SDR = "Standard Dimension Ratio" is used by many PE pipe manufacturers as a method of rating pressure piping. SDR is a ratio of pipe diameter to wall thickness and can be expressed as SDR = D/s
SDR 11 means that "D" outside diameter of the pipe
is eleven times the thickness "s" of the wall.
With a high SDR ratio the pipe wall is thin compared to the pipe diameter and will have a low pressure rating.
With a low SDR ratio the pipe wall is thick compared to the pipe diameter and will have a high pressure rating.