When you are burning carbohydrates, the volume of carbon dioxide produced is equal to the volume of oxygen you consume.
When you burn fat, however, the volume of carbon dioxide is only 70% of the oxygen volume you consume.
The ratio of CO₂ and O₂ volumes you produce and consume is called your respiratory quotient. The exact figure changes throughout the day depending on the mixture of carbohydrates, fat, protein and alcohol you are burning at any time.
This calculator uses the widely accepted RQ values of 0.85 for a mixed diet and 0.75 for low-carb ketogenic diets.