With the here proposed mathematical model, we provide a new tool to convert continuous RER data into more accurate estimations of %Gox and %Lox. It circumvents the use of non-protein respiratory quotient tables and thereby aids and simplifies by automating the conversions. The model can further be implemented into software commonly used for indirect calorimetry measurements and thereby provides %Gox and %Lox data in real-time during a running experiment.