The offset of a wheel is the distance from the center line of the rim, to the mounting face of the nave plate. This is normally given in millimetres.
Using a flat object across the front edge of the rim, measure down to the front of the nave plate. (see below) The offset can be worked out using this measurement. The exact same measurement can be taken from the back edge of the wheel to the mounting face. It is also useful to measure the thickness of the nave plate. It is always advisable to take these measurements without a tyre fitted to the wheel, as it can be difficult to get accurate results.
As with a single wheel, the offset of a twin wheel is the distance from the center line of the rim to the inner face of the nave plate.
When a wheel is fitted in twin format, the term HDS (Half Dual Spacing) is used. This is the measurement from the center line of the rim to the outer face of the nave plate. (Offset + nave thickness)
With the wheel turned on its side, use a flat object placed across the front face of the dish nave and measure down to the floor. This will give an overall height from which the half dual spacing (HDS) can be worked out. Again its easier to take these measurement with no tyre fitted to the rim for greater accuracy.