Fingu is a game and a game platform using virtual manipulatives designed to help children develop competence and fluency with basic number combinations. We present results from an intervention lasting for seven to nine weeks, in which 82 children (5, 6, and 7 years old) were allowed to play the game as part of their ordinary preschool or school activities. The results showed significant positive differences between pre- and post-tests in four arithmetic measures, with moderate to large effect sizes. In contrast, most differences between post-tests and delayed post-tests were non-significant, with low or no effect sizes.