1. An image processing apparatus, comprising: an image reading part configured to read an image of a document; and an image storage part configured to store image data read by the image reading part; wherein the image reading part includes a part configured to read a range of a part of the image of the document; the image processing apparatus further includes: a first calculating part configured to calculate image data of a one surface of the document based on a size of the image data of the range read by the image reading part; and a second calculating part configured to calculate the number of pages of image data of the document which can be stored by the image storage part based on the result of calculation by the first calculating part.