Computational thinking (CT) is a set of abilities, which has lately acquired great relevance because it brings to students the components to handle in-formation and develop analytical skills, which allows them to perform ac-cording to today's society. In this study, the authors describe the development of a formative assessment tool for Scratch programming. This web tool allows school teachers to design challenges for their students, supports the assessment of CT dimensions, gives feedback to students through the challenges' results and a gamified strategy, and keeps a track of the development of CT for each student.