The LESD project (Linguistic Engineering for Software Development) aims to develop a set of computer tools for analysing and reasoning about aerospace software functional specifications in English. These tools will help to control the software quality at its first step: the specification phase. As specification quality factors LESD considers traceability, modifiability, completeness, consistence, and verifiability. This paper deals with one of them: the modifiability factor, defining the concept and describing the mathematic model used to evaluate this factor. An example is given to illustrate the modifiability measurement.