Plumbers have there own code for sizes of pipe, which can be confusing. Most are measured by the inside diameter ( ID ) 3/8 is half inch outside diameter (OD ) this tool size is for outside diameter ; I noticed also that all tubing is not equal some are slightly thicker than others. The slightly thicker tubing worked better in this tool. I bought two rolls of 3/8 tubing which is 1/2 inch OD one from HD and the other from lws the HD tubing was just fraction of a mm bigger OD but that made a big difference in how straight that I could get the tubing.
