An image forming apparatus is disclosed. The image forming apparatus comprises an input unit through which a condition for reading image data of a document is input, a reading unit to read the image data under the reading condition, a first storage unit to store the image data read by the reading unit, a second storage unit to store information indicative of a relationship between the reading condition and an estimated volume of the image data to be stored in the first storage unit, a data volume estimating unit that estimates a volume of the image data to be read based on the reading condition and the information stored in the second storage unit, and a determining unit to determine whether to read the image data based on the estimated data volume and a volume of the image data that have already been stored in the first storage unit.