BMR (Basal Metabolic Rate) Calculators

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Basal metabolic rate (BMR) is the minimum rate of energy expenditure required to maintain basic physiological functions at rest — breathing, circulation, maintaining body temperature, and cellular metabolism. BMR is measured under strict conditions (fasted, resting, thermoneutral) and expressed in kcal/day. It represents approximately 60–70% of total daily energy expenditure (TDEE) in sedentary individuals. The most accurate prediction equations are the Harris-Benedict (1919, revised 1984) and Mifflin-St Jeor (1990) equations, which use height, weight, age, and sex to estimate BMR.

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Mifflin-St Jeor Equation (Preferred)

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 (35-year-old woman, 65 kg, 165 cm): BMR = (10×65) + (6.25×165) − (5×35) − 161 = 650 + 1031.25 − 175 − 161 = 1345.25 kcal/day.

Total Daily Energy Expenditure (TDEE)

TDEE = BMR × Physical Activity Level (PAL): Sedentary (desk job, little exercise): × 1.2. Light activity (1–3 days/week): × 1.375. Moderate (3–5 days/week): × 1.55. Very active (6–7 days/week): × 1.725. Extra active (athlete, physical job): × 1.9.

Factors Affecting BMR

  • Body composition: Lean muscle mass has higher metabolic rate than fat; higher muscle mass → higher BMR
  • Age: BMR decreases ~2% per decade after age 20 (due to muscle mass loss)
  • Sex: Men typically have higher BMR (~5–10% higher) due to more lean mass
  • Thyroid hormones: Hypothyroidism significantly reduces BMR; hyperthyroidism increases it
  • Temperature: Fever increases BMR ~7% per 0.5°C rise; cold exposure increases BMR (thermogenesis)

Glossary

BMR (Basal Metabolic Rate)
Minimum energy expenditure to maintain vital functions at complete rest; Mifflin-St Jeor: men = 10W+6.25H−5A+5; women = 10W+6.25H−5A−161; represents 60–70% of TDEE.
TDEE (Total Daily Energy Expenditure)
BMR × physical activity factor (1.2–1.9); total energy expended in a day including exercise and NEAT; determines caloric requirements for weight maintenance, loss, or gain.
Mifflin-St Jeor Equation
The most accurate BMR prediction formula; uses weight (kg), height (cm), age (years), and sex; preferred over Harris-Benedict for clinical and nutrition applications.

Frequently Asked Questions

Basal Metabolic Rate (BMR) is the energy your body expends at complete rest to maintain essential functions. The most accurate prediction equation is Mifflin-St Jeor: Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(yr) + 5. Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(yr) − 161. Example (40-year-old man, 80 kg, 178 cm): BMR = (10×80) + (6.25×178) − (5×40) + 5 = 800 + 1112.5 − 200 + 5 = 1717.5 kcal/day. This is the minimum energy needed if lying still for 24 hours.

BMR (Basal Metabolic Rate): measured under strict conditions — fasted (12–14 hours), at complete rest, in a thermoneutral environment, awake. Very difficult to measure precisely. RMR (Resting Metabolic Rate): measured under less strict conditions (fasted 4–6 hours, resting); approximately 10% higher than BMR; what most equations actually estimate. TDEE (Total Daily Energy Expenditure): all energy expended in a day = RMR + physical activity + thermic effect of food (~10% of calories eaten) + NEAT (non-exercise activity thermogenesis — fidgeting, posture, walking). TDEE = BMR × activity factor (1.2 to 1.9). For weight management: if caloric intake < TDEE → weight loss; if intake > TDEE → weight gain.

BMR declines with age by approximately 1–2% per decade after age 20, primarily due to sarcopenia (age-related muscle loss). Muscle tissue is metabolically active (~13 kcal/kg/day for resting muscle), while fat tissue is less so (~4.5 kcal/kg/day). As muscle mass decreases with age and is replaced by fat, BMR falls. Additional factors: reduced activity of organs (kidney, liver); hormonal changes (declining sex hormones, growth hormone). Practical implications: if caloric intake remains constant while BMR decreases, weight gain occurs (the classic 'middle-age spread'). Resistance training can partially offset age-related BMR decline by preserving or building muscle mass. Protein intake optimization (1.2–1.6 g/kg/day) also helps preserve muscle in older adults.

Conditions that significantly affect BMR: Thyroid disorders: Hypothyroidism reduces BMR by 20–40% (weight gain, fatigue, cold intolerance); Hyperthyroidism increases BMR by 20–80% (weight loss, heat intolerance, tachycardia). Fever: each 0.5°C (1°F) rise increases BMR by ~7%; serious illness can raise BMR by 30–50% (ICU nutrition planning uses BMR × stress factor). Burns/trauma: major burns increase metabolic rate 40–100% above normal; nutrition support must match this increased need. Malnutrition/starvation: BMR adapts downward by 15–30% during severe calorie restriction (metabolic adaptation) — a major barrier to weight loss maintenance. ICU/hospital: indirect calorimetry (measuring VCO₂ and VO₂) is the most accurate BMR measurement tool used in critically ill patients to guide feeding.