Two people — same height, same weight, same age, same sex. Peer-reviewed research confirmed they can differ by 200 to 400 calories per day in actual metabolic rate with no explainable reason. A 26% unexplained variance exists in BMR between individuals even after controlling for every measurable factor. This calculator shows your estimate with all three major formulas side by side, explains which one to use in your situation, and tells you when to stop trusting the math and start trusting your actual results.
✓Mifflin-St Jeor (1990) — Harris-Benedict revised (1984) — Katch-McArdle lean mass formula — NCBI validated
Calculate Your Basal Metabolic Rate
BMR (Mifflin-St Jeor)
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calories per day at complete rest
Formula Comparison
Mifflin-St Jeor (recommended)0
Harris-Benedict (revised 1984)0
Katch-McArdle (lean mass — most accurate for athletes)—
Difference (Mifflin vs Harris)0
TDEE (Maintenance)
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Weight Loss Target
0
Lean Mass (if BF% known)
—
Sedentary (×1.2)
0
Lightly Active (×1.375)
0
Moderately Active (×1.55)
0
Very Active (×1.725)
0
BMR estimates have ±10% accuracy for 70% of people (Mifflin-St Jeor). A 26% unexplained variance in BMR exists between individuals even at identical stats (Johnstone 2005). Thyroid conditions, prior dieting, and genetics can cause significant deviation. Use as a starting point — adjust by 100-150 calories based on actual weight change over 3-4 weeks. Not medical advice.
✓Mifflin-St Jeor (1990) — Johnstone et al. (2005) — NCBI peer-reviewed sources — May 2026
The Three BMR Formulas — Which One to Use and Why They Differ
Alex is 35 years old, 175 pounds, 5 feet 10 inches tall, male. The Mifflin-St Jeor formula gives him a BMR of 1,847 calories. The Harris-Benedict formula gives 1,942 calories. A difference of 95 calories per day — not huge, but across a year that is 34,675 calories — almost 10 pounds of fat if Alex used the wrong formula to set his deficit and consumed 95 extra calories daily. For athletes who know their body fat percentage, the Katch-McArdle formula cuts the error further because it directly uses the metabolically active lean tissue rather than inferring it.
Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle — Side by Side
Male, 35 years, 175 lbs (79.4 kg), 5'10" (177.8 cm):
Mifflin: (10×79.4) + (6.25×177.8) − (5×35) + 5 = 794 + 1,111.25 − 175 + 5 = 1,735 cal/day
Harris: 88.362 + (13.397×79.4) + (4.799×177.8) − (5.677×35)
= 88 + 1,064 + 853 − 199 = 1,806 cal/day
Difference: 71 calories/day (Mifflin is lower — more accurate for modern population)
With 15% body fat: Lean mass = 79.4 × 0.85 = 67.5 kg
Katch-McArdle: 370 + (21.6 × 67.5) = 370 + 1,458 = 1,828 cal/day
Mifflin-St Jeor predicts BMR within 10% for approximately 70% of people. Harris-Benedict is systematically higher for most modern adults — it was developed in 1919 on a much smaller, less diverse sample and tends to overestimate. The revised 1984 version is better but still trails Mifflin-St Jeor in accuracy when validated against indirect calorimetry (the gold standard). Use Mifflin-St Jeor unless you have accurate body fat data, in which case Katch-McArdle is the most accurate of the three.
When to Use Which Formula
Situation
Best Formula
Why
Accuracy
General adult, no body fat data
Mifflin-St Jeor
Best population-level validation, 1990 cohort
±10% for 70% of people
Athlete with known body fat %
Katch-McArdle
Uses lean mass directly — skips imprecise composition inference
Most accurate for athletes
Historical reference / research
Harris-Benedict
Most widely cited in older literature
Slightly lower than Mifflin
Obese individual (BMI 35+)
Mifflin-St Jeor
Harris-Benedict especially inaccurate at extremes
±10–15%
Thyroid condition
None reliably
Formula errors can reach 10–40% — use actual results
Unreliable
The 26% Variance, Age-Related Decline, and Why Your BMR May Be Wrong
The 26% Unexplained Variance — The Most Important BMR Fact Nobody Tells You
In 2005, researchers at the University of Aberdeen published a landmark meta-analysis in the American Journal of Clinical Nutrition. After controlling for fat-free mass, fat mass, age, sex, and circulating thyroid hormones — every measurable factor they could identify — 26% of the variation in BMR between individuals remained unexplained. Two people with genuinely identical stats can differ by 200 to 400 calories per day in actual metabolic rate, and science does not fully know why.
