Lay the wheel face down on a flat surface
Measure the overall wheel width from inner bead seat to inner bead seat
Divide the wheel width by 2 to find the centerline
Measure the mounting pad height from the flat hub-mounting surface to the flat surface
Subtract the centerline measurement from the mounting pad measurement
If the mounting pad is toward the outside of the wheel, the offset is positive
If the mounting pad is toward the inside of the wheel, the offset is negative
Use the result in millimeters
Confirm the measurement with a wheel offset gauge if available