In the top left corner of the dimensions dialog box there is a menu to switch from mm to inches. Be aware that this will only change the display of dimensions. Dimensions must still be input in millimeters. However, if you only know the value you want to enter in inches, you can type that value and then add *25.4 after the value in inches. For example, to input 12 inches, just type 12*25.4.
Showing dimensions in inches
In the top left corner of the dimensions dialog box there is a menu to switch from mm to inches. Be aware that this will only change the display of dimensions. Dimensions must still be input in millimeters. However, if you only know the value you want to enter in inches, you can type that value and then add *25.4 after the value in inches. For example, to input 12 inches, just type 12*25.4.