Pipe fitting sizes refer to the dimensions and specifications of various components used to connect and control the flow of fluids or gases in a piping system. These fittings are essential for joining pipes together, changing direction, branching off, or controlling the flow within a system. The sizing of pipe fittings is based on standard industry measurements to ensure compatibility and ease of installation.

Here are some common pipe fitting sizes and terms you may come across:
1. Pipe Diameter: The nominal pipe size (NPS) is a numerical designation used to identify the approximate inner diameter of a pipe. It is expressed in inches or millimeters. Common NPS sizes for pipes include 1/2", 3/4", 1", 1.5", 2", 3", 4", 6", 8", 10", 12", and larger.
2. Pipe Schedule: The pipe schedule refers to the thickness of the pipe wall. It is denoted by a schedule number such as SCH 40, SCH 80, SCH 160, etc. The higher the schedule number, the thicker the pipe wall.

Source: Pipe Fitting Size Chart