Constants for the semi empirical equation for the prediction of pressure drops in elbows of ventilation systems were determined, and the application of this equation to design problems was discussed. Air flow through smooth ducts, used for 6, 4, and 3 inch (in) elbows, and through coarse ducts, used for 6 and 4in elbows, was studied. The volumetric flow in a wind tunnel was monitored by a calibrated impact tube and static pressure tap. Ninety degree elbows were initially generated with turning ratios of 1.5 to 4.5 in steps of 0.25. To verify the initial results, 180 degree turns with turning ratios of 2, 3, 3.35, 4, 10, and 16 were tested. The constants and the equation describing the pressure losses in elbows were given. The authors state that this single equation can be used for elbows with different duct surface characteristics, diameters, and turning ratios. A simplification of the equation was given which made the calculation of pressure losses quick and easy. It was found that the measured elbow pressure losses did not agree with theoretical values using traditionally used constants, and the apparent design safety factors varied widely. The authors conclude that the equations presented can be used by an industrial hygienist to calculate the pressure losses in the elbows of ventilation systems.