Severity of hypoxaemia can be assessed using the partial pressure of arterial oxygen to fraction of inspired oxygen ratio (FiO2). However, in patients breathing through non-rebreather reservoir bag oxygen mask, accuracy of bedside FiO2 estimation methods remains to be tested. In a post-hoc analysis of a multicentre clinical trial, three FiO2 estimation methods were compared with FiO2 measured with a portable oxygen analyser introduced in the oxygen mask. Among 262 patients analysed, mean (SD) measured FiO2 was 65% (13). The 3%-formula (21% + oxygen flow rate in L/min × 3) was the most accurate method to estimate FiO2 Other methods overestimated FiO2 and hypoxaemia severity, so they should be avoided.