Question: how is pex tubing measured?
Answer: PEX tubing sizes are denoted by CTS (copper tubing size), not OD (Outside Diameter) or ID (Inside Diameter). ... PEX made to US standards must comply with a SDR-9 (Standard Dimension Ratio), which is the ratio of OD (Outside Diameter) to pipe wall thickness (9:1). Exceptions are nominal sizes 1/2" and below.