<i>Background:</i> Vital capacity is a key parameter in the determination of lung function, usually assessed by means of a forced expiration maneuver. This maneuver can be exhausting, and patients often cannot complete it. <i>Objectives:</i> This study evaluates a method to estimate forced vital capacity (FVC) based on the extrapolation of volume-time curves from forced expiration. <i>Methods:</i> The algorithm was applied to 2,363 volume-time curves from patients with and without respiratory disease. 416 of these spirograms originated from incomplete maneuvers. For each spirogram, estimated (FVC_est) and measured FVC were compared with inspiratory vital capacity. <i>Results:</i> Reliable FVC_est were obtained for 82% of all and for 76% of the incomplete maneuvers. Regardless of the category of respiratory disease and acceptability of forced expiration, FVC_est were close to inspiratory vital capacities. <i>Conclusions:</i> When assessing the lung function of patients who cannot complete forced expiration, this method could help to reduce the duration of maneuvers required to provide a reliable estimate for vital capacity.