Respiratory Quotient Calculator
The Respiratory Quotient (RQ) is the ratio of carbon dioxide produced (VCO2) to oxygen consumed (VO2) during metabolism. It is a key indicator of the metabolic fuel being used at rest or during exercise. Pure fat oxidation yields an RQ of 0.70, mixed substrates yield approximately 0.85, and pure carbohydrate oxidation yields 1.00. Enter your measured VCO2 and VO2 values (in the same units, typically mL/min or L/min) to calculate RQ and interpret your primary fuel substrate.
Respiratory Quotient formula
RQ = VCO2 / VO2
Where VCO2 is the volume of carbon dioxide produced and VO2 is the volume of oxygen consumed, measured in the same units (mL/min or L/min). Reference ranges: 0.70 (pure fat), 0.80 (protein), 0.85 (mixed), 1.00 (pure carbohydrate).
RQ interpretation table
- RQ = 0.70: Pure fat oxidation
- RQ = 0.80: Primarily fat with some protein
- RQ = 0.85: Mixed substrate (fat and carbohydrate)
- RQ = 1.00: Pure carbohydrate oxidation
- RQ above 1.00: Net lipogenesis or anaerobic exercise with CO2 buffering
Respiratory Quotient calculator: frequently asked questions
What is the Respiratory Quotient?
The Respiratory Quotient (RQ) is the ratio of carbon dioxide (CO2) produced to oxygen (O2) consumed during metabolism: RQ = VCO2 / VO2. It indicates which fuel substrate the body is using. An RQ of 0.70 indicates pure fat oxidation, 0.85 indicates a mixed substrate, and 1.00 indicates pure carbohydrate oxidation.
What does an RQ above 1.0 mean?
An RQ above 1.0 indicates net lipogenesis (conversion of carbohydrates to fat for storage) or high-intensity anaerobic exercise where excess CO2 is produced from bicarbonate buffering of lactic acid. This is sometimes called the non-protein Respiratory Exchange Ratio (RER) when measured at the mouth rather than the cell.
What is the difference between RQ and RER?
RQ (Respiratory Quotient) is measured at the cellular level and reflects true metabolic substrate use. RER (Respiratory Exchange Ratio) is measured at the mouth during gas exchange testing and equals VCO2/VO2 at the lungs. At steady state, RER approximates RQ, but during exercise or non-steady state, they diverge due to CO2 buffering.
What are normal RQ values for different substrates?
Fat: RQ = 0.70. Protein: RQ = approximately 0.80. Carbohydrate: RQ = 1.00. A fasted individual typically has an RQ near 0.70-0.75, reflecting fat as the primary fuel. After a high-carbohydrate meal, RQ rises toward 1.00.
How is RQ used clinically?
In clinical nutrition, RQ is measured by indirect calorimetry to assess whether a patient is being over-fed or under-fed, and to determine the proportion of calories from fat and carbohydrate. An RQ below 0.70 or above 1.00 may indicate measurement error. RQ guides nutritional support in ICU patients.
Official sources
- National Institutes of Health, National Library of Medicine: Physiology, Respiratory Quotient.
- American College of Sports Medicine (ACSM): ACSM Guidelines for Exercise Testing and Prescription.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.