How do I know the size of my copper tubing?

Copper pipes are classified by OUTSIDE diameter and their "nominal size", i.e., "what you call it".

A copper pipe with a 5/8 outside diameter is CALLED a "1/2-inch"; the "5/8" nominal size would have an outside diameter of 3/4.

Make sure you get the proper type of pipe: Type K or L are used for domestic water lines, Type M is used for hydronic heating systems (thinner wall, not as much pressure).

The outside diameter of Type K, L, M is the same for any given nominal size, only the INSIDE diameter changes, meaning you can use all the same fittings (elbow, tee, valve, couplings) regardless of which Type you have.

5/8 is a normal size, according to the Copper Tubing Sizes for Plumbing (called the Nominal Size). does the Outside Diameter read 3/4? that might be what the cutter reads. (though tubing is always designated by its nominal size.)

Match up your tubing cutter to the outside diameter, OD, that is what tubing goes by in AC sizing. Plumbing sizes go by inside diameter, ID. normally when going by id, you jump up one size for od, 1/2" Id is 5/8" od.

http://411plumb.com/wp-content/uploads/2009/07/Rid...

get tubing cutters like these. they fit in tight spaces and are adjustable. much more useful.

Cut you off apiece and take it with you and match it up at the store.

you measure the inside of the tube, not the outside