1. A developer consumption determining method for determining a developer consumption of an image forming apparatus which forms an image by developing a latent image formed on an image bearing member by scanning with light beams in a main scanning direction while rotating the image bearing member in a sub-scanning direction to form light beam spots on the image bearing member, which are overlapped in the main scanning direction and the sub-scanning direction and include writing pixels, comprising the steps of: dividing one of the writing pixels into a plurality of scanning sub-pixels in the main scanning direction and a plurality of overlapping sub-pixels in the sub-scanning direction; determining a weighting coefficient for each of a plurality of surrounding scanning sub-pixels of one of the plurality of overlapping sub-pixels, which are on a scanning line adjacent to the one of the plurality of overlapping sub-pixels, and are located within a predetermined distance from the one of the plurality of overlapping sub-pixels, on the basis of respective positional relationships of the plurality of scanning sub-pixels with the one of the plurality of overlapping sub-pixels; determining respective emission coefficients for the plurality of scanning sub-pixels on the basis of one of a plurality of durations and exposures of the light beams applied to each of the plurality of scanning sub-pixels; summing products of the weighting coefficients of the plurality of scanning sub-pixels and the respective emission coefficients to determine a total exposure for the one of the plurality of overlapping sub-pixels; repeating for all of the plurality of overlapping sub-pixels the steps of determining the weighting coefficient, determining respective emission coefficients, and summing products; summing the total exposures for all the overlapping sub-pixels to determine a developer consumption attributed to the overlapping of light beam spots in the sub-scanning direction; and determining the developer consumption of the image forming apparatus by using the developer consumption attributed to the overlapping of light beam spots in the sub-scanning direction.