An image forming apparatus includes a conveyor to convey a recording medium; an image forming device including a recording head to form a test pattern including a first mark and a pair of second marks; an imaging device; and a processor. The processor includes a pattern forming unit to cause the image forming device to form one of the first and second marks and form the other after the conveyor conveys the recording medium by a predetermined conveyance amount, a position detector to detect positions of the first and second marks in the captured image, a ratio calculator to calculate a ratio between a distance between the second marks and deviation of the first mark in the captured image, and a distance calculator to calculate an actual distance of the deviation based on a theoretical distance between the second marks and the ratio.