SDR

SDR stands for Standard Dimension Ratio and is the relationship between a plastic pipe's outside diameter and its wall thickness. A lower SDR number means thicker walls and higher pressure rating; SDR 13.5 can handle more PSI than SDR 21, which is thinner and cheaper. The rating directly affects what pressure the pipe can safely handle at a given temperature, so you'll see SDR called out in specs and code tables whenever plastic pipe—PVC, CPVC, PEX, or polyethylene—hits pressurized service.

In the field, SDR matters most on water lines and compressed air runs. A customer's water main might call for SDR 13.5 PVC because municipal water can run 80+ PSI, while a low-pressure drain or vent might use SDR 21 or thinner because it costs less and the application doesn't demand it. PEX tubing for radiant heating or potable water also gets rated by SDR, and the number tells you its burst pressure under standard conditions.

Always check the engineer's specs or local code tables before ordering—don't assume SDR from the application alone. The pressure rating also drops with temperature, so high-temperature applications may require a lower SDR to stay safe. Mixing SDR ratings in a system is fine as long as each segment meets its intended use.