INTRODUCTION Ventilator settings in children under anaesthesia remain difficult because of the changes in the physiology and the high dead space. OBJECTIVE To determine the alveolar minute-volume to sustain normocapnia in children under mechanical ventilation. DESIGN A prospective observational study. SETTINGS This study was performed between May and October 2019 in a tertiary care children's hospital. PATIENTS Children between 2 months and 12 years, weighing between 5 and 40 kg, admitted for general anaesthesia. INTERVENTION Volumetric capnography was used to estimate the alveolar and dead space volume (Vd). MAIN OUTCOME MEASURES Total and alveolar minute ventilation in (ml kg-1 min−1) over 100 breaths. RESULTS Sixty patients were included comprising 20 per group: 5 to 10 kg (group 1), 10 to 20 kg (group 2), 20 to 40 kg (group 3). Seven patients were excluded for aberrant capnographic curves. After normalisation to weight, the median [IQR] tidal volume per kilogram was similar between the three groups: 6.5 ml kg−1 [6.0 to 7.5 ml kg−1], 6.4  ml kg−1 [5.7 to 7.3  ml kg−1], 6.4  ml kg−1 [5.3 to 6.8  ml kg−1]; P = 0.3. Total Vd (in ml kg−1) was negatively correlated to weight (r = -0.62, 95% confidence interval -0.41 to -0.76, P < 0.001). The total normalised minute ventilation (ml kg−1 min−1) to obtain normocapnia was higher in group 1 than in group 2 and in group 3; 203  ml kg−1 min−1 [175 to 219 ml kg−1 min−1], 150  ml kg−1 min−1 [139 to 181  ml kg−1 min−1] and 128  ml kg−1 min−1 [107 to 157  ml kg−1 min−1]; P < 0.001 (mean ± SD), but (mean ± SD) alveolar minute ventilation was similar between the three groups; 68 ± 21  ml kg−1 min−1. CONCLUSION Total dead space volume (including apparatus dead space) represents a major component of tidal volume in children less than 30 kg, when using large heat and moisture exchanger filters. The total minute ventilation necessary to achieve normocapnia decreased with increasing weight, while the alveolar minute ventilation remained constant. TRIAL REGISTRATION ClinicalTrials.gov, identifier: NCT03901599