BMR Calculator
Calculate your Basal Metabolic Rate using three validated formulas. See how Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle compare — and understand which is most accurate for your situation.
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Tips & Notes
- ✓BMR is only the foundation — multiply it by your activity factor (1.2 to 1.9) to get TDEE, which is your actual daily calorie need. Never eat at BMR level as your daily target.
- ✓If you know your body fat percentage from a DEXA scan or accurate skinfold test, use the Katch-McArdle result — it is more accurate than Mifflin for athletes and anyone with above-average muscle mass.
- ✓BMR declines approximately 1–2% per decade after age 30 due to muscle mass loss. At age 60, a person who has not trained has a BMR roughly 15–20% lower than at age 30 for the same body weight.
- ✓Suspected very low metabolism despite eating little may indicate thyroid dysfunction — hypothyroidism can reduce BMR by 200–500 kcal/day. A simple TSH blood test rules this out.
- ✓The three formulas here typically agree within 100–150 kcal. If they diverge significantly (more than 200 kcal), double-check your inputs — small errors in weight or height have an outsized effect.
Common Mistakes
- ✗Using BMR as the daily calorie target — BMR is the at-rest minimum; a normal day of movement and digestion burns 400–1,000 kcal above BMR, so eating at BMR creates a very large unintended deficit.
- ✗Applying the Mifflin-St Jeor formula to elite athletes, who may have significantly higher lean mass than the formula assumes — Katch-McArdle with a measured body fat percentage gives a more accurate result.
- ✗Treating the three formula results as equally valid for your situation — Mifflin is best for most adults, Harris-Benedict tends to slightly overestimate, and Katch-McArdle is most accurate when body fat % is verified.
- ✗Not accounting for age-related BMR decline when calorie targets are set years apart — a target accurate at age 30 will likely need downward adjustment at age 45 even at the same body weight.
- ✗Assuming low BMR is the reason for weight gain without verifying — most people who believe they have a low metabolism actually have higher TDEE than they realize; the issue is calorie intake tracking, not metabolism.
BMR Calculator Overview
BMR is the foundation of every calorie-based nutrition plan. Getting this number right — and understanding its limitations — determines whether your targets are realistic or destined to fail.
Mifflin-St Jeor formula (current gold standard):
Mifflin-St Jeor (most accurate for most adults — validated in 2005 research): Male: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5 Female: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161 Harris-Benedict (Revised 1984 — still widely used): Male: BMR = 88.362 + (13.397 × weight) + (4.799 × height) − (5.677 × age) Female: BMR = 447.593 + (9.247 × weight) + (3.098 × height) − (4.330 × age)
EX: Male, age 38, 84 kg, 179 cm Mifflin: BMR = (10 × 84) + (6.25 × 179) − (5 × 38) + 5 = 840 + 1,118.75 − 190 + 5 = 1,774 kcal/day Harris: BMR = 88.362 + (13.397 × 84) + (4.799 × 179) − (5.677 × 38) = 88.362 + 1,125.35 + 859.02 − 215.73 = 1,857 kcal/day Difference: 83 kcal — both are useful estimates; Mifflin tends to be closer for most people.
Harris-Benedict and Katch-McArdle formulas:
Katch-McArdle (most accurate when body fat % is known — ideal for athletes): BMR = 370 + (21.6 × Lean Body Mass in kg) LBM = Total weight × (1 − Body fat % / 100)
EX: Same male (84 kg) with 18% measured body fat LBM = 84 × (1 − 0.18) = 84 × 0.82 = 68.9 kg Katch-McArdle BMR = 370 + (21.6 × 68.9) = 370 + 1,488.2 = 1,858 kcal/day Why this matters: if this person were actually 14% body fat (more muscle), LBM = 72.2 kg Katch BMR = 370 + (21.6 × 72.2) = 370 + 1,559.5 = 1,930 kcal/day — 72 kcal higher. Mifflin cannot detect this difference because it does not account for body composition.
