DR stands for Dimension Ratio which is the average outside diameter (note: DR is also used for PVC pipe) divided by the minimum wall thickness. A Standard Dimension Ratio (SDR) is a specific DR based on ISO preferred number series The use of SDR’s enables manufacturers to produce pipe to a set of standardized DR’s. SDR’s include 9, 11, 13.5, 17, 21, 26, and 32.5. All SDR’s are DR’s, but not all DR’s are SDR’s.