The discharge coefficient is defined as the ratio of actual discharge to theoretical discharge. The coefficient of discharge equals the actual discharge divided by the theoretical discharge. The discharge coefficient is related to the mass flow rate of a fluid via a straight tube of a constant cross-sectional area.
A discharge coefficient is the ratio of the mass flow rate at the discharge end of the nozzle to that of a model nozzle, which expands an identical working fluid under the same original conditions to the same exit pressure. The principle of discharge coefficient is used in determining the irrecoverable losses in regard to a given piece of equipment in a fluid system. This principle is otherwise known as the resistance or constriction of a piece of equipment.