Metabolic Rate Calculator
Calculate mass-specific metabolic rate from an organism's oxygen consumption, body mass, and respiratory quotient. Used in comparative physiology, exercise science, and ecology to quantify energy expenditure.
Last updated: September 2026
Formula below · 2 sources (genome.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Metabolic rate reflects how quickly an organism converts chemical energy into heat and work, and can be estimated by indirect calorimetry from oxygen consumption. Using the Weir equation, each litre of O₂ consumed releases 3.941 + 1.106 × RQ kcal, where the respiratory quotient RQ = VCO₂/VO₂ reflects the fuel being oxidised: about 4.69 kcal/L at RQ 0.7 (fat), 4.88 kcal/L at 0.85 (mixed diet) and 5.05 kcal/L at 1.0 (carbohydrate) — roughly 20 J per mL. The calculator therefore returns Metabolic Rate (kcal/kg/hr) = (O₂ consumed in mL/hr ÷ 1000) × (3.941 + 1.106 × RQ) / body mass in kg. Note that RQ only nudges the energy equivalent by a few percent; it is not a multiplier on the whole rate. One kcal/kg/hr is about 1 MET, the resting rate of an adult human. This approach is standard in exercise physiology, ecology, and toxicology.
How to use
Example 1 — Resting adult. A 70 kg person consumes 250 mL of O₂ per minute at rest (15,000 mL/hr) with an RQ of 0.85. Energy per litre = 3.941 + 1.106 × 0.85 = 4.881 kcal/L. Total = 15 L/hr × 4.881 = 73.2 kcal/hr. Per kilogram: 73.2 / 70 = 1.05 kcal/kg/hr — about 1 MET, as expected at rest (≈1,760 kcal/day). Example 2 — Laboratory mouse. A 25 g mouse (0.025 kg) consumes 60 mL O₂/hr with RQ 0.9: 0.06 × (3.941 + 0.995) = 0.296 kcal/hr, or 11.85 kcal/kg/hr — more than ten times the human mass-specific rate, as Kleiber's law predicts for a small endotherm.
Frequently asked questions
What is the respiratory quotient and how do I choose the right value?
The respiratory quotient (RQ) is the ratio of CO₂ produced to O₂ consumed during cellular respiration, and it indicates which macronutrient is being primarily oxidized. A value of 1.0 corresponds to pure carbohydrate metabolism, 0.7 to pure fat metabolism, and approximately 0.82 for protein. In practice, a resting human oxidizing a mixed diet typically has an RQ of around 0.82–0.85. Use measured RQ from expired gas analysis when available, or use 0.85 as a reasonable default for a mixed-substrate resting state.
Why is oxygen consumption used to estimate metabolic rate?
Oxygen is required for aerobic cellular respiration, the primary pathway by which organisms extract energy from nutrients. Because the relationship between O₂ consumed and energy released is well-characterized for known substrates, measuring O₂ uptake provides an accurate indirect estimate of metabolic heat production without the need for direct calorimetry. This technique, called indirect calorimetry or respirometry, is non-invasive, technically straightforward, and widely applicable to humans, animals, and even microorganisms. It is the gold standard in exercise physiology laboratories and ecological field studies.
How does body mass affect the interpretation of metabolic rate?
Larger animals consume more oxygen in absolute terms, but mass-specific metabolic rate (per kilogram) tends to decrease with increasing body size—a phenomenon described by Kleiber's law, where metabolic rate scales roughly to body mass to the power of 0.75. Dividing raw oxygen consumption by body mass partially corrects for size differences and allows meaningful comparisons between individuals or species. However, for precise interspecies comparisons, allometric scaling (using the 0.75 exponent rather than 1.0) is more appropriate than simple mass-specific division.