To find the offset of a wheel you will need to measure the wheel for overall width and measure its front space (see section on "How to measure backspace"). Once you have these two measurements you need to subtract the front space measurement from ½ one half of the overall wheel width. (Ex. A 16x7" wheel is approximately 8" wide overall, half of 8" is 4" then 4"-3"= 1"). If the resultant number is positive then the wheel has a positive offset. If the number is negative then the wheel has a negative offset.
Offset is stated in millimeters so that it can be differentiated from back and front spacing. To convert inches into millimeters multiply by 25.4 (1"x25.4 = 25.4mm). The wheel in the diagram has a 25mm positive offset.