Indirect Calorimetry — Resting Energy
Free indirect calorimetry calculator: turn measured REE or raw VO₂/VCO₂ into resting energy expenditure, respiratory quotient and substrate oxidation.
CalcVita. (2026). Indirect Calorimetry — Resting Energy. CalcVita. Retrieved June 3, 2026, from https://calcvita.com/en/calculators/indirect-calorimetry
Suggested article
Indirect Calorimetry: Measuring Metabolism Beats Guessing
Predictive formulas estimate your calorie needs from height, weight and age. Indirect calorimetry measures them directly from the air you breathe — and the difference can be large.
Read the full article →What indirect calorimetry is
Indirect calorimetry estimates how much energy your body burns by measuring the gases you exchange while breathing: the oxygen you consume (VO₂) and the carbon dioxide you produce (VCO₂). Because fat, carbohydrate and protein each consume oxygen and release CO₂ in fixed ratios, those two gas flows can be converted into a daily calorie figure. This calculator applies the classic Weir (1949) equation to turn VO₂ and VCO₂ into resting energy expenditure (REE).
What VO₂, VCO₂, REE and RQ mean
VO₂ is the volume of oxygen consumed per minute and VCO₂ the volume of carbon dioxide produced per minute. REE (resting energy expenditure) is the calories your body burns at rest over 24 hours — typically 60–75% of total energy expenditure. The respiratory quotient (RQ) is VCO₂ divided by VO₂. RQ reveals which fuel you are oxidising: values near 0.70 indicate mostly fat, around 0.85 a mixed diet, and near 1.00 mostly carbohydrate.
Why measuring beats a predictive equation
Equations such as Mifflin-St Jeor or Harris-Benedict only estimate REE from age, sex, height and weight. They can be off by 10–20% in any individual because they cannot see your actual muscle mass, metabolic adaptation, illness or training status. The ESPEN 2023 practical guideline on clinical nutrition in the intensive care unit recommends measured energy expenditure by indirect calorimetry, when available, as the reference for setting calorie targets — precisely because a real measurement removes that guesswork. This tool shows your measured REE next to the Mifflin-St Jeor prediction so you can see the gap.
How to read substrate oxidation
From VO₂, VCO₂ and (optionally) urinary nitrogen, the Frayn (1983) equations estimate how many grams of fat and carbohydrate you are burning per day, plus the share of energy each fuel provides. A low RQ with high fat oxidation is typical of fasting, low-carb intake or endurance work; a high RQ reflects recent carbohydrate intake or overfeeding. If you do not provide measured urinary nitrogen, protein oxidation is assumed from a percentage of energy and flagged as estimated.
More calculators
Keep exploring helpful tools
Basal metabolic rate (BMR)
Calculate your basal metabolic rate with Mifflin-St Jeor and Harris-Benedict.
TDEE calculator
Estimate daily energy needs based on activity.
Macro planner
Plan protein, fat and carbs for your goal.
Protein calculator
Your daily protein needs by weight, activity level and goals (ISSN).