This article presents a set of formulas for estimating the impact of reformed ethanol fuel on volumetric efficiency in I.C. engines. Both partial and complete usages of reformed ethanol fuel are discussed.Analysis suggests that volumetric efficiency drastically decreases as the level of fuel reforming increases in S.I. engines. Such impact on volumetric efficiency is reduced when the reformed fuel is partially used in diesel engines.A formula for calculating the impact of intake air temperature variation on volumetric efficiency is also presented. This formula is validated against engine simulation on factors such as engine load, speed, displacement, intake air temperature and reformed fuel temperature.Validation results show a maximum of 0.5% error, indicating an accurate approximation and hence high feasibility of this formula.The calculation predicts a decrease of 3-7% in volumetric efficiency caused by temperature variation due to high temperature reformed ethanol fuel, which is consistent with engine simulations.