In this work, a R1SM approach was applied for the calculation of ideal-gas thermodynamic properties of five amino acids with aliphatic side chains: glycine, alanine, valine, leucine, and isoleucine. The first step of the calculation was an extensive conformational analysis that located several conformers not reported previously. A new systematic and user-friendly nomenclature of the conformers was introduced, and the stable conformers were clearly assigned with the previously used labeling where possible. Stability and calculated relative energies of the conformers were compared between various levels of theory and with several experimental studies, demonstrating a good performance of the selected B3LYP-D3/6-311+G(2df,p) level of theory. As a second step, the theoretically calculated vibrational frequencies were compared to the previously reported experimental spectra to verify the performance of the applied double-linear scaling factor. Finally, ideal-gas heat capacities, enthalpies, and absolute entropies were calculated, accounting for all stable conformers using the R1SM model. The resulting thermodynamic data are presented for the first time, since they cannot be determined experimentally and their rigorous calculation requires a complex thermodynamic model.