Weir Indirect Calorimetry CalculatorFree online fitness & health calculator — instant results, no sign-up
Estimate daily energy expenditure from measured oxygen use and carbon dioxide output.
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.
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About the Weir Indirect Calorimetry Calculator
The Weir Indirect Calorimetry Calculator estimates daily energy expenditure from steady-state oxygen consumption and carbon dioxide production measured in liters per minute. It uses the abbreviated Weir equation to convert these respiratory gas measurements into kcal/day. The result represents the measured metabolic rate extrapolated over 24 hours, not necessarily your total expenditure during a day of changing activities.
- Steady-state oxygen consumption (L/min)
- The volume of oxygen your body consumes per minute during a stable measurement period, entered in L/min with a default of 0.25.
- Steady-state carbon dioxide production (L/min)
- The volume of carbon dioxide your body produces per minute during the same stable measurement period, entered in L/min with a default of 0.2.
Formula and method
EE = (3.941 × VO2 + 1.106 × VCO2) × 1440; with the defaults: EE = (3.941 × 0.25 + 1.106 × 0.2) × 1440 = 1,737.288 ≈ 1,737.29 kcal/day.
- EEEstimated energy expenditure in kcal/day, extrapolated from the measured steady-state rate.
- VO2Steady-state oxygen consumption in L/min.
- VCO2Steady-state carbon dioxide production in L/min.
- 3.941 and 1.106Abbreviated Weir equation coefficients applied to oxygen consumption and carbon dioxide production, respectively, to obtain kcal/min.
- 1440The number of minutes in 24 hours, used to convert kcal/min to kcal/day.
Multiply oxygen consumption by 3.941 and carbon dioxide production by 1.106, then add the two contributions to obtain energy expenditure in kcal/min. Multiply that rate by 1440 to express it as kcal/day and round the displayed result to two decimal places. No urinary nitrogen correction is applied.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Steady-state oxygen consumption0.25 L/min
- Steady-state carbon dioxide production0.2 L/min
Results
- Energy expenditure1,737.29 kcal/day
- NoteAbbreviated Weir equation ignores urinary nitrogen; requires valid steady-state gas measurements.
How to use the Weir Indirect Calorimetry Calculator
- 1Obtain oxygen consumption and carbon dioxide production from a calibrated indirect calorimetry system during a valid steady-state measurement period.
- 2Confirm that both measurements are in L/min and come from the same measurement period; convert mL/min to L/min by dividing by 1000 if necessary.
- 3Enter oxygen consumption in the first field and carbon dioxide production in the second field.
- 4Run the calculation; the default inputs of 0.25 L/min and 0.2 L/min give 1,737.29 kcal/day.
- 5Read the Note row and interpret the result in light of the measurement conditions, such as rest or sustained activity.
Reading your result
The Energy expenditure row shows the measured metabolic rate expressed as a 24-hour equivalent: 1,737.29 kcal/day with the default inputs. If the gases were measured under appropriate resting conditions, this can estimate resting energy expenditure rather than total daily energy expenditure. The Note row states: “Abbreviated Weir equation ignores urinary nitrogen; requires valid steady-state gas measurements.” This identifies both an omitted correction and a measurement requirement.
Limitations
Accuracy depends on calibrated equipment, leak-free gas collection, appropriate gas-volume corrections, and genuinely steady-state measurements. Rapidly changing exercise intensity, hyperventilation, or other non-steady-state conditions can make respiratory gas exchange an unreliable basis for this estimate. Omitting urinary nitrogen simplifies the calculation but does not explicitly account for protein oxidation. Extrapolating a short measurement across 24 hours assumes that rate persists; it does not automatically account for daily activity, meals, sleep, or changes in illness and recovery. The result is not a personalized calorie-intake prescription.
Common questions
Can I enter oxygen consumption reported in mL/kg/min into the Weir equation?+
Not directly. Multiply mL/kg/min by body mass in kilograms to obtain mL/min, then divide by 1000 to obtain the L/min required by this calculator. Carbon dioxide production must also be entered in L/min.
Does the default result of 1,737.29 kcal/day represent my total daily calorie needs?+
No. It is the 24-hour equivalent of the metabolic rate calculated from VO2 = 0.25 L/min and VCO2 = 0.2 L/min. Whether it approximates resting expenditure or another metabolic state depends on how those gases were measured; it does not include an automatic activity adjustment.
Why does the calculator require steady-state gas measurements?+
During transitions in activity or breathing, exhaled carbon dioxide can reflect changes in the body's carbon dioxide stores rather than current metabolic production alone. Stable measurements help the Weir equation better represent ongoing energy expenditure.
Why is urinary nitrogen mentioned if there is no nitrogen input?+
The full Weir approach includes a urinary nitrogen correction related to protein metabolism. This calculator uses the abbreviated equation, so it needs only oxygen consumption and carbon dioxide production and does not apply that correction.
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.
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Results are estimates for informational purposes and are not financial, medical or legal advice.