Recent functional neuroimaging studies have begun to clarify how the human brain performs the everyday activities that require mental calculation. We used fMRI to test the hypotheses that there are specific neural networks dedicated to performing an arithmetic operation (e.g. + or -) and to performing processes that support more complex calculations. We found that the right inferior parietal lobule, left precuneus and left superior parietal gyrus are relatively specific for performing subtraction; and bilateral medial frontal/cingulate cortex are relatively specific for supporting arithmetic procedure complexity. We also found that greater difficulty level was associated with activation in a brain network including left inferior intraparietal sulcus, left inferior frontal gyrus and bilateral cingulate. Our results suggest that the network activated by the simplest calculation serves as a common basis, to which more regions are recruited for more difficult problems or different arithmetic operations.