Measuring Screw Gauge

When measuring the gauge of Self Drilling Metal Screws, (with a drill point) The gauge should be measured fom the drill point or inbetween the thread as per this image from hobson.

tek screw

For other screws including Self Taping, Wood, andSelf Drilling Timber Screws, the gauge is takenas a measure of the major diameter of the screw, as per below.

Thanks to Hobson Engineering as a source for the above and below information