Suspected contributors include genetic differences in mitochondrial efficiency (how well cells convert fuel to energy versus heat), gut microbiome variation, and differences in sympathetic nervous system baseline activity. This is not a failure of the formula — it is a fundamental biological reality. The practical implication: treat your calculated BMR as a starting hypothesis, not a confirmed fact. If your weight does not respond as predicted over 3 to 4 weeks of consistent, accurately tracked eating, adjust the number rather than assuming the behavior is wrong.
Age and BMR — Why the Decline Is Muscle Loss, Not Just Aging
BMR declines approximately 1 to 2% per decade after age 30. The formula captures some of this through the age variable. But the mechanism matters enormously. The decline is driven primarily by muscle loss — adults lose 3 to 8% of muscle mass per decade without intervention — not by aging itself in isolation. Each pound of muscle burns approximately 6 to 7 calories per day at rest. A person who loses 20 pounds of muscle between age 30 and 60 burns 120 to 140 fewer calories per day at rest purely from that muscle loss.
This matters because it means the decline is largely preventable. Resistance training that maintains or builds lean mass directly preserves BMR. A 55-year-old who has maintained their muscle mass through consistent resistance training has a meaningfully higher BMR than a sedentary person of the same age and body weight — and the formula does not capture this distinction because it cannot see body composition without body fat input.
How to raise your BMR with resistance training: Every pound of muscle added raises BMR by approximately 6 to 7 calories per day. 10 pounds of new lean mass over a year of consistent training adds 60 to 70 calories per day to your BMR. Modest daily but meaningful over decades — and it compounds: more muscle means higher BMR means easier fat loss means leaner body composition means higher BMR. Progressive resistance exercise with adequate protein (0.7 to 1.0g per pound of lean mass) is the only evidence-based method for meaningfully and durably increasing BMR.
Thyroid Disease and BMR — The Common Reason the Math Is Wrong
Hypothyroidism reduces BMR by 10 to 40% below what any formula predicts. An underactive thyroid produces insufficient T3 and T4, downregulating virtually every metabolic process including basal calorie burning. If you have gained weight persistently despite controlled eating, feel constantly cold, experience fatigue disproportionate to your sleep, and have dry skin or hair loss — these are classic symptoms of hypothyroidism and your BMR calculator result is likely a significant overestimate.
Estimated prevalence of hypothyroidism in the US is 4.6% overt and up to 15% subclinical. Millions of people are running their metabolism at significantly below-formula levels without knowing it. A simple TSH blood test from any primary care physician confirms thyroid function. If thyroid disease explains your metabolic discrepancy, treating it restores BMR — sometimes dramatically.
BMR vs RMR — the term confusion most calculators ignore: True BMR requires measurement under highly restrictive laboratory conditions — completely fasted 12+ hours, thermally neutral environment, fully rested, lying still. Almost no online calculator measures true BMR. They calculate RMR (Resting Metabolic Rate), which is 10 to 20% higher than true BMR because the body is still processing food and recovering from activity. The formulas output numbers labeled BMR but technically calculate RMR. In practical nutrition planning, this distinction is minor — both are used as the same baseline floor. Just understand that your actual basal rate is slightly lower than the calculator shows.
Frequently Asked Questions
BMR is the calories your body burns per day at complete rest — heartbeat, breathing, organ function, cell repair. It represents 60 to 75% of total daily energy expenditure for sedentary people. Knowing your BMR sets the floor below which you should not eat without medical supervision. TDEE (Total Daily Energy Expenditure) is BMR multiplied by your activity factor, giving your maintenance calories.
Mifflin-St Jeor (1990) is most accurate for most adults — within 10% for 70% of people. Harris-Benedict (revised 1984) runs 50 to 100 calories higher and is slightly less accurate. Katch-McArdle is the most accurate if you know your body fat percentage, because it uses lean mass directly instead of inferring it. For athletes with known body composition, Katch-McArdle is the best choice.
