Analytical expressions of the energy loss coefficient for abrupt and gradual diameter reduction devices, operating in ducts on forced regime are obtained. For this approach, a state of the art was carried out where it was identified that the traditionally utilized methods (that assess the loss of the devices) needed formulas, tables and/or graphs when estimating the K coefficient. These results were used as data in multiple linear regression methods until a representative equation for each case study was obtained. These equations constitue a reliable tool to determine the loss of the K coefficient without having to manipulate tables and graphs.