Structural quantum confinement in long-channel thin silicon-on-insulator MOSFETs has been quantified using the temperature dependence of the subthreshold current. The results were compared with the shifts in the threshold voltage. Data were obtained from simulations after initial verification with experimental data. This study demonstrates that, with the temperature dependence of the subthreshold current, shifts in the valence and conduction band edge can be extracted distinctively from changes in mobility and density of states (DOS), making this method more accurate in assessing the impact of structural quantum confinement than the commonly used threshold voltage method. Furthermore, we show that, with additional C-V data, a possible change in mobility and DOS can be disentangled.