The pavilion depth measurement is divided by the girdle diameter.

The pavilion depth is the measurement between the top edge of the pavilion (where the lower edge of the girdle ends) and the culet. Some might confuse this measurement as being taken from a parallel line that runs along the pavilion angle. This would be incorrect. The imaginary line would run perpendicular to the girdle plane downward to another imaginary line parallel to the girdle and at the same depth as the culet, as in the Figure below. This number would be divided by the diameter of the diamond, the result being commonly expressed as a percentage –the depth %. (For those mathematic wizards, that’s essentially the same as the tangent of the angle.) Of course without a proportionscope, the pavilion depth would be very difficult to measure.