Respiratory Exchange Ratio Calculator
The respiratory exchange ratio (RER) reveals which fuels your body is burning by comparing the volume of carbon dioxide produced to the oxygen consumed. Enter VCO2 and VO2 in the same units (mL/min or L/min) and the calculator returns the RER and the estimated carbohydrate-versus-fat balance. An RER near 0.70 indicates fat oxidation, near 1.00 carbohydrate oxidation, and values above 1.00 signal high-intensity, non-steady-state conditions. It is a standard output of metabolic cart and VO2 testing.
Respiratory exchange ratio formula
RER = VCO2 / VO2
Carbohydrate (%) approx = (RER - 0.70) / (1.00 - 0.70) * 100
Fat (%) approx = 100 - carbohydrate (%)
VCO2 and VO2 must be in the same units so RER is dimensionless. The fuel-mix estimate uses the standard endpoints of 0.70 for pure fat and 1.00 for pure carbohydrate, clamped to the 0 to 100 percent range; it is not valid above an RER of 1.00.
Metabolic testing context
- RER is measured during indirect calorimetry, a standard method in exercise physiology and clinical nutrition.
- An RER of 0.70 indicates near-pure fat oxidation; 1.00 indicates near-pure carbohydrate oxidation.
- RER above 1.00 reflects buffering of lactic acid and hyperventilation, not actual substrate use.
- The fuel-mix percentage is a linear approximation between the 0.70 and 1.00 endpoints.
- RER is used to confirm maximal effort in VO2max tests, where values typically exceed 1.10.
Respiratory exchange ratio: frequently asked questions
What is the respiratory exchange ratio?
The respiratory exchange ratio (RER) is the ratio of carbon dioxide produced (VCO2) to oxygen consumed (VO2) measured at the mouth during respiration. It is a non-invasive indicator of which fuels (carbohydrate or fat) the body is oxidising.
What is the RER formula?
RER equals VCO2 divided by VO2, where VCO2 is the volume of carbon dioxide produced and VO2 is the volume of oxygen consumed over the same period. Both must be in the same units (for example mL/min or L/min), so the ratio is dimensionless.
What do different RER values mean?
An RER near 0.70 indicates the body is oxidising mostly fat. An RER of about 1.00 indicates pure carbohydrate oxidation. Values between 0.70 and 1.00 reflect a mix. Values above 1.00 typically occur during intense exercise as carbon dioxide is buffered from lactic acid.
What is the difference between RER and RQ?
The respiratory quotient (RQ) is the ratio measured at the cellular or tissue level, while the respiratory exchange ratio (RER) is measured at the lungs. At rest and steady state the two are similar, but RER can exceed RQ during non-steady-state conditions such as hard exercise.
Why can RER exceed 1.0?
During high-intensity exercise, hyperventilation and the buffering of lactic acid release extra carbon dioxide that is not directly from fuel oxidation. This raises VCO2 relative to VO2, pushing the RER above 1.0, so values above 1.0 do not reflect true substrate use.
Official sources
- National Center for Biotechnology Information: Physiology, Respiratory Quotient (StatPearls, NCBI Bookshelf).
- National Institutes of Health: National Heart, Lung, and Blood Institute.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.