A. Refrigeration grade copper tubing is sized according to the outside diameter of the copper pipe. The plumbing industry uses the inside diameter (nominal) size of the pipe for their measurement of sweat fittings. To figure what size sweat fittings to use on refrigeration grade tubing you must subtract 1/8" from the outside diameter of the pipe measurements. If you have 1/4" outside diameter refrigeration grade tubing then you will need to order 1/8" (nominal) sweat fittings. Click here, to see our conversion table.