The offset of a wheel is the distance from the centre line of the wheel to the mounting face of the nave plate.
The easiest ways to calculate the offset of a wheel varies, depending on whether it is a single or twin type wheel.
Using a flat object across the front edge of the rim, measure down to the front of the nave plate. (see picture) The offset can calculated 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.