Purpose: We reasoned that the application of positive pressure through air stacking (AS) technique could cause gas compression and the absolute lung volumes could be estimated. The aim of this study was to estimate the amount of gas compression (ΔVcomp) during AS in healthy subjects positioned at 45° trunk inclination and verify if the simultaneous measurements of chest wall volume changes (ΔVCW), by optoelectronic plethysmography, and changes in lung volume (ΔVao), by pneumotachograph, combined with pressure variation at the airways opening (ΔPao) during AS are able to provide reliable data on absolute lung volumes. Methods: Twenty healthy subjects (mean age 23.5 ± 3.8 years) were studied during a protocol that included slow vital capacity and AS maneuvers. Vcomp was calculated by subtracting ΔVao and ΔVCW occurring during AS and total lung capacity (TLC) was estimated by applying Boyle–Mariote’s law using Vcomp and ΔPao. Results: During AS, 0.140 ± 0.050 L of gas was compressed with an average ΔPao of 21.78 ± 6.18 cmH2O. No significant differences between the estimated TLC (−0.03 ± 3.0% difference, p = 0.6020), estimated FRC (−2.0 ± 12.4% difference, p = 0.5172), measured IC (1.2 ± 11.2% difference, p = 0.7627) and predicted values were found. Conclusion: During AS, a significant gas compression occurs and absolute lung volumes can be estimated by simultaneous measurements of ΔVCW, ΔVao and ΔPao.