The commercial process of heterogeneously catalyzed biodiesel production process is in its final steps due to its advantages over the old homogeneously catalyzed one, and in order to provide an economic and fast way to investigate the vegetable oil conversion into biodiesel, a relation between conversion and a parameter that can be easily measured like viscosity is developed in this paper. The variation of viscosity with increasing conversion was proved to be equation was developed to predict conversion from dynamic viscosity values based on the experimental data. A conversion of 91% was obtained at 60 oC reaction temperature, 12:1 methanol to oil molar ratio, 600 rpm in 2 hours of reaction time. The method reported in this paper is meant to save time and money since the most widely used chromatography methods are expensive and time consuming.