The purpose of this study was to obtain a value of volumetric interfacial surface area in a stirred batch reactor for vegetable oil transesterification that could be used as a similarity criterion in scaling up the reactor. Computational fluid dynamics was used to study mixing in the stirred batch reactor. The experimental results in a laboratory reactor of 935.7 dm3 liquid with a 200-rpm impeller rotational speed were inputted into a computational fluid dynamics model of a small reactor to calculate the volumetric interfacial surface area. This same surface area was inputted into a computational fluid dynamics model of a large reactor to scale up the reactor. The simulation results showed that the volume average volumetric interfacial surface area in the small reactor was 45,120 m2/m3. Using the same value for a large reactor of 1.31 m3 liquid, the simulation results showed that the required impeller rotational speed would be 285 rpm.