Adaptable design aims to extend the utilities of designs and products. Adaptability is classified into product and design adaptabilities. Product adaptability indicates that functionality and life can be extended for both economical and environmental benefits. Design adaptability improves design reuse by using existing designs to develop new designs more efficiently. To evaluate adaptable design, it is necessary to develop a method for quantitative measurement of the adaptability of products. A new method has been developed that first analyzes the independencies of functions and functional modules and then evaluates the adaptability of interfaces with two indices, as well as the performances of adaptive requirements. The effectiveness of the proposed method is demonstrated by an illustrative example of personal computer motherboards. The results show that the method can evaluate and reveal product adaptability for improving design and providing innovative design.