A calibration system includes an imaging device, an image forming apparatus, and a table generating section. The image forming apparatus prints a test chart containing patches of colors on a sheet. The imaging device captures images of a reference chart containing patches corresponding to the patches in the test chart and the test chart on the sheet together to generate a picture. The table generating section: calculates a weighting coefficient for each of the patches in the picture based on a color intensity of the patch; calculates, as a representative value for each of gradations of color values of the patches in the picture, a weighted average of color values of the patches in the gradation based on the weighting coefficient; and generates a gamma correction table for correcting gradation characteristics of the image forming apparatus to gradation characteristics in accordance with the reference chart based on the representative values.