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, wherein: the second calculation portion calculates the toner consumption based on data values regarding a plurality of pixel attributes in image data and respective parameters for the plurality of pixel attributes; the third calculation portion calculates the values of the respective parameters for the plurality of pixel attributes from the data values regarding the plurality of pixel attributes calculated within each of a plurality of periods and the toner consumption calculated by the first calculation method within each of the plurality of periods; the plurality of pixel attributes comprises a gradation attribute; the value of the parameter for the gradation attribute is fixed; the second calculation portion calculates the toner consumption based on the data value regarding at least one remaining pixel attribute and the parameter for the at least one remaining pixel attribute; and the third calculation portion calculates the value of the respective parameter for the at least one remaining pixel attribute from the data value regarding the at least one remaining pixel attribute calculated within each of at least one period and the toner consumption calculated by the first calculation method within each of the at least one period.