Despite the importance of microsphere size for controlled drug delivery, little work has been done to quantitatively predict the distribution of microspheres from manufacturing techniques. This work presents a quantitative study that describes the size distribution of poly(lactide-co-glycolide) (PLG) microspheres. A fluid mechanics based correlation for the mean microsphere diameter is formulated based on the theory of emulsification in turbulent flow under non-coalescing conditions. The correlation was constructed and validated with experimentally obtained mean microsphere diameters prepared at different stirring speeds. In addition, a Rosin Rammler distribution function was found to give an accurate representation of the microsphere distribution. The spread of the microsphere size distribution was found to decrease with stirring speed. With the validation of the mathematical correlation, it is now possible to have a good estimate of the average microsphere size prior to microsphere preparation. This is directly relevant to the pharmaceutical industry where microspheres of specified mean diameter and size distribution are desirable.