Respiratory Quotient Calculator
Enter CO₂ produced and O₂ consumed to compute the respiratory quotient RQ, judging whether the body or cells burn carbohydrates, protein, or fat as the main fuel.
Input Data
Results
At a glance:The respiratory quotient (RQ) is the ratio of 'CO₂ volume produced' to 'O₂ volume consumed' during aerobic metabolism: RQ = V(CO₂) ÷ V(O₂). Because different energy substrates differ in chemical composition, their gas-exchange ratio at complete oxidation also differs: pure carbohydrate RQ ≈ 1.0, protein ≈ 0.8, fat ≈ 0.7. Thus RQ can infer what fuel the body or cells are mainly burning, and is a core index of energy metabolism and respiratory physiology in indirect calorimetry.
Formula
Respiratory quotient: RQ = CO₂ produced ÷ O₂ consumed.
Glucose complete oxidation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, RQ = 6 ÷ 6 = 1.0.
$$RQ = \dfrac{V_{CO_2}}{V_{O_2}}$$$$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O \Rightarrow RQ = 1.0$$How to Use
- Enter CO₂ volume produced and O₂ volume consumed per unit time (same unit, both L or both mL).
- The tool instantly computes the respiratory quotient RQ.
- Compare with the substrate table: ≈1.0 means carbohydrate-based, ≈0.8 mixed/protein, ≈0.7 fat-based.
Respiratory quotient of common metabolic substrates
| Energy substrate | Typical RQ | Note |
|---|---|---|
| Carbohydrate (glucose) | 1.00 | 6CO₂ ÷ 6O₂, oxygen-rich, less O₂ needed |
| Mixed diet | 0.80–0.85 | Common resting range for most people |
| Protein | 0.80 | ≈0.8 after subtracting urinary nitrogen |
| Fat | 0.70 | Oxygen-poor molecule, needs more O₂ to oxidize |
| Vigorous exercise / hyperventilation | >1.00 | Extra CO₂ from anaerobic metabolism or over-breathing |
RQ below 0.7 is common in prolonged fasting, ketosis, or alcohol metabolism; above 1.0 often reflects anaerobic metabolism, lipogenesis, or hyperventilation.
Case Studies
Resting subject mainly on carbohydrate
Indirect calorimetry: 6 L CO₂ produced and 6 L O₂ consumed per minute.
RQ = 6 ÷ 6 = 1.0.
RQ near 1.0 shows carbohydrate is the main fuel now, matching the post-carbohydrate-meal state.
Fat as fuel after endurance exercise
Another measurement: 16 L CO₂ produced, 23 L O₂ consumed.
RQ = 16 ÷ 23 ≈ 0.696.
RQ near 0.7 means fat is the main energy source, common after fasting or prolonged moderate exercise.
FAQ
Is RQ the same as respiratory exchange ratio (RER)?
They share the formula (CO₂ ÷ O₂) but differ in context. RQ is the true cellular-level metabolic gas-exchange ratio, theoretically between 0.7 and 1.0; RER is the whole-body gas ratio measured at the mouth/nose, which can temporarily exceed 1.0 during intense exercise due to anaerobic metabolism and hyperventilation. For estimating energy substrate, the measured ratio is commonly used.
Why is fat's RQ lower than carbohydrate's?
Fat molecules have a lower oxygen proportion and more hydrogen; complete oxidation needs relatively more O₂ to produce the same amount of CO₂, so the CO₂/O₂ ratio is smaller (about 0.7). Glucose itself contains more oxygen and needs less extra O₂, so its RQ is near 1.0.
Can RQ exceed 1.0?
Yes, but it usually does not mean pure substrate oxidation. RQ (or RER) above 1.0 is common when anaerobic metabolism releases extra CO₂ as lactate is buffered, during lipogenesis (fat synthesis), or when hyperventilation 'washes out' CO₂. At resting steady state, true cellular RQ generally does not exceed 1.0.
What is RQ practically used for?
Clinical and sports science use RQ to estimate the carbohydrate vs fat proportion of energy expenditure, guiding nutrition and training; ICUs assess patient metabolic state and ventilator settings; research uses it for metabolic rate, ketogenic-diet, and exercise-physiology analysis. Combined with O₂ consumption it can also estimate energy expenditure (kcal).
Must CO₂ and O₂ use the same unit?
Yes. RQ is a volume ratio; as long as CO₂ and O₂ use the same unit (both L or both mL, same time interval), the ratio is correct. If units differ, convert to the same unit before dividing, otherwise the result is distorted.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.