Washable baghouses are increasingly used to filter the small size fractions of milk powder, known as fines, which become entrained with spray dryer outlet air streams in milk powder plants. Surprisingly, very little is known about the quantity of powder that becomes fines, an important parameter for both the control of agglomeration rates and baghouse design. The aim of this work was to develop a method to measure the concentrations of powder in the outlet stream of an industrial spray dryer. A novel application of an optical scintillation instrument working beyond its previously recommended concentration range is reported here that successfully provides estimates of the fines fraction (the ratio of fines flow to total powder flow). This method was calibrated with isokinetic sampling using a thimble filter to independently determine powder flow rates. Results were also compared with those from a convenient mass-balance method based on powder size distributions of samples from around the plant. Results showed that the fines fractions ranged from 49 ± 8% to 86 ± 2% depending on the powder type and plant operating conditions. These values are much greater than what was traditional thought (10–20%). It was concluded that the optical scintillation instrument readings could yield reliable values of fines flows. Also, it was concluded that the fines fraction in a milk powder plant with washable baghouses is a function of the total dry powder flow rate, the dryer feed milk concentrate total solids, and the powder particle size and density.