Respiratory Exchange Ratio CalculatorFree online fitness & health calculator — instant results, no sign-up
Calculate respiratory exchange ratio from measured carbon dioxide and oxygen flow.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
Sign in to save results and favorites.
About the Respiratory Exchange Ratio Calculator
The Respiratory Exchange Ratio Calculator divides measured carbon dioxide production by oxygen consumption to calculate respiratory exchange ratio (RER). It uses gas flow rates in liters per minute and returns a dimensionless ratio that can help contextualize fuel use during steady-state exercise. With the default measurements of 0.8 L/min carbon dioxide production and 1 L/min oxygen consumption, RER is 0.8.
- Carbon dioxide production (L/min)
- The measured rate of carbon dioxide released in exhaled air, expressed in liters per minute (L/min).
- Oxygen consumption (L/min)
- The measured rate of oxygen taken up by the body, expressed in liters per minute (L/min).
Formula and method
RER = V̇CO₂ / V̇O₂ = 0.8 L/min / 1 L/min = 0.8
- RERRespiratory exchange ratio, a dimensionless ratio of carbon dioxide production to oxygen consumption.
- V̇CO₂Carbon dioxide production rate in L/min.
- V̇O₂Oxygen consumption rate in L/min.
Divide carbon dioxide production by oxygen consumption using measurements from the same sampling interval and expressed on the same gas-volume basis. Oxygen consumption must be greater than zero. Because both flow rates use L/min, their units cancel, leaving a dimensionless RER.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Carbon dioxide production0.8 L/min
- Oxygen consumption1 L/min
Results
- Respiratory exchange ratio0.8
- NoteExercise intensity and non-steady-state breathing affect substrate interpretation.
How to use the Respiratory Exchange Ratio Calculator
- 1Obtain carbon dioxide production and oxygen consumption measurements from the same sampling interval, typically using a metabolic cart or respiratory gas analyzer.
- 2Enter carbon dioxide production in L/min; the default is 0.8.
- 3Enter oxygen consumption in L/min; the default is 1 and the value must be greater than zero.
- 4Calculate the ratio: 0.8 divided by 1 gives an RER of 0.8.
- 5Read the RER result alongside the note about exercise intensity and non-steady-state breathing before drawing conclusions about fuel use.
Reading your result
The 'Respiratory exchange ratio' output is the calculated ratio, not a percentage; the default result of 0.8 means carbon dioxide production is 0.8 times oxygen consumption. Under steady-state conditions with protein contribution treated as small, values near 0.70 are associated with predominantly fat oxidation and values near 1.00 with predominantly carbohydrate oxidation; 0.8 is consistent with mixed fuel use. The 'Note' output states: 'Exercise intensity and non-steady-state breathing affect substrate interpretation.' This warns that the ratio alone does not establish exact fuel-use proportions.
Limitations
RER measured at the lungs approximates cellular respiratory quotient only under suitable steady-state conditions. During intense exercise, bicarbonate buffering can generate additional exhaled carbon dioxide, while changes in ventilation and body carbon dioxide stores can shift the measured ratio independently of fuel oxidation. Results also depend on instrument calibration, sampling alignment, and consistent gas-volume corrections. Zero oxygen consumption makes the calculation undefined, and negative flow values are not valid inputs for this interpretation. This calculator does not measure calories burned, diagnose disease, or determine exact fat and carbohydrate oxidation rates.
Common questions
Does an RER of 0.8 mean that 80% of my energy comes from fat?+
No. With the default inputs, 0.8 is simply 0.8 L/min of carbon dioxide production divided by 1 L/min of oxygen consumption. It is consistent with mixed fat and carbohydrate use under steady-state assumptions, but it is not a fuel-use percentage.
Why can RER exceed 1 during hard exercise?+
Intense exercise can increase carbon dioxide output through bicarbonate buffering, and changing ventilation can alter carbon dioxide elimination. An RER above 1 therefore does not mean carbohydrate supplies more than 100% of energy.
Can I enter oxygen consumption reported in mL/kg/min?+
Not directly. This calculator requires absolute oxygen consumption in L/min. Multiply the mL/kg/min value by body mass in kilograms, then divide by 1,000 to convert it to L/min.
Can I combine carbon dioxide and oxygen measurements taken at different times?+
Use measurements from the same sampling interval. Combining values from different exercise stages or recovery periods can produce a ratio that does not represent respiratory gas exchange at either time.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
You might also like
BMI Calculator
Body mass index with category, healthy weight range, BMI Prime and Ponderal Index.
Calorie Calculator
Daily calories to maintain, lose or gain weight.
Body Fat Calculator
Estimate body fat using the U.S. Navy tape method.
BMR Calculator
Basal metabolic rate with Mifflin-St Jeor and Harris-Benedict.
Ideal Weight Calculator
Ideal weight from four well-known formulas.
Pace Calculator
Running pace, speed and finish time.
Results are estimates for informational purposes and are not financial, medical or legal advice.