The offset of a wheel is the distance from the hub mounting surface to the centre line of the wheel. The wheel offset is measured in millimetres and results in a positive, negative, or zero offset. Negative offset is when the hub mounting surface is toward the back or brake side of the wheels centerline.
Example: 7x15 wheel with a -6 mm offset would have a backspace of 3.75" which leaves 4.25" from the mounting hub to the very front of the wheel including the lip. 3.75 + 4.25 = 8"
Use the chart below to convert an offset to backspace