Hydrant flows may be specified directly in flow units or as an emitter coefficient. Because hydrant flow is a function of pressure and the user does not usually know the pressure at the hydrant beforehand, it is more accurate to specify the emitter coefficient. For standard North American hydrants that comply with AWWA Standard C502 or C503, the emitter coefficient would be 150-180 gpm/psi0.5 (11-14 L/s/m0.5) for the 2.5 in. (63 mm) outlet and 380-510 gpm/psi0.5 (30-40 L/s/m0.5) for the 4.5 in. (115 mm) outlet depending on the model of hydrant, size of barrel and length of barrel. See Advanced Water Distribution Modeling and Management (p 451-453) for more discussion on this. In terms of flow units, free discharge from a hydrant can vary from 500 to 1500 gpm (32-95 L/s) depending primarily on the strength of the distribution system at that point.