Although correlations between magnitude processing and math skills are well established, direct tests of interventionsthat improve magnitude processing are scarce, and the few extant studies have depended on lab-based tasks. Advancesin interactive technology create novel opportunities to design learning experiences that also permit directly testing causalmechanisms in more naturalistic contexts. To capitalize on these opportunities, we developed Fractions War, an iOS appfor tablets to train fractions magnitude representations. Players turn over pairs of cards that create a fraction, and indicatewhich player’s fraction has the larger magnitude to gain points. Cards can be altered to present comparisons betweensymbolic fractions (2/7), nonsymbolic ratios (2 diamonds over 7 hearts), or mixed representations (traditional cards). Weexamine hallmarks of fraction magnitude processing (e.g. the numerical distance effect) using in-game data and discussongoing work testing the effectiveness of Fractions War for improving fractions magnitude processing.