In this graphic from Forgeline you can see that the rim width is taken from the inside of the opposing bead grooves and not from the outside edges of the rim. For example, an 18x8 wheel is actually 9 inches overall (0.5 inch for each rim edge). The offset of a wheel is the distance from its hub-mounting surface to the centerline of the wheel. Given this, it can be one of three types. Zero offset would mean the hub mounting surface is even with the centerline of the wheel. Positive offset would indicate that the hub-mounting surface is toward the front side of the wheel. Negative offset is where the hub-mounting surface is toward the back, or brake side of the wheel’s centerline. Wheels with a “deep dish” have negative offsets. Offset is typically measured in millimeters. Backspace is the distance from the hub face to the back outer edge of the rim (not the tire bead) and is expressed in inches.