An image forming apparatus includes a fixing member and a pressing member forming a fixing nip therebetween, a heat generator, and a processor. The heat generator is disposed to heat a print medium passing through the fixing nip via the fixing member. The processor is configured to control the heat generator to start heating at a timing when a non-fixed image portion formed on the print medium is expected to reach the fixing nip, based on image data of an image to be fixed, a conveyance speed of the print medium, and an estimated heat capacity of the print medium.