Standard Dimensional Ratio

Published: July 31, 2018 | Last updated: July 5, 2023

What Does Standard Dimensional Ratio Mean?

The standard dimensional ratio (SDR) is a measurement standard for pipe based on the ratio of the pipe's outer diameter and the pipe's wall thickness. The formula for the standard dimensional ratio is:

SDR = (OD)/(wall thickness)

The standard dimensional ratio is often found in the specifications for polyvinyl chloride (PVC) and high-density polyethylene (HDPE) pipe. The SDR is more precise than the older, historical "schedule" system used for describing pipe wall thickness and thus, resistance to internal pressure.


Trenchlesspedia Explains Standard Dimensional Ratio

The SDR can be important to both designers and horizontal directional drillers when it comes to avoiding the loss of tools and equipment at the bottom of the hole.

For example, if 6-inch, Schedule 40 pipe is specified but 6-inch Schedule 80 pipe is used because the bore is deeper underground and the extra wall thickness of Schedule 80 pipe is needed to resist the pressure of the earth, the pipe's carrying capacity is diminished and the nominal size of the bore will give the driller no indication of the interior diameter of the pipe.

The SDR tells both the designer and the driller outside diameter and the wall thickness and allows them to make intelligent decisions regarding the pipe used.


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