There has been a growing need felt by practitioners, researchers, and evaluators to obtain a common measure of science engagement that can be used in different out-of-school time (OST) science learning settings. We report on the development and validation of a novel 10-item self-report instrument designed to measure, communicate, and ultimately help promote science engagement among youth: the Common Instrument (CI). When administered to 7,521 elementary and middle school students participating in OST science programming, CI items were found to have good psychometric properties -- showing strong item discrimination and a range of difficulties, little difference in item functioning by grade-level, gender, or race/ethnicity, good unidimensionality, stability over time, and a small standard error of measurement over a large variety of science engagements. Given its properties, the CI is a reasonable way to collect data on science engagement in a wide range of OST science programs. Common measures, like the CI, that are reliable and valid provide a common language that enables programs to describe their strengths and challenges and make decisions about adapting and improving their approaches. Common measures are also essential for collective impact initiatives that need brief, easy-to-administer instruments to assess progress and impact of their change-making processes.