An image forming apparatus includes a feeding roller and a recording unit. The feeding roller is rotatably disposed to convey the recording medium in a conveying direction, and includes a rotational axle that is eccentric and a peripheral surface having an actual external dimension that is different from a theoretical external dimension. The recording unit has a head length in the conveying direction. A method of compensating for an error of a conveyance distance of a recording medium in the image forming apparatus includes (1) printing a first image on the recording medium; (2) rotating the feeding roller N turns, N being a positive integer; (3) printing a second image on the recording medium after the feeding roller has been rotated N turns; and (4) determining difference of the actual external dimension from the theoretical external dimension, based on both the first image and the second image.