Respiratory Quotient from Gas Volumes Calculator
Enter the volume of CO₂ produced and O₂ consumed and get the respiratory quotient plus the fuel it points to — carbohydrate, fat or mixed substrate oxidation.
How it works
RQ = CO₂ produced ÷ O₂ consumed · Carbohydrate ≈ 1.0, protein ≈ 0.8, fat ≈ 0.7 · %carbohydrate ≈ (RQ − 0.707) ÷ (1.0 − 0.707) × 100
The respiratory quotient is the ratio of carbon dioxide released to oxygen consumed during respiration, and it reveals which fuel is being burned. Carbohydrate oxidation produces one CO₂ per O₂ (RQ = 1.0), fat needs more oxygen so its RQ is about 0.7, and protein sits near 0.8. A non-protein RQ between 0.7 and 1.0 maps almost linearly to the proportion of energy coming from carbohydrate versus fat.
Worked example
A subject releases 250 mL of CO₂ while consuming 300 mL of O₂, so RQ = 250 ÷ 300 = 0.83. That falls in the mixed range and works out to about (0.83 − 0.707) ÷ 0.293 ≈ 42% of energy from carbohydrate and 58% from fat.
Sources: Respiratory quotient — Wikipedia
Frequently asked questions
Why is the RQ for fat lower than for carbohydrate?
Fats are highly reduced and hydrogen-rich, so oxidising them consumes proportionally more oxygen for the CO₂ they release. That drives the ratio down to about 0.7, compared with 1.0 for glucose.
Can the respiratory quotient exceed 1.0?
Yes, briefly. Values above 1.0 occur during hyperventilation, intense exercise, or when carbohydrate is being converted to fat — extra CO₂ is blown off relative to oxygen used. Sustained values above 1.0 are not from steady-state substrate oxidation.
Is respiratory quotient the same as respiratory exchange ratio?
They use the same CO₂/O₂ calculation, but RQ refers to gas exchange at the cell or tissue level in steady state, while the respiratory exchange ratio (RER) is measured at the mouth and can be skewed by breathing changes.
Related calculators
Educational estimate, not medical or laboratory advice — verify formulas, units and assumptions for your specific organism or protocol.