The purpose of capability growth index is to give designers and development managers an early heads-up as to where they stand in achieving the metric under scrutiny, such as MTBF. The Capability Growth Index measure is based on the widely used quality process capability indices, Cp and Cpk. The problem reliability practitioners have in the area of design for reliability is that the major reliability metrics are mean time between failure, mean time to repair and operational availability. Mean time between failure, mean time to repair and operational availability are moments of a distribution, are usually not s-normal, and thus, a capability growth index based upon a process capability methodology will lead to incorrect results. This paper examines the development of a design margin index for use with mean time between failure and operational availability.