Which type of copper tubing has the smallest inside diameter?

Type M copper tubing is designed specifically for applications where the internal diameter needs to be relatively small yet still provide adequate flow. It is commonly used in residential plumbing systems for water supply lines and is slightly thinner than Type K and Type L copper tubing, which have larger wall thicknesses and, consequently, larger internal diameters.

While Type K has the thickest walls and is most often used for underground applications due to its strength and resistance to pressure, and Type L is suitable for both hot and cold water systems with a medium wall thickness, Type M meets the requirements for lighter-duty applications without sacrificing too much flow capability.

Type DWV, on the other hand, is typically used for drainage, waste, and vent applications and is not usually referred to in terms of pressure applications, thereby differing fundamentally in purpose and construction from the other types.

Given this context, Type M is the tubing that ends up having the smallest inside diameter compared to the others when considering the context of typical applications and specifications in plumbing.