What is BMR and how is it calculated?
BMR (Basal Metabolic Rate) is the number of calories your body burns at complete rest to maintain basic functions like breathing and circulation. The Mifflin-St Jeor equation is most accurate: Men: 10×weight(kg) + 6.25×height(cm) − 5×age − 5. Women: same formula + 161 instead of −5.
Formula & Methodology
Mifflin-St Jeor (1990) — Gold Standard
Men: BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age) + 5 Women: BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age) − 161 Validated against measured resting metabolic rate in 498 individuals. The American Dietetic Association recommends this as the preferred formula for estimating BMR in healthy adults. Mean error vs. indirect calorimetry: ±10%.
Harris-Benedict (Revised, 1984)
Men: BMR = 88.362 + (13.397 × weight_kg) + (4.799 × height_cm) − (5.677 × age) Women: BMR = 447.593 + (9.247 × weight_kg) + (3.098 × height_cm) − (4.330 × age) A revision of the original 1919 equations by Roza and Shizgal. Tends to overestimate BMR by 5–8% in sedentary individuals compared to Mifflin-St Jeor, but remains widely used in clinical nutrition and athletic contexts.
Katch-McArdle — Body Composition Based
BMR = 370 + (21.6 × Lean Body Mass in kg) LBM = Body Weight × (1 − Body Fat%) The only widely-used formula that accounts for individual body composition. Requires body fat percentage as input. Since fat tissue has very low metabolic activity, using lean body mass eliminates the influence of adiposity — making it the most accurate formula for lean athletes and obese individuals.
Oxford Equations (2005) — Age-Stratified
Men (18–30): BMR = (15.057 × weight_kg) + 692.2 Women (18–30): BMR = (14.818 × weight_kg) + 486.6 Derived from a large multi-national dataset of over 10,000 subjects. Oxford equations provide age-stratified constants — different coefficients for each decade — making them particularly valuable for adults over 60 where younger-population formulas may be less accurate.
Key Terms
- BMR (Basal Metabolic Rate)
- The minimum calories required to maintain basic physiological functions — breathing, circulation, cell production, and temperature regulation — at complete rest in a thermoneutral environment after 12 hours of fasting. BMR accounts for roughly 60–75% of total daily energy expenditure.
- RMR (Resting Metabolic Rate)
- Similar to BMR but measured under less strict conditions — no 12-hour fast required. RMR is typically 10–20% higher than true BMR and is the value measured in most clinical metabolic tests. The two terms are frequently used interchangeably in practical nutrition contexts.
- TDEE (Total Daily Energy Expenditure)
- BMR multiplied by an activity factor to estimate total daily calorie burn. Common multipliers: Sedentary ×1.2, Light Activity ×1.375, Moderate ×1.55, Very Active ×1.725, Extra Active ×1.9. TDEE is the baseline for designing a calorie deficit or surplus.
- Lean Body Mass (LBM)
- Total body weight minus fat mass. Includes muscle, bone, organs, and body water. Because metabolically active tissue is almost exclusively lean mass, LBM is a better predictor of BMR than total body weight — especially in individuals with high or very low body fat percentages.
- Metabolic Age
- The chronological age at which the average healthy person has the same BMR as yours. A metabolic age below your actual age indicates higher metabolic efficiency (more lean mass, better fitness). Useful as a fitness benchmark rather than a clinical diagnostic tool.
- Adaptive Thermogenesis
- The involuntary reduction in BMR that occurs during prolonged calorie restriction, beyond what is predicted by weight loss alone. Commonly called "metabolic adaptation." Diet breaks and refeeds can partially reverse this response and restore metabolic rate.
Worked Examples
Example 1: Male, Age 35 — Mifflin-St Jeor
Inputs: Male, age 35, weight = 85 kg, height = 180 cm.
BMR: (10 × 85) + (6.25 × 180) − (5 × 35) + 5 = 850 + 1,125 − 175 + 5 = 1,805 kcal/day.
TDEE (Moderate Activity ×1.55): 1,805 × 1.55 = 2,798 kcal/day. A 500 kcal daily deficit targets ~0.5 kg/week fat loss at a 2,300 kcal daily intake.
Example 2: Female, Age 28 — Mifflin-St Jeor
Inputs: Female, age 28, weight = 62 kg, height = 163 cm.
BMR: (10 × 62) + (6.25 × 163) − (5 × 28) − 161 = 620 + 1,018.75 − 140 − 161 = 1,337.75 kcal/day.
TDEE (Light Activity ×1.375): 1,338 × 1.375 = 1,840 kcal/day. A 300 kcal deficit produces ~0.27 kg/week fat loss — a sustainable, muscle-preserving rate.
Example 3: Lean Athlete — Katch-McArdle
Inputs: Male, 90 kg, body fat percentage = 12%.
LBM: 90 × (1 − 0.12) = 79.2 kg lean mass.
BMR (Katch-McArdle): 370 + (21.6 × 79.2) = 370 + 1,710.72 = 2,081 kcal/day. Mifflin-St Jeor estimates ~2,040 kcal — close at 12% body fat, but the gap widens significantly at extreme body compositions (very lean or obese).
BMR Formula Comparison
| Formula | Year | Requires Body Fat % | Accuracy vs. Calorimetry | Best For |
|---|---|---|---|---|
| Mifflin-St Jeor | 1990 | No | ±10% (best overall) | General healthy adult population |
| Harris-Benedict (Rev.) | 1984 | No | ±10–15% (overestimates) | Clinical nutrition, legacy comparison |
| Katch-McArdle | 1996 | Yes (required) | ±7% for athletes | Lean athletes, significantly obese individuals |
| Oxford 2005 | 2005 | No | ±10% (age-stratified) | Adults over 60, non-Western populations |
What Is BMR and How Should You Use It?
Your Basal Metabolic Rate is the foundation of all calorie planning. It represents the energy your body needs just to exist — to keep your heart beating, lungs breathing, and cells functioning — with zero physical activity. Understanding BMR is the critical first step to any effective diet or training program because all calorie targets are derived from it.
BMR vs. TDEE: The Key Distinction
BMR is what you burn at complete rest. TDEE (Total Daily Energy Expenditure) is what you burn across a full day, including movement, exercise, and the energy cost of digesting food. TDEE is typically 20–90% higher than BMR depending on activity level. The relationship: TDEE = BMR × Activity Multiplier. Eating at TDEE maintains your current weight. Eating below creates a deficit for fat loss; eating above creates a surplus for muscle gain. Never design a diet around BMR alone — always use TDEE.
Which Formula Should You Use?
For most healthy adults, Mifflin-St Jeor is the most accurate starting point — developed and validated on a modern Western population, it consistently outperforms the original Harris-Benedict in head-to-head studies. If you know your body fat percentage, the Katch-McArdle formula is more precise because it removes the influence of metabolically inert fat tissue. Use Katch-McArdle if you are a lean athlete or significantly overweight, as these populations deviate most from average-body-composition assumptions in other formulas.
Understanding the Limitations
BMR formulas are population averages with a mean error of ±10%. Individual genetics, thyroid function, hormonal status, and certain medications can cause your true BMR to deviate significantly from any estimate. Treat your calculated BMR as a starting hypothesis, not a definitive measurement. Track your actual weight change for 2–3 weeks at a TDEE-based calorie target, then adjust intake based on real-world results. Expect to refine your calorie targets 2–3 times before finding your precise maintenance level.