The most common type of PEX tubing is made for copper tubing measured by the outside diameter and is available in 3/8 inch, 1/2 inch, 5/8 inch, 3/4 inch and 1 inch.
PEX tubing is also sometimes referred to as PEX pipe, but the terms are interchangeable. While the most common type of PEX sold is marked as CTS-OD for copper tubing size, outside diameter, the second most common type of PEX, is marked IPS-ID for iron pipe size, inside diameter. Iron pipe size PEX for residential use also comes in sizes ranging from 3/8 inch to 1 inch. The mark for size on both types of PEX is located on the outside of the tube.
Charts for PEX tube fitting, tools and compatibility are available at PEXinfo.com.