Scale efficiency can be used to determine how close an observed DMU is to the most productive scale size. It may be calculated as the ratio of the measure of technical efficiency calculated under the assumption of constant returns to scale (CRS) to the measure of technical efficiency calculated under the assumption of variable returns to scale (VRS). A measure of technical efficiency under VRTS is given by the input-oriented BCC model and a measure of technical efficiency under CRTS is given by the solution to the input-oriented CCR model. Finally, scale efficiency (SE) for each DMU is given by:

SE is not greater than one. For a BCC-efficient DMU with CRS characteristics, i.e., in the most productive scale size, its scale efficiency is one. The CCR score is called the (global) technical efficiency (TE), since it takes no account of scale effect as distinguished from PTE. On the other hand, BCC expresses the (local) pure technical efficiency (PTE) under variable returns-to-scale circumstances. Using these concepts, the above relationship demonstrates a decomposition of efficiency as:

This decomposition, which is unique, depicts the sources of inefficiency, i.e., whether it is caused by inefficient operation (PTE) or by disadvantageous conditions displayed by the scale efficiency (SE) or by both.