The structure of many science programs at the college level assumes that students are able to draw on and integrate ideas from multiple disciplinary contexts. However, most assessment tools focus on learning in the context of a single discipline. We describe the development and validation of an instrument to measure how well students are able to combine energy ideas from different disciplines into a coherent understanding of a phenomenon. The final version of the instrument consists of a pair of multiple-choice online assessments, along with a metric calculated from the assessment scores: the cross disciplinary learning index (CDLI). The items on both assessments were found to have satisfactory psychometric properties for our sample. Furthermore, CDLI scores correlated with other relevant factors such as amount of science coursework. The CDLI is an easy-to-use metric that could be a useful component of program-level assessment for science, technology, engineering, and mathematics majors. Furthermore, the broader measurement approach, involving a pair of assessments set in different disciplinary contexts, provides a model for assessing cross-disciplinary learning that could be utilized for other cross-cutting scientific concepts.