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.
About this calculator
Metabolic rate reflects how quickly an organism converts chemical energy into heat and work, and can be estimated indirectly from oxygen consumption via indirect calorimetry. The formula used here is: Metabolic Rate = (O₂ Consumed (ml/hr) × 20.1 × RQ) / Body Mass (kg), where 20.1 J/ml is the approximate energy equivalent of one milliliter of oxygen consumed, and the respiratory quotient (RQ) adjusts for the fuel substrate being oxidized. RQ = 1.0 indicates pure carbohydrate catabolism, RQ = 0.7 indicates pure fat oxidation, and intermediate values reflect mixed substrates. Dividing by body mass gives a mass-specific rate in J/(hr·kg), enabling fair comparison across animals of different sizes. This approach, known as respirometry, is standard in exercise physiology, ecology, and toxicology. The 20.1 J/ml factor is a weighted average for typical mixed diets and may be refined if substrate composition is precisely known.
How to use
A 70 kg person consumes 1200 ml of oxygen per hour at rest, with an RQ of 0.85 (mixed diet). Apply the formula: Metabolic Rate = (1200 × 20.1 × 0.85) / 70. First calculate the numerator: 1200 × 20.1 = 24,120; 24,120 × 0.85 = 20,502. Then divide: 20,502 / 70 ≈ 292.9 J/(hr·kg). Enter 1200 for oxygen consumed, 70 for body mass, and 0.85 for RQ in the calculator to reproduce this result. This value represents resting metabolic rate per kilogram and can be scaled by body mass to get total energy expenditure of about 20,502 J/hr.
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.