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.

Very good product.

If you are planning to run copper tubing in your next build this tool is definitely not optional, it's another MUST HAVE. Soft copper tubing that comes in a coil must be cut then straightened before bending and this tool does it extremely well. I saw a video on youtube that demonstrated the KwixUK tool and I was sold. After you make your copper tube perfectly straight then you can use the other bending tool that primochill sells to make your bends into show quality build.