BMR Calculator

Calculate your Basal Metabolic Rate using both the Mifflin-St Jeor and Harris-Benedict equations to see your resting calorie burn.

Results

Visualization

How It Works

Basal Metabolic Rate is the calorie cost of keeping you alive at complete rest in a thermoneutral environment after 8 hours of sleep and 12 hours of fasting. BMR accounts for 60-75% of total daily energy expenditure, with the brain alone consuming about 20% (roughly 300-400 kcal/day in adults). This calculator runs both Mifflin-St Jeor (Mifflin et al., Am J Clin Nutr 1990) and revised Harris-Benedict (Roza and Shizgal, 1984) formulas. The Academy of Nutrition and Dietetics evidence analysis (Frankenfield et al., J Am Diet Assoc 2005) found Mifflin-St Jeor predicted measured BMR within 10% in 82% of normal-weight adults and 70% of obese adults, outperforming Harris-Benedict (which overshoots by 5-15% in modern populations) and the WHO/FAO/UNU equations. The Katch-McArdle formula (BMR = 370 + 21.6 x LBM) outperforms both for very lean or muscular individuals because it uses lean body mass directly rather than total weight as a proxy.

The Formula

Mifflin-St Jeor: men BMR = 10W + 6.25H - 5A + 5; women BMR = 10W + 6.25H - 5A - 161. Revised Harris-Benedict: men BMR = 88.362 + 13.397W + 4.799H - 5.677A; women BMR = 447.593 + 9.247W + 3.098H - 4.330A. Katch-McArdle: BMR = 370 + 21.6 x LBM (kg). W=weight kg, H=height cm, A=age years, LBM=lean body mass kg.

Variables

  • W — Body weight in kilograms
  • H — Height in centimeters
  • A — Age in years
  • LBM — Lean body mass in kilograms (Katch-McArdle only)

Worked Example

Diego, 42, 178 cm, 84 kg male, sedentary office worker. Mifflin: BMR = (10 * 84) + (6.25 * 178) - (5 * 42) + 5 = 840 + 1112.5 - 210 + 5 = 1,747.5 kcal/day. Harris-Benedict: BMR = 88.362 + (13.397 * 84) + (4.799 * 178) - (5.677 * 42) = 88.362 + 1125.35 + 854.22 - 238.43 = 1,829.5 kcal/day. The two formulas differ by 82 kcal (4.7%). If Diego knows from a DEXA scan he has 22% body fat (LBM = 65.5 kg), Katch-McArdle: BMR = 370 + (21.6 * 65.5) = 370 + 1414.8 = 1,784.8 kcal/day. Indirect calorimetry would likely confirm a value between 1,700-1,850 kcal. The Mifflin estimate (1,748) sits closest to Katch-McArdle (1,785).

Methodology

BMR represents the minimum energy required to sustain life at complete rest. James Arthur Harris and Francis Gano Benedict published the original predictive equations in 1919 from indirect calorimetry of 239 subjects, and these served as the clinical standard for 70 years despite drift from changing body compositions. Roza and Shizgal revised the Harris-Benedict equations in 1984 using data from a more contemporary sample. Mifflin and St Jeor published their equations in 1990 based on indirect calorimetry of 498 subjects (251 men, 247 women, ages 19-78, BMI 17-42). The American Dietetic Association's 2005 evidence analysis (Frankenfield et al.) compared four predictive equations against measured RMR in 1,287 subjects and identified Mifflin-St Jeor as the most accurate. The Katch-McArdle formula, derived from lean body mass rather than total weight, better captures the metabolically active tissue component and outperforms anthropometric formulas in athletic and very obese populations where the LBM-to-total-weight ratio diverges from population norms. Indirect calorimetry remains the gold standard for measurement, calculating energy expenditure from oxygen consumption (VO2) and carbon dioxide production (VCO2) using the abbreviated Weir equation: BMR (kcal/day) = (3.94 x VO2 + 1.11 x VCO2) x 1440. Population standard deviation around predicted values runs 8-12%.

When to Use This Calculator

Hospital dietitians calculate BMR (and add stress factors of 1.2-1.5 for trauma, sepsis, or major surgery) to set parenteral and enteral nutrition delivery rates. Underfeeding ICU patients increases mortality; overfeeding causes hyperglycemia, hepatic steatosis, and CO2 retention complicating ventilator weaning. Endocrinologists order indirect calorimetry when suspected hypothyroidism or pituitary disease produces clinical signs (cold intolerance, weight gain, fatigue) but borderline thyroid panels; a measured BMR 25%+ below predicted suggests treatment-relevant hypothyroidism. Bariatric surgery teams calculate BMR pre- and post-operatively to monitor for excessive metabolic adaptation, which signals inadequate intake or extreme rapid weight loss. Olympic and professional athletes get measured BMR rather than estimated to design competition-period nutrition plans where 100-200 kcal precision matters for body composition and performance.

Common Mistakes to Avoid

Confusing BMR with TDEE leads people to eat at BMR thinking it covers daily needs; this creates a 25-90% deficit and triggers metabolic adaptation. Using Harris-Benedict in 2026 overshoots BMR by 5-15% versus measured values; switch to Mifflin-St Jeor for adults outside critical care. Treating BMR as fixed across a weight loss phase ignores the predictable decline as fat-free mass falls; recalculate every 4-5 kg of weight change. Failing to factor in thyroid medications, stimulants, beta-blockers, or corticosteroids when interpreting BMR estimates can introduce 100-300 kcal errors. Using a body weight measured after a salty meal or strenuous workout produces a temporarily inflated weight that overstates BMR by 30-80 kcal; weigh in the morning, fasted, after using the bathroom.

