An image forming apparatus includes an image reading section that reads an image on paper to generate image data, and an image forming section that forms on paper an image based on image data. The image forming section forms on paper a dimension correction image including marks. The image forming section includes a computation unit that performs computation using dimension correction image data, obtained by reading the dimension correction image, to generate dimension correction data, and a memory that stores the dimension correction data. The image forming section corrects image data using the stored dimension correction data and forms an image based on the corrected image data. The computation unit has a first computation mode of generating first dimension correction data based on distances between the marks and a second computation mode of generating second dimension correction data based on distances between edges of paper and the marks.