This study investigated whether near-infrared spectroscopy (NIRS) can be used to measure the processing of arithmetic problems in school children. Another aim was to assess whether distinct formats of such problems would lead to different neural processing. Two large samples of school children from different age groups were examined while calculating or reading arithmetic problems that were either presented in numeric or in word format. As expected, we found that, compared to reading, calculation resulted in greater average oxygenation in parietal and posterior frontal regions. Neither format nor age had a significant effect on brain oxygenation. We were able to demonstrate that NIRS measurements can readily be conducted with children and in school settings, which is an indication for the ecological validity of this measurement technique.