard density at this altitude1.00649 kg/m³

Density altitude2,626 m8,614 ft — the altitude at which the standard atmosphere has this density, and what an aircraft actually performs to

Density altitude excess626 mthe air is thinner than standard, so the aircraft behaves as though it were this much higher — hot and high is the dangerous combination

Pressure altitude2,000 mwhat an altimeter set to 1013.25 hPa would read

Rule of thumb check135.74 knotsabout 2% per 1000 ft — close to the exact figure at moderate altitude

Oxygen partial pressure166.542 hPa78.5% of the sea-level value — the composition does not change with altitude, only the pressure does

Speed of sound there332.53 m/sfalls with temperature, not with pressure — which is why the Mach number for a given true airspeed rises with altitude

The formula

p = p₀(T/T₀)^5.2559 below the tropopause

Density altitude is what an aircraft feels

Aerodynamic performance depends on air density, and density falls with both altitude and temperature while humidity lowers it further. Density altitude is the altitude at which the standard atmosphere would have the density you actually have — so an aircraft at a 1,500 m airfield on a hot day may be performing as though it were at 3,000 m. "Hot and high" is the classic accident setup, and humidity makes it slightly worse still.

Moisture reducing density surprises people. Water's molar mass is 18 against nitrogen's 28, so vapour displacing air makes the mixture lighter. It is a small effect but it goes the opposite way to intuition.

An airspeed indicator measures dynamic pressure, ½ρv², so in thin air it under-reads the true airspeed. True airspeed grows as the inverse square root of density — roughly 2% per thousand feet, which is the standard mental correction. Indicated airspeed is nonetheless the right thing to fly by, since stall and structural limits also depend on dynamic pressure.

The 5.2559 exponent in the barometric formula is not a fit: it is gM/RL for the standard 6.5 K/km lapse rate. The model is therefore exact within its own assumptions, and departs from reality only where the real lapse rate does.