An image forming apparatus includes a effective resolution determinator for forming a plurality of patch patterns different in resolution by the output engine, checking by sensor whether or not the patch patterns formed by the output engine are developed with developer, and determining that a resolution of the patch pattern with highest resolution recognizable by the sensor is an effective resolution which can be outputted by the output engine; and a dot counter for counting number of dots constituting an image of not less than effective resolution in the printing data, by which an error in estimated toner consumption, caused by dots too small to be developed, is reduced.