An input efficiency index gives a numeric assessment of the degree to which a given input combination falls short of being efficient in producing a fixed amount of output. This paper presents a system of axioms which characterise a certain family of efficiency indices containing the well-known Farrell and Färe-Lovell indices. The family of indices satisfying the axioms can be obtained by minimizing a suitable function, called a performance evaluation, and different choices of performance evaluation will result in different indices