Social capital describes students' access to, and ability to mobilize, relationships that help them further their potential and their goals. Just like skills and knowledge, relationships offer resources that drive access to opportunity. Most schools and programs wholeheartedly agree that relationships matter. But far fewer actually measure students' social capital. Oftentimes, relationships, valuable as they may be, are treated as inputs to learning and development rather than outcomes in their own right. In turn, schools routinely leave students' access to relationships and networks to chance. To address this gap, a host of early innovators across K-12, postsecondary, and workforce development are making important strides toward purposefully building and measuring students' social capital in an effort to expand access to opportunity. Drawing on those emerging practices, this paper offers a framework for measuring social capital grounded in both research and practice.