Tri Clamp sizes are determined by the outer diameter of the pipe or tubing, not the outer diameter of the flange or Tri Clamp itself. The flange of a Tri Clamp fitting typically has about a ½ inch greater diameter than the outside diameter of the pipe. This can become especially confusing when the Tri Clamp fitting has no pipe attached to it, as is the case with adapters, For instance, a 1.5" Tri Clamp to 1/2" Hose Barb Adapter is not 1.5" inches in any dimension, but the Tri Clamp end will fit any other 1.5" Tri Clamp fitting.