Practical Tips

  • Use Katch-McArdle if you know your body fat percentage from DEXA, Bod Pod, or hydrostatic weighing. It's more accurate than Mifflin or Harris-Benedict for people outside the 12-30% body fat range, where weight-based formulas drift.
  • Each kilogram of skeletal muscle adds about 13 kcal/day to BMR; each kilogram of fat adds about 4.5 kcal/day. Putting on 4 kg of muscle through 1-2 years of consistent training raises BMR by roughly 50 kcal/day, modest but compounding.
  • Severe caloric restriction (below 1,000 kcal/day for women, 1,200 for men) drops BMR 15-25% within 4-8 weeks via reduced T3 thyroid hormone and lower sympathetic tone. Recovery takes months. The Minnesota Starvation Experiment (Keys et al., 1950) documented 40% BMR reductions in semi-starvation.
  • Hypothyroidism lowers BMR by 15-40%; hyperthyroidism raises it 25-50%. If your measured BMR diverges sharply from formula estimates, get a TSH/free T4 panel before assuming the formula is wrong.
  • BMR drops about 0.5-1.0% per year between ages 60-90, mostly from organ mass and lean tissue loss (Pontzer et al., Science 2021). Before 60, total energy expenditure stays remarkably stable when controlled for body size.
  • Pregnancy raises BMR by 8-15% in the second trimester and 20-25% in the third (Lof et al., Br J Nutr 2005). Add roughly 340 kcal/day in the second trimester and 450 kcal/day in the third to standard formula estimates.

Frequently Asked Questions

Which BMR formula should I use?

Mifflin-St Jeor for general use. The Academy of Nutrition and Dietetics 2005 evidence analysis identified it as the most accurate for healthy and overweight adults. Use Katch-McArdle if you have a recent DEXA or hydrostatic weighing body fat percentage; it's more accurate for very lean (under 10% men, under 18% women) or very muscular populations. Avoid Harris-Benedict; it consistently overestimates BMR by 5-15% in modern populations because the original 1919 sample had different body compositions.

Is BMR the same as resting metabolic rate (RMR)?

No. BMR is measured under strict laboratory conditions: 8 hours of sleep, 12 hours fasted, supine in a thermoneutral room, no movement. RMR is measured with relaxed conditions (subject seated, recently woken, perhaps having eaten 3-4 hours prior) and runs 5-10% higher. Most online calculators and clinical tools report RMR but call it BMR. The difference is small enough that the labels are used interchangeably outside research settings.

Can I raise my BMR?

Yes, but modestly. Resistance training 3-4x per week and 1.6+ g/kg protein can add 2-4 kg of muscle over 12-18 months in untrained adults, raising BMR by 25-50 kcal/day. Cold exposure activates brown adipose tissue and adds 50-200 kcal/day in cold-acclimated subjects. Adequate sleep (7-9 hours) prevents the 5-10% BMR drop that comes with chronic sleep restriction (Spaeth et al., Sleep 2013). The lever that matters most isn't BMR; it's NEAT and structured exercise, which together can move TDEE by 500-1,000 kcal/day.

Why does my measured BMR differ from formula estimates?

Population standard deviation around predicted BMR is roughly 8-12%, so individuals can fall 150-300 kcal in either direction. Causes include body composition extremes (very lean or very obese), thyroid status (hypothyroidism lowers BMR 15-40%, hyperthyroidism raises it 25-50%), genetic variation in mitochondrial uncoupling proteins, and muscle fiber type distribution. Indirect calorimetry in a metabolic ward gives the actual number; expect to pay $100-300 at a clinic offering RMR testing.

Does BMR drop with age?

Yes, but mostly after age 60. The 2021 Science paper by Pontzer et al. analyzing doubly-labeled water data from 6,400 people found total energy expenditure adjusted for fat-free mass stays remarkably stable from age 20 to 60, then declines at about 0.7% per year. The popular 1-2% per decade figure for younger adults reflects sarcopenic muscle loss in inactive populations rather than an intrinsic metabolic slowdown. Trained adults at 65 can have BMRs equal to sedentary adults at 30.

What's the lowest safe BMR-based intake?

Don't sustain intake below your BMR for more than 4-6 weeks. Eating below BMR for 8+ weeks triggers metabolic adaptation: T3 thyroid hormone drops, leptin falls, sympathetic tone reduces, and BMR can decline 15-25%. The Minnesota Starvation Experiment showed BMR reductions of 40% at 50% of maintenance intake. Very-low-calorie diets (800 kcal/day) used for medically supervised obesity treatment require physician monitoring of electrolytes, blood pressure, and gallbladder function.

Do thyroid medications affect BMR calculations?

Yes, significantly. Levothyroxine treatment for hypothyroidism normalizes BMR back to formula estimates over 6-12 weeks once TSH is in range. Over-replacement (suppressed TSH from too high a dose) raises BMR 5-15% above formula values. Beta-blockers like metoprolol lower BMR by 4-8% by reducing sympathetic activation. Stimulant medications including amphetamines, methylphenidate, and high-dose caffeine raise BMR 5-12%. If you're on these medications, expect predicted BMR to be off by 100-200 kcal/day.

Sources

Last updated: May 04, 2026 · Last reviewed: May 2026 — NutritionCalcs Editorial Team · About our methodology

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