Dimensional analysis is traditionally one of the first topics covered in a general chemistry course. Chemists use dimensional analysis as a tool to keep track of units and guide them through calculations. Although unit conversions are taught in a variety of subjects over several grade levels, many students have not mastered this topic by the time they enter college. To properly equip beginning chemistry students, a collaborative active-learning activity was developed. This article describes the activity and reports data of the effects it had on students' performance in a first-semester general chemistry course at a large research institution. (Contains 5 tables and 2 figures.)