How to Measure Fitting Sizes

Regardless of the fitting type being measured, the OD of the tube will be the fitting or gasket size you will need. If you look at the diagram below, most people will measure the Outside Diameter/OD (item A in the diagram) and assume that is the size of the fitting, but this is incorrect. Others will measure the Inside Diameter/ID (item I in the diagram), but this is incorrect also. The proper way to measure for a fitting size would be to measure the OD of the tube (item O in diagram). For example, a 2-inch Tri-Clamp Ferrule will have an OD (item A in diagram) measurement of 2.5", ID (item I in diagram) of 1.87", and a Tube (item O in diagram) size of 2".