Respiratory Quotient (RQ) Calculator
Respiratory Quotient (RQ) Calculator
VCO₂ divided by VO₂, with the substrate values and the 0.67 to 1.3 physiologic range — and an honest account of what RQ can and cannot be used for, because the overfeeding teaching is much weaker than it is usually stated.
Respiratory quotient
VCO₂ over VO₂VCO₂ 200 mL/min, VO₂ 250 mL/min
Formula
Physiologic range in humans: 0.67 to 1.3
- the unit cancels
- RQ is dimensionless, so VCO₂ and VO₂ may be in mL/min, L/min or L/day as long as they are in the SAME unit and come from the same measurement period
- substrate anchors
- carbohydrate 1.0, protein about 0.8, lipid about 0.7, a mixed diet about 0.8. These are oxidation values; a measured RQ is a weighted mixture of them, and the mixture cannot be recovered from the single number
- 0.67 to 1.3
- the range measured in humans. Outside it, the Academy of Nutrition and Dietetics advises suspecting a measurement error and repeating; inside it, an RQ “should not be used alone to reject” a measurement. That validity check is the use of RQ with the strongest support. Above 1.0, net lipogenesis produces CO₂ without consuming O₂, but the inference does not run reliably backwards: sensitivity for overfeeding was 21% in one paediatric series
- what moves it that is not nutrition, and where sources differ
- hyperventilation and hypoventilation, an acidosis being blown off, sodium bicarbonate, an unsteady state, a leak, and a high FiO₂. One ICU review concludes RQ “is highly variable and unpredictable in critically ill patients, limiting its validity as an indicator of energy substrate oxidation”, and the Academy rates “do not rely solely on measured RQ” as Strong; a 2024 ESPEN tutorial takes the opposite view and calls it “a good marker of over- or underfeeding”. The position with the quantitative evidence behind it is the first
Worked example
VCO₂ 200 mL/min, VO₂ 250 mL/min
200 ÷ 250 = 0.80 — the 0.75 to 0.95 band, mixed substrate oxidation, the value expected on an ordinary mixed intake
In L/min: 0.200 ÷ 0.250 = 0.80. The unit cancels, which is why only one unit field exists
Inside 0.67 to 1.3, so the measurement passes the one validity check the Academy endorses — and an RQ in range is not grounds to reject a measurement by itself
Change VCO₂ to 255 and RQ becomes 1.02: carbohydrate-predominant, above the 1.0 of pure carbohydrate oxidation. That is consistent with net lipogenesis and does not establish overfeeding — sensitivity 21% in one series
Change VCO₂ to 160 and RQ becomes 0.64, or to 330 and it becomes 1.32: both outside the physiologic range, and both measurements to repeat rather than findings
The same two gases through the Weir equation give a measured energy expenditure of 1,737 kcal/day abbreviated — one measurement, two questions
Substrate respiratory quotients, and the range
| Substrate or state | RQ | Source |
|---|---|---|
| Carbohydrate oxidation | 1.0 | “close to 1 for carbohydrate metabolisation” |
| Protein oxidation | about 0.8 | “0.8 for protein” |
| Lipid oxidation | about 0.7 | “0.7 for lipids” |
| Mixed diet | about 0.8 | Also the value assumed when expenditure is estimated from VCO₂ alone |
| Physiologic range in humans | 0.67 to 1.3 | “This ratio is measured between 0.67 and 1.3 in humans”; the Academy of Nutrition and Dietetics uses the same bounds |
| Net lipogenesis | above 1.0 | CO₂ produced without O₂ consumed; the mechanism behind a high RQ |
What RQ has actually been shown to do
| Question | Published performance | Cohort |
|---|---|---|
| Is this measurement valid? | The use with the strongest support. Outside 0.67 to 1.3, suspect an error and repeat (rated Consensus). Inside it, do not reject on RQ alone (rated Fair) | Academy of Nutrition and Dietetics recommendations |
| Is this patient being underfed? | RQ below 0.85: sensitivity 63%, specificity 89%, negative predictive value 90% | Critically ill children |
| Is this patient being overfed? | RQ above 1.0: sensitivity 21%, specificity 97%, positive predictive value 93% | The same paediatric cohort |
| What mixture of fuels is being burnt? | “Highly variable and unpredictable in critically ill patients, limiting its validity” | ICU review |
| Any of the above, per a dissenting source | “A good marker of over- or underfeeding at the different stages of disease” | 2024 ESPEN case-based tutorial |
Good at one job, oversold at the other two
The respiratory quotient is carbon dioxide produced divided by oxygen consumed, and it comes free with every indirect calorimetry measurement because the cart already needs both gases. The physiology is solid: oxidising carbohydrate yields one molecule of CO₂ per molecule of O₂ and so an RQ of 1.0, lipid about 0.7, protein about 0.8, and converting carbohydrate into fat produces CO₂ without consuming O₂ and can push the ratio above 1.0. Across humans the measured value runs between 0.67 and 1.3.
