This work presents a correlation method that uses 1H NMR spectra for determining the conversion during transesterification of vegetable oil with 10 alcohols: methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, 1-pentanol, isopentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-decanol, 1-dodecanol, and benzyl alcohol. Conversions ranging from 70 to 99%, depending on primary alcohol used, are obtained using organic catalyst N,N′,N″-tris­(3-(dimethylamino)­propyl)­guanidine. Catalysts, alcohols, and intermediate products can cause signal overlapping, debilitating the use of the conventional 1H NMR method. Thus, our method uses the correlation between the triplet signal of α-carbonylmethylene and two signals of unsaturated parts of the fatty acid chain, where signal overlapping does not occur, hence, adding robustness and flexibility to the method. The method is applicable for unsaturated oils and is not affected by the amounts of residual alcohol, catalyst, or intermediate products, making it ideal for conversion determination and in situ monitoring of complex mixtures. In addition, we present a systematic analysis of the 1H NMR spectra of biodiesels produced with the mentioned alcohols.