Copper tubing size charts are available at PetersenProducts.com, Sizes.com and EngineeringToolBox.com. Most charts list the external diameter, the wall thickness and the temper. Wall thickness of copper tubing is indicated by the letters K, L and M, and the temper may be either annealed or drawn.
Anneal tempered tubing is soft or flexible, while drawn temper is rigid. The wall thickness of the tubing dictates the amount of pressure it can handle and the applications it can be used for. M grade tubing has the thinnest walls and is only suitable for low pressure applications, although some plumbing codes prohibit its use. L is the median wall thickness and the most common type. The thickest is K, which is most suitable for underground applications.