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In the US, biodiesel producers usually follow the 19.8:1 methanol-to-FFA molar ratio for free fatty acid (FFA) esterification, as suggested by the National Renewable Energy Laboratory (NREL) without optimization studies. In this paper, both laboratory studies and industrial practices of the esterification process were compared, and an optimization study of a used vegetable oil with 5% FFA was conducted. The optimal conditions of this oil, i.e., methanol-to-FFA molar ratio of 40:1, and sulfuric acid usage of 10%, fell out of the suggested range of 19.8:1. The activation energy of the esterification reaction is 20.7 kJ/mol at the optimized condition and 45.9 kJ/mol at the 19.8:1 methanol to FFA ratio. It was found that the 19.8:1 methanol-to-FFA molar ratio worked well only within the FFA range of 15-25% while the suggested 5% sulfuric acid worked well only within the FFA range of 15-35%. Outside these ranges, especially at FFA levels less than 15%, optimization study is necessary. Regression models of methanol and acid dosing have been utilized in two industrial scale biodiesel producing facilities and have successfully reduced the FFA level to less than 0.5%. (C) 2014 Elsevier B.V. All rights reserved.