Respiratory Quotient Calculator
The respiratory quotient from oxygen consumed and carbon dioxide produced — which tells you whether the body is burning carbohydrate, fat or a mix — with the energy expenditure it implies.
Burning carbohydrate produces one CO₂ for every O₂ consumed — an RQ of 1.
How the respiratory quotient calculator works
Burning carbohydrate produces one CO₂ for every O₂ consumed — an RQ of 1.0. Fat needs more oxygen per carbon and gives about 0.7. A measured RQ between the two says what mix is being oxidised, and that is exactly what indirect calorimetry does.
The energy per litre of oxygen also depends on the fuel — about 5.05 kcal for carbohydrate and 4.7 for fat — so RQ plus oxygen consumption gives energy expenditure.
Formula: RQ = VCO₂ / VO₂; fat fraction ≈ (1 − RQ) / 0.3
Worked examples
| Inputs | Respiratory quotient | Note |
|---|---|---|
| RQ 0.85 at rest | 0.85 | half fat, half carbohydrate |
| Hard exercise | 0.9833 | nearly all carbohydrate |
| Fasted | 0.7167 | mostly fat |
FAQFrequently asked questions
What is the respiratory quotient?
Carbon dioxide produced divided by oxygen consumed. It is 1.0 for carbohydrate, about 0.7 for fat and 0.8 for protein, so a measured value says what is being burned.
Why does fat give a lower RQ?
Fat is more reduced — fewer oxygen atoms per carbon — so burning it needs more O₂ per CO₂ released.
What is indirect calorimetry?
Measuring energy expenditure from gas exchange. Oxygen consumption times the energy per litre for the measured RQ gives kilocalories, without measuring heat at all.
Why does RQ rise during hard exercise?
Because muscles switch to carbohydrate at high intensity, and lactate buffering releases extra CO₂. Values above 1.0 briefly are normal near maximum effort.
What does an RQ above 1 at rest mean?
Net conversion of carbohydrate to fat, as after a large meal — or hyperventilation blowing off CO₂. Protein is ignored here, which shifts the fat estimate slightly.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- Wellcome Sanger Institute — a leading UK genomics institute
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.