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BMR Calculator

Calculate Basal Metabolic Rate (BMR) using the Mifflin-St Jeor equation — calories burned at complete rest. Includes TDEE calculation with activity multipliers and comparison with Harris-Benedict and Katch-McArdle equations.

Physiological Data

30
180 lbs

Basal Metabolic Rate

1,783
Calories / Day

Your BMR represents the number of calories your body burns at rest just to survive (breathing, circulating blood, cellular growth).

Note: This does not include calories burned from daily movement or exercise.

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What is Mifflin-St Jeor Basal Metabolic Equation?

BMR quantifies the minimum caloric energy required by the human body to sustain critical biological processes (breathing, circulation, cellular repair) in a state of complete physical rest.

Mathematical Foundation

Mifflin-St Jeor Equation (Male)
BMR=(10⋅W)+(6.25⋅H)−(5⋅A)+5BMR = (10 \cdot W) + (6.25 \cdot H) - (5 \cdot A) + 5
WW= Body weight in kilograms
HH= Standing height in centimeters
AA= Age in years
Mifflin-St Jeor Equation (Female)
BMR=(10⋅W)+(6.25⋅H)−(5⋅A)−161BMR = (10 \cdot W) + (6.25 \cdot H) - (5 \cdot A) - 161
WW= Body weight in kilograms
HH= Standing height in centimeters
AA= Age in years

Laws & Principles

  • Metabolic Aging: Each decade of life, the human metabolic rate naturally declines by approximately 2%. This reflects the gradual shift in body composition from metabolically active muscle fibers toward lower-activity adipose tissue, as well as declining hormonal stimulation.
  • BMR vs. TDEE: BMR is not your daily calorie target. It only reflects calories burned at absolute rest. Total Daily Energy Expenditure (TDEE) multiplies BMR by an activity factor (Sedentary = 1.2 to Very Active = 1.9) to account for real-world movement and exercise expenses.

Step-by-Step Example Walkthrough

" Computing the daily caloric floor for a 30-year-old male weighing 180 lbs (81.6 kg) standing 5'10" (177.8 cm). "

  1. Apply weight term: 10 × 81.6 = 816.
  2. Apply height term: 6.25 × 177.8 = 1,111.25.
  3. Apply age term: 5 × 30 = 150.
  4. Apply male constant: +5.
  5. Final BMR: 816 + 1,111.25 - 150 + 5 = 1,782 calories/day.
Final Result: 1,782 Cal/day — minimum energy required just to sustain biological function at rest.

What is Basal Metabolic Rate (BMR): Equations, TDEE & Metabolic Science?

Basal Metabolic Rate (BMR) is the number of calories your body requires at complete rest to maintain basic physiological functions: breathing, circulation, cell production, temperature regulation, and organ function. BMR accounts for 60–75% of total daily energy expenditure (TDEE) for most sedentary to lightly active individuals. It is distinct from Resting Metabolic Rate (RMR), which is measured under less strict conditions (RMR is typically 10–20% higher than BMR due to residual digestive activity). Three equations are commonly used to estimate BMR: Mifflin-St Jeor (most accurate for average adults, ±10% of measured BMR), Harris-Benedict Revised (1984 revision, ±15%), and Katch-McArdle (most accurate for lean individuals when body fat % is known).

