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; 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, and a range setting part configured to set the range, wherein the image reading part reads the range as a range defined by a reading starting position and a reading ending position from the head end of the document in the reading direction, wherein the range setting part can set a plurality of the ranges, wherein the image reading part reads the plural ranges in a case where the plural ranges are set by the range setting part, wherein the first calculating part calculates, in a case where image data of the plural ranges are read by the image reading part, an image data size of the one surface of the document, based on sizes of the image data of the plural ranges wherein the first calculating part is configured to calculate image data of the one surface of the document based on the size of the image data of a first range read by the image reading part and the size of the image data of a second range read by the image data, the first range being different than the second range, and wherein the number of pages of image data which can be stored by the image storage part L is based on a memory free capacity of the image storage part N, the size of the image data of the first range m, the size of the image data of the second range n, an area of the document, and a sum of a reading area, such that L=N/((m+n)* document area/sum of the reading area).