Although inspiratory capacity (IC) is strongly associated with the disease severity of chronic obstructive pulmonary disease, there was no appropriate equation to compute predicted values for IC. Furthermore, whether assessment of IC can identify the risk of prolonged postoperative stay (PPS) in patients undergoing thoracic surgery also remains unclear. To evaluate whether %IC predicted, for which the new equation to compute the predicted values for IC was utilized, could be applied to identify the risk of PPS, we retrospectively analysed the cases of 412 patients who underwent thoracic surgery in Nagoya University Hospital. The multivariate analysis demonstrated that %IC predicted < 85% was one of the most critical risk predictors for PPS (odds ratio, 1.65; 95% confidence intervals, 1.03-2.648) and, in particular, was independent of percentage predicted forced expiratory volume in 1 s (%FEV1) < 80%. A combined assessment of ICFEV1 Low, defined as %IC predicted <85% or %FEV1 <80%, was able to identify more than double the number of patients with PPS, compared with %FEV1 <80% alone (65.9 vs. 28.5%, respectively). This is the first study to demonstrate the significance of %IC predicted in screening for the risk for PPS in patients undergoing thoracic surgery.