Mathematical Foundation

Mifflin-St Jeor Equation (Recommended — Most Accurate for General Adults)
BMRmale=10W+6.25H−5A+5BMRfemale=10W+6.25H−5A−161\text{BMR}_{\text{male}} = 10W + 6.25H - 5A + 5 \qquad \text{BMR}_{\text{female}} = 10W + 6.25H - 5A - 161
WW= Weight in kilograms. Each additional kg of body weight adds 10 kcal/day to BMR.
HH= Height in centimeters. Each additional cm adds 6.25 kcal/day.
AA= Age in years. BMR decreases by 5 kcal/day per year of age due to gradual loss of metabolically active lean tissue.
+5/−161+5 / -161= Sex constant. Males: +5; Females: -161. The 166 kcal difference reflects average differences in lean mass, hormonal profile, and organ size between sexes at the same weight and height.
Katch-McArdle Equation (Best When Body Fat % Is Known)
BMR=370+21.6×LBM\text{BMR} = 370 + 21.6 \times \text{LBM}
LBM\text{LBM}= Lean Body Mass in kg = Total body weight × (1 − body fat fraction). Example: 80 kg at 20% body fat: LBM = 80 × 0.80 = 64 kg. BMR = 370 + 21.6 × 64 = 1,752 kcal. This equation has no sex term because sex differences in BMR are entirely explained by differences in lean mass.
21.621.6= Metabolic rate coefficient of lean tissue: approximately 21.6 kcal per kg of lean mass per day. Fat tissue contributes negligible metabolic rate (~4.5 kcal/kg/day). This is why body composition matters more than total weight for metabolic rate.
TDEE (Total Daily Energy Expenditure)
TDEE=BMR×Activity Factor\text{TDEE} = \text{BMR} \times \text{Activity Factor}
Activity Factor\text{Activity Factor}= Sedentary (desk job, no exercise): 1.2. Lightly active (1–3 days/week light exercise): 1.375. Moderately active (3–5 days/week): 1.55. Very active (6–7 days/week hard training): 1.725. Extremely active (2×/day training, physical job): 1.9. TDEE is the calorie level required to maintain current weight. Deficit of 500 kcal/day → ~0.5 kg/week loss. Surplus of 500 kcal/day → ~0.5 kg/week gain.

Laws & Principles

  • Accuracy comparison: Mifflin-St Jeor predicts measured BMR within ±10% for 82% of individuals in validation studies (Frankenfield et al., 2005). Harris-Benedict (1919 original) was developed from a small, unrepresentative sample and overestimates BMR by 5% in normal-weight and up to 20% in obese individuals. The 1984 Roza-Shizgal revision improved accuracy. Katch-McArdle is the most accurate when body fat % is accurately measured, as it eliminates the fat mass component entirely.
  • BMR declines with age: approximately 1–2% per decade after age 30, primarily due to sarcopenia (muscle loss, ~0.5–1% per year without resistance training). The age coefficient (-5A) captures this trend. Resistance training significantly counteracts age-related BMR decline by preserving lean mass.
  • Adaptive thermogenesis: When caloric intake drops significantly below TDEE (aggressive caloric restriction), BMR can fall by 10–25% beyond what is predicted by weight loss alone. This metabolic adaptation occurs within 2–4 weeks of severe restriction and persists for months to years ('metabolic damage' or 'starvation response'). It is the primary reason why individuals on very-low-calorie diets plateau sooner than equations predict and why slow, moderate deficits (~500 kcal/day) are more sustainable than aggressive restriction.
  • Thermic effect of food (TEF): Digesting food itself costs approximately 10% of caloric intake (protein 20–30%, carbohydrates 5–10%, fat 0–3%). TEF is NOT included in BMR. Full TDEE = BMR × activity factor + TEF. Some TDEE calculators include TEF implicitly within activity multipliers; others treat it separately.

Step-by-Step Example Walkthrough

" 35-year-old woman, 5‘6” (167.6 cm), 145 lbs (65.8 kg), moderately active (gym 4×/week). Calculate BMR and TDEE. "

  1. Mifflin-St Jeor: BMR = 10(65.8) + 6.25(167.6) - 5(35) - 161
  2. = 658 + 1,047.5 - 175 - 161 = 1,369.5 kcal/day
  3. TDEE = BMR × 1.55 (moderately active) = 1,369.5 × 1.55 = 2,123 kcal/day
  4. Fat loss target (-500 kcal/day): 2,123 - 500 = 1,623 kcal/day
  5. Muscle gain target (+250 kcal/day): 2,123 + 250 = 2,373 kcal/day
Final Result: BMR = 1,370 kcal/day; TDEE = 2,123 kcal/day. At maintenance intake, weight is stable. To lose 0.5 kg/week: eat 1,623 kcal/day (do NOT go below 1,200 kcal/day for women to avoid nutrient deficiency). Going below 1,200 kcal triggers adaptive thermogenesis and muscle catabolism, making sustainable loss harder.

