In aviation, equivalent airspeed (EAS) is calibrated airspeed (CAS) corrected for the compressibility of air at a non-trivial Mach number. It is also the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the true airspeed (TAS) and altitude at which the aircraft is flying. In low-speed flight, it is the speed which would be shown by an airspeed indicator with zero error. It is useful for predicting aircraft handling, aerodynamic loads, stalling etc.

$${\displaystyle \mathrm {EAS} =\mathrm {TAS} \times {\sqrt {\frac {\rho }{\rho _{0}}}}}$$

where ρ is actual air density and ρ₀ is standard sea level density (1.225 kg/m³ or 0.00237 slug/ft³).

EAS is a function of dynamic pressure:

$${\displaystyle \mathrm {EAS} ={\sqrt {\frac {2q}{\rho _{0}}}}}$$

where q is the dynamic pressure ${\displaystyle q={\tfrac {1}{2}}\,\rho \,v^{2}.}$

EAS can also be obtained from the aircraft Mach number and static pressure.

$${\displaystyle \mathrm {EAS} ={a_{0}}M{\sqrt {P \over P_{0}}}}$$