A 2005 peer-reviewed meta-analysis confirmed a 26% unexplained variance in BMR between individuals even after controlling for all measurable factors. Two people with identical stats can differ by 200 to 400 calories per day. Additional reasons: thyroid conditions (10 to 40% reduction), metabolic adaptation from prior severe dieting, genetics, and gut microbiome differences. If your weight consistently does not respond as predicted, adjust the number based on actual results rather than assuming the formula is definitively correct for you.
BMR declines 1 to 2% per decade after 30, primarily through muscle loss (3 to 8% of muscle mass per decade). Each pound of muscle lost costs 6 to 7 calories per day of BMR. A person losing 20 pounds of muscle by age 60 burns 120 to 140 fewer calories per day at rest. Resistance training that maintains lean mass directly preserves BMR — the age decline is largely preventable through consistent progressive training and adequate protein intake.
Yes — primarily through building lean muscle mass. Each pound of new muscle adds 6 to 7 calories per day to BMR. Adding 10 pounds of lean mass over a year of resistance training raises BMR by 60 to 70 calories per day. Other supports: eating adequate protein (its 20 to 35% thermic effect effectively raises metabolic rate), avoiding severe calorie restriction that triggers adaptive downregulation, and optimizing thyroid function through adequate micronutrient intake.
Yes, significantly. Hypothyroidism can reduce actual BMR by 10 to 40% below formula predictions. Symptoms suggesting thyroid involvement: persistent weight gain despite controlled eating, constant fatigue, cold intolerance, dry skin, hair loss. Estimated 4.6% overt and up to 15% subclinical hypothyroidism prevalence in US adults. A simple TSH blood test from your doctor confirms thyroid function. If thyroid disease explains metabolic discrepancy, treatment can restore BMR substantially.
Step 1: Calculate BMR. Step 2: Multiply by activity factor to get TDEE. Step 3: For weight loss, subtract 250 to 500 from TDEE — never eat below BMR long-term. Step 4: Eat consistently at the target for 2 full weeks. Step 5: Adjust by 100 to 150 calories based on actual weight change. The calculation is a starting hypothesis — your body's actual response over 3 to 4 weeks is the real data.
BMR = 370 + (21.6 × lean mass in kg). Lean mass = total weight × (1 − body fat%/100). For a 180-pound person at 20% body fat: lean mass = 144 lbs = 65.3 kg. BMR = 370 + (21.6 × 65.3) = 1,780 calories. More accurate than Mifflin-St Jeor for athletes and lean individuals because it directly uses metabolically active lean tissue rather than inferring body composition from height, weight, age, and sex.
BMR: measured under strict laboratory conditions — fasted 12+ hours, thermally neutral, completely rested, lying still. RMR: measured under more relaxed conditions — rested but not necessarily fasted. RMR is 10 to 20% higher than true BMR. Online calculators calculate RMR but label it BMR — they are used interchangeably in nutrition practice. Practical difference is minor for calorie planning purposes.
Varies significantly with body size. A 30-year-old male at 5'10" and 196 pounds: approximately 1,900 to 2,000 calories. A 30-year-old female at 5'4" and 170 pounds: approximately 1,550 to 1,650 calories. Athletes with high lean mass run 15 to 25% higher. Small-framed individuals at healthy weight: 1,400 to 1,600 for women, 1,700 to 1,900 for men. There is no universal "normal" — these are population midpoints with substantial individual variation.
Metabolic adaptation is the involuntary reduction in BMR during sustained calorie restriction — beyond what weight loss alone explains. After 4 to 8 weeks of dieting, your body downregulates thyroid hormone, reduces NEAT, and improves metabolic efficiency — reducing actual BMR by 10 to 20% below formula predictions at your current weight. Partly reversible with diet breaks at maintenance for 1 to 2 weeks every 8 to 12 weeks. Prior significant dieting may mean your current BMR is lower than a fresh calculation suggests.
Mifflin-St Jeor is within 10% accuracy for approximately 70% of people — the best of the mainstream formulas. The remaining 30% may have significantly different actual metabolic rates. Error sources: thyroid conditions, unusual body composition for height and weight, significant prior dieting history, medications (beta-blockers reduce BMR, stimulants increase it), and the fundamental 26% unexplained inter-individual variance confirmed in peer-reviewed research. Use the result as a starting point and calibrate from actual results.