This paper uses a rich longitudinal dataset to measure the evolution of the gender differences in numeracy among school-age children in Indonesia. Girls outperformed boys by 0.08 standard deviations when the sample was around 11 years old. Seven years later, the gap has widened to 0.19 standard deviations, equivalent to around 18 months of schooling. I find no evidence that households invest more resources in girls relative to boys. However, there is suggestive evidence that schools play a role in fostering the gender gap in numeracy. Given the importance of numeracy in later life, there may be some scope for public policies to address the widening numeracy gap between genders.