An image forming apparatus includes a forming section that forms an image on a side of paper; a fixing section that applies heat to fix the image onto the side of the paper; a paper reversing section that reverses a front and back of the paper having the image fixed onto a first side in the fixing section; a first measuring section that is provided between a registration roller and the forming section to measure a first moisture content of the paper before an image is formed on the first side, and a second moisture content of the paper before an image is formed on a second side, after the image is fixed on the first side; and a calculating section that calculates amount of change of the size of the paper on the basis of the difference between the first moisture content and the second moisture content.