Ethanol is the biofuel with the greatest global consumption. Brazil is the largest sugar cane ethanol producer in the world. In the 2017-2018 harvest, 27.9 billion liters of ethanol were produced. Accurate determination of the fermentation efficiency value of an industrial unit is therefore essential for evaluation of the performance of an ethanol production process. The present work proposes a new methodology to calculate the fermentation efficiency discounting the volume occupied by yeast cells. Data from fed-batch fermentations performed at bench and industrial scales were used to estimate the ethanol fermentation efficiency, using classical industrial methodologies as well as the new method. The results showed that the former overestimated the ethanol fermentation efficiency values, while the new method provided greater accuracy. The use of this new technique could justify investments in the fermentation unit, with the aim of improving the process.