Biodiesel is a renewable fuel derived from vegetable, animal, or waste oils. It is synthesized by the transesterification of triglycerides with either methanol or ethanol. The fatty acid profile of biodiesel differs depending upon the feedstock of the derived fuel and is the major factor in determining the properties of the fuel. Using biodiesel blends decreases hydrocarbon, particulate matter, and CO emissions while increasing fuel consumption and NOx emissions. This effect is dependent upon not only the blend ratio but also the fatty acid profile of the biodiesel. Current techniques to determine the fatty acid profile of biodiesel samples are both time-consuming and expensive; however, by measurement of three simple physical properties (viscosity, density, and the refractive index) and a comparison of the values to known standards, it is possible to create a model that can accurately predict both the average chain length and degree of unsaturation.