A developer consumption determining method for determining a developer consumption of an image forming apparatus includes the steps of dividing one of writing pixels into a plurality of sub-pixels in a main scanning direction, determining a weighting coefficient for each of surrounding sub-pixels of one of the sub-pixels located within a predetermined distance from the one of the sub-pixels on the basis of positional relationships, determining respective emission coefficients for the surrounding sub-pixels on the basis of one of durations and exposures of the light beams applied to the surrounding sub-pixels, summing products of the weighting coefficients of the surrounding sub-pixels and the respective emission coefficients to obtain total exposure for the one of the sub-pixels, repeating for all of the sub-pixels the steps of determining the weighting coefficient, determining respective emission coefficients, and summing products, and summing the total exposures for all the sub-pixels to determine the developer consumption.