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Respiratory Quotient Calculator

Biology

Calculate the respiratory quotient (RQ) from CO2 produced and O2 consumed. Instantly see which macronutrient — fat, protein, or carbohydrate — is being metabolized.

Fat
0.7
Protein
0.8
Carbs
1.0

Approximate reference values: fat ≈ 0.70, protein ≈ 0.80, carbohydrate ≈ 1.00. Real-world RQ reflects a mix of substrates being oxidized simultaneously.

Respiratory Quotient

0.91
Carbohydrate

What is a Respiratory Quotient?

The Respiratory Quotient Calculator computes RQ — the ratio of carbon dioxide produced to oxygen consumed during cellular respiration — from your measured or given gas volumes. RQ is a classic indicator of which macronutrient (fat, protein, or carbohydrate) is being predominantly metabolized for energy.

Enter the volume of CO2 produced and O2 consumed (in any consistent unit), and the calculator returns the RQ value along with an interpretation of the likely primary substrate. To explore the ATP energy output once carbohydrate metabolism is confirmed, see the ATP Yield Calculator.

How to use this Respiratory Quotient calculator

  1. Enter CO2 produced — the measured or given volume of carbon dioxide output over the measurement period.

  2. Enter O2 consumed — the measured or given volume of oxygen consumed over the same period, in the same unit.

  3. Read the RQ value — automatically computed as CO2 produced ÷ O2 consumed.

  4. Check the substrate interpretation — the result names the likely dominant fuel source based on where your RQ falls on the reference scale.

Formula & Methodology

Respiratory Quotient formula:
RQ = Volume of CO2 produced ÷ Volume of O2 consumed

Reference ranges (approximate, standard physiology textbook figures):
- RQ ≈ 0.70 — fat (lipid) oxidation
- RQ ≈ 0.80 — protein oxidation (or mixed fat/protein)
- RQ ≈ 1.00 — carbohydrate oxidation
- RQ > 1.00 — may indicate lipogenesis, heavy anaerobic exercise, or hyperventilation

Worked example:

If CO2 produced = 200 L and O2 consumed = 220 L:

RQ = 200 ÷ 220 = 0.91

This falls between the protein (0.8) and carbohydrate (1.0) benchmarks, suggesting a mixed diet with carbohydrate as a leading substrate.

Note: These reference values assume steady-state aerobic metabolism. During intense exercise, at rest under stress, or with certain metabolic conditions, CO2 production can be influenced by factors other than substrate oxidation (like bicarbonate buffering of lactic acid), which can push the measured ratio (often called RER in this context) above the pure-substrate values shown here.

Frequently Asked Questions

The respiratory quotient is the ratio of carbon dioxide produced to oxygen consumed during cellular respiration (RQ = VCO2 ÷ VO2), and it indicates which macronutrient — carbohydrate, protein, or fat — is being predominantly metabolized for energy at that moment.
An RQ of approximately 1.0 indicates that carbohydrates are the primary fuel being oxidized, because burning glucose consumes and produces equal molar volumes of O2 and CO2 (C6H12O6 + 6O2 → 6CO2 + 6H2O).
An RQ of approximately 0.7 indicates that fat is the primary fuel being metabolized. Fat oxidation produces less CO2 relative to O2 consumed because fatty acid molecules contain proportionally more hydrogen and less oxygen than carbohydrates, requiring more O2 per CO2 released.
An RQ of approximately 0.8 indicates protein is being used as a significant energy source, sitting between the fat (~0.7) and carbohydrate (~1.0) values, since protein's amino acid structure gives it an intermediate O2-to-CO2 ratio during oxidation.
Yes — an RQ above 1.0 can indicate lipogenesis (converting excess carbohydrate into fat, which releases extra CO2), heavy anaerobic exercise producing CO2 from bicarbonate buffering of lactic acid, or hyperventilation, rather than pure aerobic substrate oxidation.
RQ is typically measured using indirect calorimetry, where a person breathes into a metabolic cart that measures the volume of O2 inhaled and CO2 exhaled over a period of time, from which VCO2 ÷ VO2 is calculated directly.
RQ helps estimate what fuel source (fat vs. carbohydrate) a patient's body is using at rest or during exercise, which is useful in nutrition assessment, monitoring critically ill patients on parenteral nutrition, and evaluating metabolic flexibility in athletes.
RQ and RER are calculated the same way (CO2 produced ÷ O2 consumed) but are used in different contexts: RQ traditionally refers to steady-state cellular metabolism, while RER is the term typically used during exercise testing, where CO2 production can also reflect non-metabolic buffering of lactic acid at high intensities.
At rest and during low-to-moderate intensity exercise, the body relies more on fat oxidation (RQ closer to 0.7–0.8), while at higher exercise intensities the body shifts toward carbohydrate metabolism (RQ approaching 1.0) because carbohydrates can be broken down for energy faster without oxygen.
Once glucose is confirmed as the primary substrate (RQ near 1.0), you can estimate the total energy yield from that glucose using the [ATP Yield Calculator](/atp-yield-calculator/), which applies the standard ~30 ATP net per glucose textbook figure.
In most real physiological states, the body oxidizes a mix of fat, protein, and carbohydrate simultaneously rather than a single pure fuel, so RQ values between 0.7 and 1.0 (e.g., 0.85) usually reflect a blended substrate mixture rather than pure metabolism of one macronutrient.
Any consistent volume unit works since RQ is a ratio — liters, milliliters, or moles are all valid as long as CO2 produced and O2 consumed are measured in the same unit and under the same conditions (temperature, pressure, time period).
Also known as
RQ calculatorrespiratory quotient formulaCO2 to O2 ratio calculatormetabolic substrate calculator