From that grew a piece of bedside teaching — an RQ above 1.0 means carbohydrate overfeeding, below 0.7 means underfeeding — which is far weaker than its confident delivery suggests. The quantitative work is mostly one paediatric intensive care series, and it is not encouraging: an RQ above 1.0 detected overfeeding with a sensitivity of 21%, missing four overfed patients in five, and an RQ below 0.85 detected underfeeding with a sensitivity of 63%. Both specificities were high — 97% and 89% — so the direction it is useful in is the one it is rarely used for: a high RQ that does turn up is worth chasing, but a normal RQ excludes very little. The Academy of Nutrition and Dietetics rates “do not rely solely on measured RQ to evaluate level or composition” of feeding as a Strong recommendation, because “RQ has poor accuracy to evaluate under- and over-feeding”.
Part of the problem is that RQ is not purely metabolic. Measured VCO₂ depends on ventilation too, so hyperventilation, an acidosis being blown off, sodium bicarbonate, an unsteady state and a ventilator change mid-measurement all move the ratio with no change in substrate use at all. One ICU review calls it “highly variable and unpredictable in critically ill patients”. Not every source agrees: a 2024 ESPEN tutorial calls RQ “a good marker of over- or underfeeding at the different stages of disease”. Both positions are on this page, and the one with sensitivity and specificity attached is the sceptical one.
What RQ is genuinely good for is the job it is least often given: checking the measurement. Outside 0.67 to 1.3, suspect an error and repeat; inside it, do not reject a measurement on RQ alone. That is why this calculator sits beside the Weir equation calculator, which turns the same two gases into kcal/day. For the substrate-delivery side of the question the parenteral macronutrient calculator gives the carbohydrate load in mg/kg/min against published ceilings, which is a more direct way to ask whether a patient is receiving too much glucose than reading it off a ratio. This page computes a figure and prints the published targets beside it with the guideline each comes from. The prescription is the clinician’s, taken with the patient in front of them.
Frequently asked questions
What is a normal respiratory quotient?
About 0.8 on a mixed intake. The substrate values are 1.0 for carbohydrate oxidation, about 0.8 for protein and about 0.7 for lipid, and the range measured in humans is 0.67 to 1.3. A value outside that range is a reason to suspect a measurement error and repeat, rather than a finding about the patient.
Does an RQ above 1.0 mean overfeeding?
It is consistent with it and does not establish it. Net lipogenesis does raise RQ above 1.0, but in critically ill children an RQ above 1.0 detected overfeeding with a sensitivity of only 21% — four overfed patients in five were missed — although specificity was 97%, so a high value that does appear is worth investigating. Hyperventilation, sodium bicarbonate and an acidosis being blown off all raise measured VCO₂ with no change in substrate use.
Can a low RQ tell me a patient is underfed?
Poorly. An RQ below 0.85 detected underfeeding with a sensitivity of 63% and a specificity of 89% in critically ill children. The Academy of Nutrition and Dietetics rates “do not rely solely on measured RQ to evaluate level or composition” of feeding as a Strong recommendation.
What units should VCO₂ and VO₂ be in?
Any unit, as long as both are in the SAME one. RQ is a ratio and the unit cancels, so 200 and 250 mL/min give the same 0.80 as 0.200 and 0.250 L/min. Mixing the two is the one unit error available on this page, and it is wrong by a factor of a thousand.
What is RQ actually useful for?
Checking the measurement. Outside 0.67 to 1.3, suspect an error and repeat; inside it, do not reject a measurement on RQ alone. Those are the two recommendations with the clearest support. An assumed RQ of about 0.85 is also what allows energy expenditure to be estimated from VCO₂ alone when a full cart is not available.
Related calculators
References
- Academy of Nutrition and Dietetics Evidence Analysis Library. Energy Expenditure: respiratory quotient recommendations. An RQ within 0.67 to 1.3 “should not be used alone to reject” a measurement (Fair); outside it, “suspect an error and repeat the RMR measurement” (Consensus); “RQ has poor accuracy to evaluate under- and over-feeding” (Strong).
- Hulst J, van Goudoever H, Zimmermann L, Hop W, Büller H, Tibboel D, Joosten K. Adequate feeding and the usefulness of the respiratory quotient in critically ill children. Nutrition. 2005;21(2):192–198. An RQ below 0.85 for underfeeding: sensitivity 63%, specificity 89%. An RQ above 1.0 for overfeeding: sensitivity 21%, specificity 97%.
- Oshima T, Graf S, Heidegger CP, Genton L, Pugin J, Pichard C. Can calculation of energy expenditure based on CO₂ measurements replace indirect calorimetry? Crit Care. 2017;21:36. Prints the modified Weir equation with a 1.11 VCO₂ coefficient and the 0.67 to 1.3 window, and states that RQ “is neither a reliable indicator of the feeding status”.
- Pardo E, Constantin JM. Indirect calorimetry in mechanically ventilated patients to assess nutritional targets. ICU Management & Practice. 2021;21(6):297–300. “This ratio is measured between 0.67 and 1.3 in humans. The ratio is close to 1 for carbohydrate metabolisation, 0.8 for protein and 0.7 for lipids.”
- Berger MM, De Waele E, Gramlich L, et al. How to interpret and apply the results of indirect calorimetry studies: a case-based tutorial. Clin Nutr ESPEN. 2024;63:856–869. Takes the opposite view of RQ, calling it “a good marker of over- or underfeeding at the different stages of disease”.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/