The main difference between type M and L is the wall thickness and pressure rating. The two most common sizes are half-inch and three-quarter-inch. Type M half-inch pipe has a wall thickness of 0.028 inches while type L is 0.040 inches. Type M three-quarter-inch pipe has a wall thickness of 0.032 inches and type L is 0.045 inches.
Type L costs more than type M. Type L with its thicker walls weighs more and requires more copper, thereby costing more to manufacture and also to transport.
Copper pipes of the same type and size all have the same outside diameter, which allows them to use the same fittings for connections. The inside diameter varies by type due to the different thickness of the walls. For example, half-inch type M has an inside diameter of 0.569 inches and type L is 0.545 inches.
Copper pipe for indoor residential water supply lines may be either type M or type L, unless local codes specify otherwise. Both are rated well above common water pressure levels found in municipal water supply systems. The higher cost of type K pipe usually restricts it to the buried pipe bringing water into a home.
The cost difference of type M versus type L is enough that it makes sense for most homeowners to use type M instead of type L for large projects. Always check your local code requirements for building materials and permits before starting projects or buying building materials.
