1. An image forming apparatus that consumes toner to form an image, the apparatus comprising: a first calculation portion that calculates a toner consumption by a first calculation method; a second calculation portion that calculates the toner consumption by a second calculation method according to a predetermined calculation formula after a predetermined switching criterion is satisfied; and a third calculation portion that calculates a value of a parameter in the predetermined calculation formula of the second calculation method based on the toner consumption calculated by the first calculation method.