Metabolic Rate Calculators

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Metabolic rate is the rate at which an organism converts nutrients to energy, measured in kilocalories (kcal) or watts per unit time. Basal metabolic rate (BMR) is the minimum energy required to maintain vital functions at rest in a post-absorptive state. Resting metabolic rate (RMR) is slightly higher and is measured under less strict conditions. Total daily energy expenditure (TDEE) adds physical activity and the thermic effect of food to BMR. These measures are fundamental to nutrition science, exercise physiology, weight management, and clinical dietetics.

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Mifflin-St Jeor BMR Equation

The most accurate validated formula for estimating BMR:

  • Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) + 5
  • Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) − 161

Example: 30-year-old woman, 65 kg, 165 cm: BMR = 650 + 1031.25 − 150 − 161 = 1370 kcal/day.

Total Daily Energy Expenditure (TDEE)

TDEE = BMR × Activity Factor

  • Sedentary (office job, little exercise): × 1.2
  • Lightly active (exercise 1–3 days/week): × 1.375
  • Moderately active (3–5 days/week): × 1.55
  • Very active (hard exercise 6–7 days/week): × 1.725
  • Extremely active (physical job + training): × 1.9

Components of Energy Expenditure

  • BMR: 60–70% of TDEE — largest component; driven by organ mass (liver, brain, muscle)
  • Physical activity: 15–30% — most variable component
  • Thermic effect of food (TEF): 8–10% — energy used to digest and absorb food

Metabolic Rate in Biology

Metabolic rate scales allometrically with body mass: BMR ∝ M^0.75 (Kleiber's law). Small animals have higher mass-specific metabolic rates than large ones — a mouse metabolizes ~200 kcal/kg/day; an elephant ~13 kcal/kg/day. This scaling explains why small animals have higher heart rates, respiratory rates, and shorter lifespans per unit body size.

Glossary

Basal Metabolic Rate (BMR)
The minimum energy required to maintain vital functions at complete rest, fasted, in a thermoneutral environment; estimated by the Mifflin-St Jeor equation; 60–70% of total daily energy expenditure.
Total Daily Energy Expenditure (TDEE)
Total calories burned per day: TDEE = BMR × activity factor; includes BMR, physical activity, and the thermic effect of food.
Kleiber's Law
The empirical relationship that BMR scales with body mass to the 3/4 power (M^0.75) across mammalian species; explains why smaller animals have higher mass-specific metabolic rates.

Frequently Asked Questions

BMR is the minimum energy the body needs to maintain vital functions at complete rest in a warm environment, in a post-absorptive state (12+ hours fasted). It represents 60–70% of total daily energy expenditure. BMR is primarily determined by lean body mass (especially skeletal muscle and organ mass), age, sex, and genetics. The Mifflin-St Jeor equation is the most clinically validated formula for estimating BMR in adults.

TDEE = BMR × activity factor. First estimate BMR using Mifflin-St Jeor (10 × kg + 6.25 × cm − 5 × age ± constant). Multiply by: 1.2 (sedentary), 1.375 (lightly active), 1.55 (moderately active), 1.725 (very active), 1.9 (extremely active). To lose weight: eat 300–500 kcal below TDEE. To gain muscle: eat 200–300 kcal above TDEE. These are estimates — individual variation due to genetics, gut microbiome, and adaptive thermogenesis is substantial.

BMR (basal metabolic rate) is measured under strict conditions: complete physical rest, post-absorptive state (12+ hours fasted), thermoneutral environment, immediately after waking. RMR (resting metabolic rate) is measured under less strict conditions — simply at rest, not necessarily fasted. RMR is typically 10–20% higher than BMR due to the thermic effect of recent meals and activity. In practice, RMR is more commonly measured clinically and the terms are often used interchangeably.

Kleiber's law states that BMR scales with body mass to the power of 3/4: BMR ∝ M^0.75. This means metabolic rate does not scale linearly with mass — doubling body mass increases BMR by only 1.68-fold. Mass-specific metabolic rate (kcal/kg/day) therefore decreases with increasing body size. A 20g mouse has a mass-specific metabolic rate ~7× higher than a 70kg human. This scaling governs heart rate, lifespan, pharmacokinetics, and dosing calculations across mammalian species.