The adiabatic efficiency of a turbine is a measure of how efficiently it converts the enthalpy drop of the working fluid into mechanical work under adiabatic conditions (no heat transfer). It is typically denoted by the symbol  and is given by the ratio of the actual work done by the turbine to the ideal work done, assuming isentropic (adiabatic and reversible) expansion.
The formula for adiabatic efficiency () of a turbine is:
Where:
- â„Ž1Â = Enthalpy at the inlet of the turbine (kJ/kg)
- ℎ2,act = Enthalpy at the outlet of the turbine (actual) (kJ/kg)
- â„Ž2,isentropic= Enthalpy at the outlet of the turbine (isentropic) (kJ/kg)
- The SI Unit of ηad​ is dimensionless