Today, process languages are frequently used for implementing service-oriented systems and a variety of specifications for this task exist. These specifications strive for the portability of processes among different runtime environments, i.e., process engines. However, direct portability, especially of executable processes, is seldom achieved. If processes cannot be ported directly among engines, an option is to adapt them. Such an adaptation is nontrivial and hence automated support is desirable. A first step in this direction is the quantification of the design-time adaptability of a process. This quantification is the goal of this paper. We formally define software metrics for measuring the design-time adaptability of processes and validate them theoretically with respect to measurement theory and construct validity using two validation frameworks. Moreover, we implement the metrics computation for Business Process Model and Notation (BPMN) processes and demonstrate their practical applicability with an evaluation of a large set of open source processes.