Quick Answer: How do you calculate BMR and TDEE?

BMR (Mifflin-St Jeor, male) = 10W(kg) + 6.25H(cm) − 5A + 5. Female: same but −161 instead of +5. Then multiply by activity factor for TDEE: sedentary ×1.2, light ×1.375, moderate ×1.55, very active ×1.725. Example: 35-year-old woman, 5’6” (167.6 cm), 145 lbs (65.8 kg), moderately active: BMR = 10(65.8) + 6.25(167.6) − 5(35) − 161 = 1,370 kcal/day. TDEE = 1,370 × 1.55 = 2,123 kcal/day. Fat loss at −500 kcal = 1,623 kcal/day target.

TDEE Activity Multipliers & Calorie Targets

Total Daily Energy Expenditure = BMR × activity multiplier. Deficit of 500 kcal/day = ~0.5 kg/week loss. Surplus of 250–500 kcal/day = ~0.25–0.5 kg/week muscle gain (with training). Example TDEE values assume BMR of 1,800 kcal (avg adult male) or 1,400 kcal (avg adult female).

Activity Level Multiplier Description Example TDEE (M / F)
Sedentary1.20Desk job, no intentional exercise. Little to no walking.2,160 / 1,680
Lightly Active1.375Light exercise 1–3 days/week (walking, light gym, yoga).2,475 / 1,925
Moderately Active1.55Moderate exercise 3–5 days/week. Most gym-going adults.2,790 / 2,170
Very Active1.725Hard exercise 6–7 days/week or physical labor job.3,105 / 2,415
Extremely Active1.90Twice-daily training (athletes) + physical labor job. Rare.3,420 / 2,660
Most people overestimate their activity level. If unsure, use one level lower than you think. TDEE calculators typically have ±15–20% error; track weight for 2–3 weeks at a given intake to calibrate actual maintenance. 1 lb of fat = ~3,500 kcal; 1 kg of fat = ~7,700 kcal.

Pro Tips & Common BMR / Calorie Target Mistakes

Do This

  • ✓Use Katch-McArdle if you know your body fat percentage — it's more accurate for both lean athletes and people with high body fat. Mifflin-St Jeor uses total weight, which means a muscular 200 lb person and an obese 200 lb person get identical BMR estimates, despite the muscular person having dramatically higher actual metabolic rate. The Katch-McArdle formula eliminates fat mass from the calculation: BMR = 370 + 21.6 × LBM (kg). For accurate body fat input, use DEXA scanning (gold standard, ±1.5%), hydrostatic weighing (±1.5%), or 7-site skinfold calipers (±3%) rather than bioelectrical impedance (BIA, ±4–8%, highly variable with hydration status).
  • ✓Validate your TDEE empirically by tracking weight for 2–3 weeks at a known intake level. Equations have ±15–20% error for individuals. If eating 2,200 kcal/day and weight is stable, your TDEE is 2,200 — regardless of what the equation says. This empirical TDEE is more actionable than any formula output. Weight fluctuates 1–2 kg daily from water, food, and glycogen; use weekly average weights to filter this noise. Track for at least 3 weeks before adjusting intake targets.

Avoid This

  • ✗Don't drop calories below 1,200 kcal/day (women) or 1,500 kcal/day (men) without medical supervision. At very low intakes, adaptive thermogenesis reduces BMR by 10–25% beyond weight-loss-driven reduction. The body also reduces non-exercise activity thermogenesis (NEAT) — unconscious movement like fidgeting and posture changes — by up to 350 kcal/day. This can stall weight loss despite severe restriction. Below these thresholds, it becomes almost impossible to meet micronutrient requirements without supplementation, and muscle catabolism accelerates significantly as the body breaks down lean tissue for gluconeogenesis.
  • ✗Don't recalculate and update calorie targets weekly as you lose weight — wait for meaningful change. BMR formulas are sensitive to weight input: losing 5 kg (11 lbs) reduces Mifflin BMR by ~50 kcal/day. Recalculating every week based on a 1–2 lb fluctuation introduces noise. Recalculate BMR and TDEE every 5–10 lbs of actual fat loss (confirmed by trend, not single weigh-in), or every 8–12 weeks. Constant calorie target micro-adjustments create obsessive tracking behavior without meaningful accuracy benefit.

