With increasing consumer demand for a greater variety of products in changing and unpredictable quantities. Manufacturing different products tailored to the needs of their clients is a real challenge that industries face today. Today's consumers are looking for products that carry part of their identity and allow them to stand out from the crowd. In this context many indicators have been proposed to evaluate the reactivity and flexibility of a production system; such as customization, scalability and convertibility of the system. Customization is one aspect of flexibility; it expresses a system's ability to produce customized products with customized functionality. This document develops a measure of customization of reconfigurable production systems. These measures do not only impact the production system but also the product design and the process design, which can therefore serve as a guide for the customization of manufactured products. A case study is presented to show the use of the proposed approach.