Basal Metabolic Rate,
to the calorie.

Field guide

What BMR really is and why three formulas exist.

Basal Metabolic Rate is the energy your body burns in 24 hours at complete rest. Not while sleeping, but while awake but motionless, after a 12-hour fast, in a thermoneutral room. It's the cost of staying alive: roughly 20% goes to your brain, 20% to your liver, 10% to your heart, and another 10% to your kidneys, organs that work constantly whether you do or not.

For most adults, BMR makes up 60–75% of total daily calorie burn. The rest is movement, exercise, and the thermic effect of digesting food. Knowing BMR is the foundation of every nutrition calculation that follows, from a maintenance calorie goal to a fat-loss plan to a clinical caloric prescription.

BMR vs. RMR: same number, different rigor

Strictly speaking, BMR requires laboratory conditions: 12-hour fast, thermoneutral room, supine, awake, no caffeine. RMR (Resting Metabolic Rate) is measured more loosely, sitting quietly, no overnight fast required and runs about 10% higher. The three predictive formulas below technically estimate RMR even though everyone calls them BMR equations. In practice, the distinction only matters in clinical research; the numbers on this page are appropriate for any non-clinical use.

The Mifflin-St Jeor equation (default)

Published in 1990 by Mifflin and colleagues, this is the modern reference. The Academy of Nutrition and Dietetics evaluated it against indirect calorimetry and concluded it is the most accurate predictive equation for the general adult population, within ±10% of measured RMR for ~80% of subjects.

Revised Harris-Benedict (1984)

The original Harris-Benedict equations were published in 1919 and were the standard for the next 70 years. They were updated in 1984 by Roza and Shizgal with modern data and corrected coefficients. Compared to Mifflin-St Jeor, Harris-Benedict tends to overestimate BMR by roughly 5%, particularly in people with higher body weight.

Katch-McArdle (when you know lean mass)

If you have a measured body-fat percentage (DEXA, BodPod, calipers, or a U.S. Navy estimate), Katch-McArdle beats both alternatives. The reason is simple: BMR scales with lean body mass, not total weight. A 90 kg lifter at 12% body fat and a 90 kg sedentary person at 30% body fat have roughly 16 kg different lean mass and roughly 350 kcal/day different BMRs. Mifflin and Harris can't see that difference. Katch can.

From BMR to TDEE: the activity multipliers

Multiply BMR by an activity factor to get Total Daily Energy Expenditure. These multipliers were published with the 1985 WHO/FAO/UNU report and have been used essentially unchanged since.

• Sedentary (×1.2): desk job, no exercise
• Lightly active (×1.375): light exercise 1–3 days/week
• Moderately active (×1.55): moderate exercise 3–5 days/week
• Very active (×1.725): hard exercise 6–7 days/week
• Extra active (×1.9): physical job + intense training

What actually drives BMR

Five factors explain almost the entire range of human BMR:

• Lean body mass: the dominant input. More muscle and organ mass means a higher BMR.
• Age: BMR drops roughly 1–2% per decade after 20, mainly because lean mass declines (sarcopenia).
• Sex: men average 5–10% higher BMR at the same height and weight, due to higher lean-mass ratios.
• Genetics: identical-twin studies put the heritable component at roughly 30–40%. Two people with identical stats can differ by 200 kcal/day from genetics alone.
• Thyroid hormone: even subclinical hypothyroidism can drop BMR by 100–200 kcal/day; treated hyperthyroidism the reverse.

What does not meaningfully affect BMR

Despite popular belief: green tea, spicy food, cold showers, and fasted cardio all change BMR by less than 5% transiently and have negligible effect on long-run daily burn. The only durable lever is changing lean mass.

Worked example

A 30-year-old male, 178 cm, 75 kg. Mifflin-St Jeor gives:

Per-hour, that's ~71.6 kcal, about 83 watts of continuous heat output, roughly the wattage of an old incandescent light bulb. At a moderately active lifestyle (×1.55), TDEE works out to ~2,662 kcal/day.

Disclaimer

Predictive BMR equations carry an individual error of roughly ±10–15%. This calculator is for educational use; medical and clinical decisions should rely on indirect calorimetry under professional supervision. If your tracked weight is drifting persistently in the wrong direction at the calorie target you derived from this number, the BMR estimate, not your willpower , most likely the imprecise variable.