Frequently Asked Questions

What is the difference between BMR and RMR?

Basal Metabolic Rate (BMR) is measured under strict conditions: at complete physical rest (lying down), in a thermoneutral environment (22–26°C), in a post-absorptive state (12–14 hours after eating), and after at least 8 hours of sleep. These conditions ensure only truly basal metabolism is measured. Resting Metabolic Rate (RMR) is easier to measure: only 4–6 hours of fasting, no post-sleep requirement, and may include some residual thermic effect of food. RMR is typically 10–20% higher than BMR. Most clinical metabolic carts measure RMR, not BMR. The equations in this calculator (Mifflin-St Jeor, Harris-Benedict) were validated against indirect calorimetry measurements of RMR in many studies, so the output is more accurately an estimated RMR. The distinction matters mostly for research; for practical weight management, the difference is within the ±15–20% error range of any equation.

Why is the Mifflin-St Jeor equation preferred over Harris-Benedict?

The original Harris-Benedict equation was published in 1919 using data from 136 men and 103 women — a small, non-representative sample by modern standards. It tends to overestimate BMR by 5% in normal-weight individuals and up to 20% in obese individuals, because it was derived before the obesity epidemic changed the average body composition distribution. The Mifflin-St Jeor equation was published in 1990 using data from 498 subjects including obese individuals, and a 2005 systematic review by Frankenfield et al. found it predicted measured RMR within 10% for 82% of subjects — compared to 74% for the revised 1984 Harris-Benedict (Roza-Shizgal revision). The original 1919 Harris-Benedict is consistently less accurate than either replacement. If your software uses “Harris-Benedict,” check whether it's the 1919 or 1984 revision — they produce meaningfully different results, particularly for overweight and obese individuals.

How does muscle mass affect BMR and why does this matter for dieting?

Skeletal muscle is metabolically active tissue that consumes approximately 13 kcal/kg/day at rest (Elia, 1992). Fat tissue consumes only ~4.5 kcal/kg/day. The brain (~20%), liver (~21%), and heart (~9%) account for more than half of BMR despite being small organs, because they are extremely energy-intensive. Adding 1 kg of lean muscle through resistance training increases BMR by ~13 kcal/day — not dramatic on its own, but significant at scale: 5 kg of new muscle = +65 kcal/day = +2,000+ kcal/month. More importantly, resistance training preserves lean mass during caloric restriction. A diet that causes weight loss through calorie restriction without resistance training loses roughly 25–30% of weight as lean mass; with consistent resistance training, this drops to 10–15%. Preserving lean mass protects long-term BMR, making weight maintenance dramatically easier after the diet ends.

What is adaptive thermogenesis and how does it affect calorie calculations?

Adaptive thermogenesis is the body's metabolic response to sustained caloric restriction: BMR drops beyond what is predicted by weight or lean mass loss alone. A landmark study (Leibel et al., 1995) found that weight-reduced individuals had BMR 15% lower than predicted for their new weight — meaning the body actively resists weight loss by lowering metabolism. This effect is mediated primarily through reductions in thyroid hormone (T3), leptin, and sympathetic nervous system activity. Practical implications: (1) Weight loss plateaus earlier than equations predict. If your calculator says 1 lb/week loss at 1,800 kcal, expect a plateau within 6–12 weeks at the same intake. (2) The suppression is greater with more aggressive deficits. (3) “Diet breaks” (2–week periods of eating at maintenance) may partially reverse adaptive thermogenesis and improve long-term results (Byrne et al., 2018 MATADOR study). (4) The suppression partially persists after weight is regained, which is a key mechanism behind the “yo-yo dieting” pattern of progressively harder weight loss and easier regain.

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