Among the various alternative fuels, vegetable-oil-based fuels have been attracting greater attention as a promising alternative to fossil diesel fuel in compression ignition (CI) engines. Fuel viscosity has a definite effect on fuel injection, spray development, and combustion processes of CI engines; hence, the viscosity estimation of new candidate fuels is significant. This paper discusses the methodologies used to estimate the viscosities of vegetable oil and biodiesel fuels, based on their fatty acid composition. While the methyl ester composition of biodiesel is directly related to the fatty acid composition of the oil, a basis for determining the triglyceride composition to estimate the straight vegetable oil viscosity is elucidated in the paper. The proposed methodologies are validated over a wide range of available viscosity data for vegetable oils and biodiesel fuels of varying composition and for varying temperatures. A comparison of the estimated viscosities with the measured values for 13 vegetable oils and 14 biodiesel samples shows an agreement within a predicted error of 10%. © 2009 American Chemical Society.