An image forming apparatus includes a conveying unit that conveys a transferred medium in a first direction, an image forming unit, a controller that controls the image forming unit to form a test pattern including first and second line segments slanting toward directions opposite to each other, wherein the first line segment is offset, in an offset direction, from a second direction orthogonal to the first direction and the second line segment is offset, in a direction opposite to the offset direction, from the second direction, a detection unit that detects passage of at least respective parts of the first and second line segments; and a calculating unit that calculates a position where the image forming unit forms an image based on a time difference between times at which the detection unit detects the passage of the at least part of the first and second line segments.