The illustration below shows the dimension referred to on this site as the Shank/Receiver Depth. This is the measurement from the end face of the receiver to the surface of the lip at the bottom of the receiver.

This dimension is one of the three used to calculate the taper of the shank or stem, the other two being the Shank/Stem Lower Diameter (that at the exit end if the mouthpiece) and the Shank-Stem/receiver diameter (the diameter of the Shank/Stem at the point where it enters the receiver or the diameter of the receiver entry point). This diameter is most-commonly adjusted when fitting a mouthpiece shank taper to a receiver. This process is discussed here.

It is best calculated by means of a digital caliper, as shown on the illustration below. If you are not familiar with this measuring device, please check the page: How to Obtain and use a Digital Caliper.