Background: Acute lung injury is a life threatening condition often requiring mechanical ventilation. Lung-protective ventilation with tidal volumes of 6 mL\/kg predicted body weight (PBW, based on sex and body height), is part of current recommended ventilation strategy. However, it is a common practice to visually estimate the body height of mechanically ventilated patients and use these estimates for ventilator settings. We aimed to determine if the common practice of estimating visual height to define tidal volume reduces the possibility of receiving lung-protective ventilation.Methods: In this prospective observational study, 28 mechanically ventilated patients had their height visually estimated by 20 nurses and 20 physicians. All medical professionals calculated the PBW and a corresponding tidal volume with 6 ml\/kg\/PBW on the basis of their visual estimation. The patients\u2019 true heights were measured and the true PBW with a corresponding tidal volume was calculated. Finally, estimates and measurements were compared.\u00a0Results: 1033 estimations were undertaken by 153 medical professionals. Most estimates were imprecise and resulting data comprised taller body heights, higher PBW and higher tidal volumes (all p\u22640.01). Using estimates for tidal-volume definition, patients are exposed to mean tidal volumes of 6.5 \u00b1 0.4 ml\/kg\/PBW. 526 estimation-based tidal volumes (51.1%) did not provide lung-protective ventilation.Conclusion:\u00a0The common practice of visually estimating body height and using these estimates for ventilator settings is imprecise and potentially harmful because it reduces the chance of receiving lung-protective ventilation. Avoiding this practice increases the patient safety. Instead, height should be measured as a standard procedure.