3/4 Copper Tubing Types and Sizes: Type K, L, and M Explained

A 3/4-inch copper tube always measures 0.875 inches across the outside — Type K, L, or M only changes the wall thickness.

Buy the wrong wall and the pipe still slides into the fitting, which is exactly why the mistake travels so far before anyone catches it. That 0.875-inch outside diameter is locked across Type K, L, and M, so a supply house will hand you any of the three and every one of them threads onto the same 3/4-inch fitting. What changes is inside: thicker walls mean higher pressure ratings, smaller flow area, and a higher price per foot. Know the wall, and you know which tube belongs in the ground, in a wall, or above a ceiling.

What Sets Type K, L, And M Apart?

Wall thickness is the whole difference, and it drives pressure rating, durability, and flow. All three types fall under ASTM B88, the standard covering seamless copper water tube, and the Outside Diameter (OD) stays fixed at 0.875 in for every one of them.

Type K carries the heaviest wall at 0.065 in, leaving a 0.745 in inside diameter. Type L sits in the middle at 0.045 in wall and 0.785 in ID. Type M is the thinnest at 0.032 in wall and the widest bore at 0.811 in. Same pipe on the outside, three different pipes on the inside.

That extra metal shows up in the pressure numbers. At 3/4-inch, Type K is rated to 724 psi annealed and 1424 psi drawn. Type L holds 495 psi annealed and 972 psi drawn. Type M comes in at 346 psi annealed and 680 psi drawn.

3/4-Inch Copper Tube Dimensions And Pressure Ratings

The table below is the number set to quote from, because nominal size tells you almost nothing once you’re standing at the counter. Fittings size to the tube OD, so all three types are generally interchangeable with the same 3/4-inch CTS fittings.

The pressure figures and dimensions above track the copper tube handbook published by the Copper Development Association, which lays out the full ASTM B88 size convention. the copper tube handbook’s ASTM B88 size tables

Which Type Belongs In Your Job?

Type L is the usual middle pick for residential water service and general supply lines, Type K is the heavy wall for underground and buried runs, and Type M is the light wall used above ground where demand is modest. Match the type to where the tube lives, not to what’s cheapest on the shelf.

• Burying it or running it underground: Type K, the heaviest wall, handles soil contact and abuse best.
• Standard residential service and supply: Type L covers most homes and remains the common default.
• Above-ground interior runs with light demand: Type M works, and its wider 0.811 in bore flows more water at the same nominal size.
• Matching fittings and valves: all three types share the 0.875 in OD, so the same 3/4-inch fittings work across K, L, and M.

One mistake shows up constantly: reaching for Type M because the OD matched and the price was lower. Wall thickness, not fitting size, is what separates durability and pressure capability — swapping the type where a heavier wall was specified quietly lowers the whole assembly’s rating.

For anyone comparing actual product options before ordering, this rundown of tested 3/4 copper tubing picks breaks down what’s currently available by type.

And don’t quote a 3/4-inch tube as 0.75 in OD. It isn’t. Nominal 3/4-inch copper measures 0.875 in across the outside, so use that figure for planning, fitting selection, and anything you hand to a supplier. Pull the exact wall thickness off the tube’s printed marking before you commit.