 The friction loss is customarily given as pressure loss for a given duct length, Δ"p" / "L", in units of (US) inches of water for 100 feet or (SI) kg / m2 / s2.For specific choices of duct material, and assuming air at standard temperature and pressure (STP), standard charts can be used to calculate the expected friction loss.The chart exhibited in this section can be used to graphically determine the required diameter of duct to be installed in an application where the volume of flow is determined and where the goal is to keep the pressure loss per unit length of duct "S" below some target value in all portions of the system under study.