Many PE pipe manufacturers use the "Standard Dimension Ratio" - SDR - method of rating pressure piping. The SDR is the ratio of pipe diameter to wall thickness and the SDR can be expressed as
SDR = D / s (1)
D = pipe outside diameter (mm)
s = pipe wall thickness (mm)
A SDR 11 means that the outside diameter - D - of the pipe is eleven times the thickness - s - of the wall.
As a consequence a high SDR pipe has a low-pressure rating and low SDR pipe has a high-pressure rating.