A measure of the closeness to equilibrium on a plate or stage within a fractional distillation column. The vapour and liquid on an ideal stage are in equilibrium. However, in practice, this may not be the case. The Murphree plate efficiency is therefore expressed as the ratio of the increase in mole fraction of vapour of a volatile component passing through a plate in a column to the same increase when the vapour is in equilibrium. In effect, more stages are therefore required to bring about a desired separation. For a binary distillation, it is presented as:

where the subscripts n and n−1 refer to the outlet and inlet vapour streams to a stage, and y* is the equilibrium vapour concentration. It was proposed in 1925 by Eger V. Murphree (1892–1962).