Process flexibility enables product designers to accommodate changing requirements in a timely and cost‐effective way. However, in order to effectively guide process flexibility in terms of investment in product design, one must be able to objectively measure process flexibility. Hence, this paper introduces a new quantitative method to measure the process flexibility of product design. Instead of directly measuring the available process flexibility, this method introduces a surrogate value (i.e., the reduced project cost resulting from process flexibility) to indirectly measure process flexibility, with consideration of requirement variations. Then, the relationships between process flexibility and impact factors of requirement variations are investigated based on the proposed method. A set of simulation experiments indicates that the method proposed and related propositions are also suitable for complex product design.