Fluid feed rate and operating pressure range are the key parameters of an injector, and vacuum pressure and evacuation rate are the key parameters for an ejector.
Compression ratio and the entrainment ratio may also be defined:
The compression ratio of the injector, , is defined as ratio of the injector's outlet pressure  to the inlet pressure of the suction fluid .
The entrainment ratio of the injector, , is defined as the amount  (in kg/h) of suction fluid that can be entrained and compressed by a given amount  (in kg/h) of motive fluid.