What size of BC Series brass compression fitting do I need to order?

First it is helpful to understand the difference between “nominal” dimensions vs. outside diameter (O.D.) dimensions. Nominal is the term used to describe most pipe and fitting measurements in the plumbing industry. Nominal refers to the inside diameter measurement of the pipe. The actual inside pipe measurements will vary slightly due to the wall thickness of the pipe, so it is referred to as "nominal" meaning a small variance of size.

The outside diameter (OD) dimensions refer to the actual outside diameter of the pipe. The outside diameter dimensions of pipe are a constant size which allow pipe fittings to be used with the same type of piping with multiple grades of wall thickness.

Nominal or Standard CTS (Copper Tube Size) can be confusing. It is important to note that the BC Series fitting part numbers are based on and described by the O.D. – NOT the nominal size!

When in stalling ½” copper pipe (NPS) a 5/8” compression fitting is ordered, it is designed to work with 5/8” O.D. (0.625”) tubing. Tubing is O.D. controlled and therefore compression fittings will work with any tubing manufactured to the CTS dimension regardless of the wall thickness (For example Type K, L & M copper tubes all have the same 5/8” O.D., but the I.D. varies K = 0.527”, L = 0.545” & M = 0.569”)

To be sure to order the right size of compression fitting, you will need to know the outside diameter of your copper pipe or tubing. Whatever the outside diameter of your copper tubing is will be the size of the compression fitting you will need. For instance, if the outside diameter of your copper tubing measures ½” (0.500”) then the compression fitting will need to be a 3/8” size -08 compression fittings whereas if it measures 5/8” (0.625”) then the compression fitting will need to be a 5/8” size -10 compression fittings. The numbers are based on the number of 1/16” in the outside diameter. -08 representing 8 -1/16 of an inch.