The first question to ask yourself is what you need the fitting for specifically. Fittings serve as pipe connectors and connect the appliances in our homes to the wider plumbing system. Some are designed to redirect, others to extend.
Size is another important factor. Fittings are usually measured by the inside diameter (ID) and outside diameter (OD) of the corresponding connection. In most cases, this is measured in inches or millimeters.
The inside diameter refers to the hollow part of the fitting. The outside diameter includes the thickness of the casing (source).