Raising awareness of educational games and game-based learning is an important step for large-scale adoption of these new educational methods. Digital game-based learning provides unique opportunities to engage students in learning, which is especially important for subjects that students struggle with, such as mathematics. Therefore, the aim of the current paper was to evaluate the usefulness of a math game competition for engaging, assessing, and training rational numbers in students from different schools. In particular, we investigated whether playing a digital game would improve students' magnitude understanding of rational numbers and whether playing behavior can be used for assessment purposes. Finnish fourth (n = 59, Mage = 10.36) and sixth-graders (n = 105, Mage = 12.34) participated in a math game competition relying on intra-classroom cooperation and inter-classroom competition. The students played a digital rational number game called Semideus, which is founded on number line estimation task mechanics in which players have to estimate the spatial position of a target number on a number line with only its start and endpoint specified. In previous empirical studies, this task mechanic has been successfully used to assess and foster students number magnitude understanding in conventional nongame based settings. Consequently, students were allowed to play the game as much as they wanted during a three-week period in order to improve their rational number knowledge and were able to check the status of the competition online. As expected, sixth grade students performed more accurately than fourth grade students in the game-based rational number magnitude estimation tasks. Moreover, results indicated that students benefited significantly from participating in the math game competition with respect to rational number knowledge. Importantly, the Semideus game was particularly effective with students who started with less rational number knowledge. Overall, the study demonstrated that participation in a math game competition seems to be a useful and engaging approach to assess and support the development of students' rational number knowledge. [ABSTRACT FROM AUTHOR]