The options in the crop tool options bar are different before and after you draw the crop selection. When you first select the crop tool, you can set values to crop to a specific width and height, and resolution. The units of measurement available for the crop size width and height are inches, centimeters, millimeters, points, and picas. To make sure you are using the right unit, type the unit or the unit abbreviation after the number in the value field, for example: 100px, 1 in or 1 inch,10cm, 200 mm, 100 pt,, or 100 pica. When you specify values for height and width, the crop selection contains only the four corner change points of the 8 change points available when no height and width values are specified.

Image size formulas
The following formulas provide information about how all the photo editing software calculate the physical size, resolution, and pixel dimensions of an image:

•Physical size = Resolution x Pixel Dimensions
•Resolution = Physical Size / Pixel Dimensions
•Pixel Dimensions = Physical Size / Resolution