There is often confusion as to which size product a client actually needs – Pipe Size or OD Tubing Size. Keep in mind that pipe size refers to a nominal – not actual – inside pipe diameter. Schedule refers to the pipe’s wall thickness. The dimensions provided for OD tubing on the other hand refer to the actual outside diameter and wall thickness.
For example: 1-1/2″ pipe size flanges have an opening to fit over 1.90″ OD – the actual outside diameter of 1-1/2″ pipe – while 1.50″ OD tubing has a true 1.50″ outside diameter and the flange is sized accordingly. See below for illustration.