BMR formula comparison — when each is most accurate:
| Factor | Effect on BMR | Magnitude |
|---|---|---|
| 1 kg of muscle gained | Increases BMR | +13–17 kcal/day |
| 1 kg of fat gained | Increases BMR slightly | +4–5 kcal/day |
| Each decade of age (after 30) | Decreases BMR | −50–100 kcal/day |
| Hypothyroidism (untreated) | Decreases BMR | −200–500 kcal/day |
| Hyperthyroidism | Increases BMR | +200–500 kcal/day |
| Sustained calorie restriction | Decreases BMR (adaptation) | −100–300 kcal/day |
BMR reference values by age, sex, and body size:
| Formula | Best for | Average error vs indirect calorimetry |
|---|---|---|
| Mifflin-St Jeor | Most adults (normal to overweight BMI) | ±10% for 80% of people |
| Harris-Benedict (revised) | General use — slightly overestimates | ±12% |
| Katch-McArdle | Athletes and people with known body fat % | ±5% when BF% is accurate |
| Direct calorimetry | Clinical research settings | Reference standard |
BMR is not fixed — it is a dynamic number that responds to body composition changes, age, dietary restriction, and health conditions. The most meaningful use of BMR is establishing your calorie floor (the minimum safe intake) and building upward from there based on your actual activity level. Eating at BMR while living an active life creates a hidden deficit that often produces faster initial weight loss followed by frustrating plateaus — because the body reduces energy expenditure to match the lower intake.
Frequently Asked Questions
For most adults, Mifflin-St Jeor is the recommended starting point. A 2005 comparison study found Mifflin to be the most accurate for normal-weight and overweight adults, predicting BMR within 10% for approximately 80% of people tested. Harris-Benedict (revised 1984 version) tends to overestimate slightly by 50–100 kcal on average. Katch-McArdle is more accurate than either for people who know their body fat percentage from a reliable method (DEXA, skinfold) — particularly athletes and individuals with muscle mass significantly above average. If you have a verified body fat percentage, use Katch-McArdle; otherwise, use Mifflin-St Jeor.
True metabolic disorders are real but rarer than most people believe. Hypothyroidism is the most common legitimate cause of significantly reduced BMR — an underactive thyroid can reduce metabolic rate by 200–500 kcal/day. Cushing syndrome, certain pituitary conditions, and some medications also reduce BMR. However, research consistently shows that when food intake and activity are accurately measured, most people who report "slow metabolism" are actually underreporting food intake and overestimating activity. Indirect calorimetry testing at a medical facility is the only way to definitively measure actual BMR and determine whether it falls outside the normal range for your body composition.
Skeletal muscle burns approximately 13–17 kcal per kg per day at rest, compared to 4–5 kcal per kg for fat tissue. This means gaining 5 kg of muscle adds roughly 65–85 kcal to daily BMR — significant over a year, but not the dramatic metabolism-boosting effect often claimed in fitness culture. The difference between someone with very high muscle mass and average muscle mass of the same body weight is typically 100–200 kcal/day in BMR. The bigger impact of muscle mass is on active calorie burn — more muscle means more power output and more calories burned during exercise and daily activity.
Formula-based BMR calculations have a ±10–15% margin of error for individuals. For a calculated BMR of 1,700 kcal, the actual measured BMR could be anywhere from 1,445 to 1,955 kcal. Factors that cause individual variation include thyroid function (the single biggest variable), genetic differences in mitochondrial efficiency, gut microbiome composition, body temperature regulation, and the proportion of different organ types (the liver and brain are metabolically expensive; they are disproportionately high contributors to BMR despite being a small fraction of body weight). Indirect calorimetry — measuring actual oxygen consumption and CO2 production — is the only way to get a truly accurate individual BMR.
Yes — this is called metabolic adaptation or adaptive thermogenesis. When you sustain a calorie deficit, BMR decreases beyond what the reduction in body weight alone would predict. Research shows adaptive thermogenesis typically reduces BMR by an additional 100–300 kcal/day after 8–12 weeks of sustained restriction, even when controlling for changes in body composition. The mechanisms include reduced thyroid hormone output, lower leptin levels, decreased sympathetic nervous system activity, and more efficient muscle contraction. Planned diet breaks of 1–2 weeks at maintenance calories can partially reverse these adaptations. This is why very long continuous cuts are less efficient than structured phases of cutting and maintenance.
Yes, but the effect is more modest than commonly believed and takes time. Resistance training is the most effective method — it builds lean mass, which directly increases BMR at a rate of 13–17 kcal per kg of muscle gained. Adding 5 kg of muscle through sustained training increases BMR by roughly 65–85 kcal/day. The larger benefit of training for metabolism is through TDEE increase during and after sessions, not through BMR changes. High-intensity exercise also produces a modest EPOC (excess post-exercise oxygen consumption) effect — elevated calorie burn for 12–24 hours after strenuous sessions — that adds to total daily expenditure beyond the session itself.