An image reading apparatus, having a conveyer, a sheet tray, an obtaining device, a memory, a determining device, and a controller, is provided. The controller determines whether information concerning pixels in the memory indicates edge positions or edge absence in in read-image data concerning a sheet, detects an upstream leading-end position and a downstream leading-end position, detects upstream continuing pixels and downstream continuing pixels, detects an upstream lateral-end position and a downstream lateral-end position, calculates a line representing an upper end of the sheet based on the upstream leading-end position and the downstream leading-end position, calculates an upstream corner position and a downstream corner position based on the upstream and downstream lateral-end positions and the line representing the upper end of the sheet, and calculates a width of the sheet based on the upstream corner position and the downstream corner position.