An apparatus and method of calculating a resolution. The method of calculating a resolution includes selecting a region having an edge from a test chart, calculating a spatial frequency response (SFR) of an image forming device that has printed the test chart from the selected region, setting a frequency that corresponds to a principal SFR among the calculated SFRs of the image forming device as a principal frequency of the SFR of the image forming device, and judging the set principal frequency as the resolution of the image forming device.