About the BMR Calculator

Basal Metabolic Rate (BMR) is the energy your body expends at complete rest, in a thermo-neutral environment, after a 12-hour fast — the calories needed to keep your heart beating, lungs breathing, kidneys filtering, and brain firing when you do nothing else. BMR is the largest single component of daily energy expenditure, typically 60-75% of total.

This calculator uses the Mifflin-St Jeor equation, published by Mifflin MD and colleagues in the American Journal of Clinical Nutrition in 1990 (PubMed 2305711). The study compared four predictive equations against indirect calorimetry in 498 healthy adults and found Mifflin-St Jeor the most accurate, with a mean error of approximately 10% — closer than the older Harris-Benedict (1919) or Owen (1986) equations.

The Academy of Nutrition and Dietetics recommended Mifflin-St Jeor as the preferred predictive equation in its 2009 position paper. Note that Mifflin-St Jeor predicts Resting Energy Expenditure (REE), not Total Daily Energy Expenditure (TDEE). To convert REE to TDEE, multiply by an activity factor (see our TDEE Calculator).

How It Works

The Mifflin-St Jeor equations:

Men:   REE = 10 × weight(kg) + 6.25 × height(cm) - 5 × age(y) + 5
Women: REE = 10 × weight(kg) + 6.25 × height(cm) - 5 × age(y) - 161

The constants come from the regression coefficients in the 1990 study. The +5 (men) and -161 (women) intercept terms reflect the gender dimorphism in lean mass at any given weight and height: men typically carry more metabolically active lean tissue per kg.

REE is the energy cost of staying alive at rest; it does not include the thermic effect of food (~10% of intake), exercise energy, or non-exercise activity thermogenesis (NEAT). To get TDEE, multiply by an activity factor of 1.2 (sedentary) up to 1.9 (extra active).

Inputs in imperial units are converted internally to kg and cm using the exact factors 0.453592 kg/lb and 2.54 cm/in before applying the formula.

Worked Examples

A 30-year-old man weighing 75 kg at 178 cm has 10×75 + 6.25×178 - 5×30 + 5 = 750 + 1112.5 - 150 + 5 = 1,717.5 kcal/day. Sedentary TDEE would be 1,717.5 × 1.2 = 2,061 kcal.

A 35-year-old woman weighing 62 kg at 165 cm has 10×62 + 6.25×165 - 5×35 - 161 = 620 + 1031.25 - 175 - 161 = 1,315 kcal/day. Lightly active TDEE is 1,315 × 1.375 = 1,808 kcal.

Compare with the older Harris-Benedict (1919), which typically overestimates REE by 5-15% in modern populations because it was derived from subjects who were leaner and more physically active than today’s average. Mifflin-St Jeor corrects this by reweighting the coefficients against a more representative sample.

A 25-year-old male endurance athlete at 70 kg and 180 cm has BMR 700 + 1125 - 125 + 5 = 1,705 kcal. With 6×/week training his Very Active TDEE reaches 1,705 × 1.725 = 2,941 kcal — the engine behind fuelling 60-mile bike rides.

When to Use This Tool

Estimate resting calorie burn before setting a fat-loss or muscle-gain target.
Compare against indirect-calorimetry readings from a registered dietitian’s clinic.
Compute TDEE (multiply by an activity factor) for nutrition planning.
Set the calorie floor for a hypocaloric diet — generally not below 1,200 kcal (women) or 1,500 kcal (men) without supervision.
Adjust estimated needs for ageing (BMR falls ~1-2% per decade after 20).
Calculate REE for a parenteral-nutrition prescription starting point in hospitalised patients.
Teach energy balance and metabolism in a nutrition or kinesiology classroom.

Limitations & Disclaimer

BMR equations estimate resting energy expenditure with ~10% mean error in healthy adults; accuracy falls in elderly, very lean, obese, or pregnant individuals. The formula does not account for thyroid status, fever, burns, recent surgery, or medications that alter metabolism. This tool is not medical advice and is not a substitute for indirect calorimetry or a registered dietitian’s assessment. See our disclaimer for full terms.

Frequently Asked Questions

What is the difference between BMR and RMR?

BMR is measured under strict conditions (12-hour fast, thermo-neutral room, immediate on waking). RMR (resting metabolic rate) is measured under less stringent conditions and is typically 5-10% higher. Mifflin-St Jeor actually predicts RMR/REE; many sources use BMR and RMR interchangeably.

Is Mifflin-St Jeor accurate for everyone?

It has a mean error of ~10% in healthy adults but is less accurate in elderly, very lean, or obese populations, and during pregnancy. Clinical indirect calorimetry remains the gold standard; predictive equations are starting points to refine, not gospel.

Why does BMR drop as I age?

Lean mass declines ~3-8% per decade after age 30 (sarcopenia), and lean mass is the single largest driver of BMR. Resistance training 2-3 times per week preserves lean mass and can blunt the age-related BMR decline by 100-200 kcal/day.

Does the thermic effect of food count?

TEF is ~10% of total intake (protein ~25-30%, carbs ~5-10%, fat ~0-3%). It is not part of BMR but is part of TDEE. A 2,000-kcal diet with average composition yields ~200 kcal of TEF per day.

Can I use this for weight-loss targets?

Yes, but never drop below ~1,200 kcal/day (women) or ~1,500 kcal/day (men) without medical supervision. The NIH recommends deficits of 500-1,000 kcal/day for 0.5-1.0 kg/week loss. See our <a href="/tools/calorie-calculator.php">Calorie Calculator</a> for guided targets.

How does this compare to Harris-Benedict?

Harris-Benedict (1919) overestimates REE by 5-15% in modern populations because it was derived from leaner subjects. The 1984 revision partially corrected this. Mifflin-St Jeor (1990) is now preferred by the Academy of Nutrition and Dietetics.

Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team