Blood desaturation after surgery in obese patients in not uncommon. Oxygen therapy is almost always necessary in these cases and we have to use low-flow catheters, Venturi masks or non-rebreathing systems, but maybe the most efficacious are the non-invasive ventilation (NIV), continuous positive pressure ventilation (CPAP) and the new high-flow oxygen therapy which offers a warm and humid high-flow mixture of O2 and air (up to 70l/min). We decided to use this device in a very obese patient scheduled for a gastroplasty but one question arose: how to calculate FiO2 without tables or mathematic calculations? We used a tool available in a website since 2020 to resolve this problem precisely. Patient’s SpO2 remained all the time between 99 and 100% and 4h after the surgery he was transferred to the ward.