Background: A universal method for determining ideal body weight (IBW) for the application of appropriate tidal volumes in children on mechanical ventilation is elusive. We sought to compare 3 commonly used IBW methods for subjects between ages 2 and 20 y., Methods: Demographic data were recorded, and the IBW was calculated based on the McLaren-Read, Moore, and body mass index methods by using growth chart data from the Centers for Disease Control and Prevention. The percentage error between each IBW method and the actual body weight were calculated and reported as median (interquartile range). We decided a priori that a ≥10% difference between the actual body weight and IBW would be clinically important. The Wilcoxon signed-rank test was used to compare the actual body weight with the IBW. Bland-Altman analysis was used to assess the individual agreement of each IBW method with the actual body weight. The Kruskal-Wallis test was used to detect differences among the IBW methods., Results: A total of 58 subjects (36% female) were analyzed. The median (interquartile range) percent weight error between the actual body weight and calculated the IBW was 14.8% (1.9-22.1%, P = .038), 13.8% (4.6-23.4%, P = .008), and 12.0% (3.9-20.5%, P = .037); the mean biases were 2.7 (95% CI -13.4 to 18.9) kg, 3.9 (95% CI -15.1 to 22.9) kg, 3.2 (95% CI -16.7 to 23.1) kg; and the numbers of subjects who would have a clinically important error were 29 (55.7%), 29 (56.9%), and 30 (51.7%) for the McLaren-Read, Moore, and body mass index methods, respectively., Conclusions: The majority of the subjects demonstrated a clinically important error between the actual body weight and the IBW. The percent error increased in subjects > 25 kg actual body weight. These data underline the importance of obtaining height measurements and calculated IBW in pediatric patients who are mechanically ventilated., Competing Interests: The authors have disclosed no conflicts of interest., (Copyright © 2018 by Daedalus Enterprises.)