Determining the pipe thread size for plumbing fitting is not as simple as you might think. Pipe thread size doesn’t necessarily mean the physical dimension of the pipe. Once you know this, it is easy to know which fitting you should use. It is also important to know the outer diameter of the fitting, also referred to as OD and the TPI or threads per inch. Each thread size has a defined number of threads per inch. It is important to know both the outer diameter and thread per inch to get the right-sized fitting for your plumbing.
Step 1 - Get the Diameter
Determine the outer diameter of your fitting. People commonly get an inner diameter which should not be the case.
Step 2 - Get the TPI
Determine the threads per inch or TPI of the required fitting. These two are required to determine the right thread size for plumbing fitting. Pipe threading commonly come in standard and tapered thread. Packaging also determines if it is a male pipe thread, or MIP, female pipe thread, or FIP, and national pipe thread, also known as NPT.
These are standard threads on galvanized, black iron, copper, PVC and other fittings for pipe. These thread fittings create a watertight seal to make sure your pipes do not leak but one should keep in mind that these fittings should be used in conjunction with Teflon thread tape or any joint compound in order to be efficient.