Two measurements are used when discussing the capabilities of a sewer jetter: GPM and PSI. GPM, or gallons per minute, pertains to the amount of water that passes through your sewer jetter. PSI, or pounds per square inch, refers to the intensity of pressure at which the water leaves the sewer jetter. Which specification matters more will depend on your application. A sewer jetter with an adequate PSI will cut through settled masses such as dirt, tree roots, hard grease, etc. Sewer jetters with a higher GPM are better at flushing out debris, larger rocks, grease logs, and more.