BMR Calculator

What is BMR?

Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to sustain basic life functions — breathing, circulation, cell repair, temperature regulation, and organ function. It represents the minimum energy required to keep you alive if you were to do nothing but rest for 24 hours.

BMR typically accounts for 60–75% of total daily calorie expenditure, making it the largest component of your energy needs.

The Three BMR Formulas

1. Mifflin-St Jeor (1990) — Recommended Default

The most validated formula for most healthy adults. Recommended by the American Dietetic Association.

Men: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5

Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161

2. Harris-Benedict (Revised 1984)

The original BMR formula, revised by Roza and Shizgal in 1984. Slightly less accurate than Mifflin-St Jeor for modern populations.

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)

3. Katch-McArdle

Uses lean body mass instead of total weight, making it more accurate for muscular athletes or people who know their body fat percentage.

BMR = 370 + (21.6 × lean body mass in kg)

Where lean body mass = weight × (1 − body fat % / 100)

Which Formula Should I Use?

Situation	Recommended Formula
Average adult	Mifflin-St Jeor
Athlete / known body fat %	Katch-McArdle
Comparison reference	Harris-Benedict

From BMR to Daily Calories

BMR alone is your calories at complete rest. To find your actual daily calorie needs, multiply BMR by your activity factor:

Activity Level	Multiplier	Daily Calories (example: 1700 BMR)
Sedentary	1.20	2,040 kcal
Lightly active	1.375	2,338 kcal
Moderately active	1.55	2,635 kcal
Very active	1.725	2,933 kcal
Extra active	1.90	3,230 kcal

What Affects BMR?

• Muscle mass — more muscle = higher BMR. Resistance training is the most effective way to raise BMR.
• Age — BMR declines ~2% per decade after age 20 due to muscle loss (sarcopenia).
• Hormones — thyroid disorders, testosterone, and oestrogen significantly affect BMR.
• Body composition — fat-free mass is the primary driver of BMR differences between individuals.
• Genetics — some individuals have inherently higher or lower metabolic rates.

How BMR Is Measured in Clinical Settings

The gold standard for measuring actual BMR is indirect calorimetry — measuring the rate of oxygen consumption and carbon dioxide production while a person rests in a controlled environment. This test requires:

• 8+ hours of fasting beforehand
• 20+ minutes of complete rest before measurement
• Controlled room temperature (~25°C)
• Measurement of O₂ consumed and CO₂ produced via a metabolic cart and breathing mask

Indirect calorimetry gives an accurate BMR ±3% — far more precise than any predictive equation. However, it requires clinical equipment and is rarely performed outside research or hospital settings.

Predictive equations like Mifflin-St Jeor achieve ±10% accuracy for most people — acceptable for practical nutritional planning but not clinical diagnosis.

BMR and Metabolic Adaptation

One of the most important concepts for anyone who has dieted for extended periods is metabolic adaptation: the reduction in BMR that occurs in response to sustained calorie restriction. Studies using indirect calorimetry have found that after 24 weeks of dieting, BMR can fall by 15–25% beyond what is expected from the reduction in body mass alone.

This phenomenon — sometimes called "metabolic slowdown" or "starvation adaptation" — is why weight loss plateaus occur even when calorie intake remains constant. The body adapts by:

• Reducing thyroid hormone production (T3 levels fall)
• Decreasing sympathetic nervous system activity
• Reducing non-exercise activity thermogenesis (NEAT)
• Improving caloric extraction from food

Practical implication: If you have been in a calorie deficit for more than 12 weeks, your actual BMR may be 10–20% lower than our calculator predicts. Diet breaks (2–4 weeks at maintenance calories) can partially restore metabolic rate before resuming a deficit.

BMR, Lean Mass, and the Katch-McArdle Advantage

The Mifflin-St Jeor equation uses total body weight as its primary input, which means it implicitly averages across typical body composition for a given sex, age, and weight. For people who deviate significantly from average composition — either very lean (athletes) or with very high body fat — this introduces error.

The Katch-McArdle formula corrects for this by using lean body mass (LBM) directly. Since metabolically active tissue (muscle, organs) is what drives BMR — not fat — LBM is a better predictor of resting calorie burn. The formula:

BMR = 370 + (21.6 × LBM in kg)

To use it, you need your body fat percentage. If you've had a DEXA scan or use a reliable body composition scale, Katch-McArdle will typically be more accurate than Mifflin-St Jeor for athletes.

Example:

• 80 kg male, 12% body fat
• LBM = 80 × (1 − 0.12) = 70.4 kg
• Katch-McArdle BMR = 370 + (21.6 × 70.4) = 1,890 kcal
• Mifflin-St Jeor (5'10", 35 years) = 1,843 kcal (6% less — meaningful if you're trying to set precise calorie targets)

BMR Across Different Life Stages

Infants and children: BMR per unit of body mass is highest during early childhood due to rapid growth. A 5-year-old burns approximately 55 kcal/kg/day — nearly double the rate of an adult.

Adolescence (12–18): Peak BMR during puberty, particularly for males, driven by the anabolic effects of testosterone and growth hormone.

Young adulthood (18–25): BMR peaks and stabilises.

Middle age (30–60): BMR declines approximately 1–2% per decade, primarily due to gradual sarcopenia (muscle mass loss). This is preventable with consistent resistance training.

Older adults (65+): Accelerated decline in BMR, often 3–5% per decade, due to significantly increased sarcopenia. Research suggests that active older adults who maintain muscle mass have BMRs 15–25% higher than sedentary peers of the same age and body weight.

Raising Your BMR

Unlike many physiological variables, BMR is genuinely modifiable through lifestyle choices:

Resistance training: The most effective intervention. Skeletal muscle consumes 13 kcal/kg/day at rest (versus ~4.5 kcal/kg/day for fat tissue). Adding 5 kg of lean muscle raises BMR by approximately 65 kcal/day.

High-protein diet: The thermic effect of protein (25–30% of protein calories burned in digestion) raises daily calorie expenditure. Replacing 15% of calories from fat/carbs with protein can raise TDEE by 80–100 kcal/day.

Adequate sleep: Sleep deprivation reduces thyroid function and growth hormone output — both critical for maintaining BMR. Aim for 7–9 hours.

Avoiding extreme calorie restriction: Crash diets (below 1,000 kcal/day) trigger rapid metabolic adaptation and muscle loss — reducing BMR significantly. Moderate deficits (300–500 kcal below TDEE) produce slower but more sustainable fat loss while protecting BMR.

Related Resources

Related Calculators

• TDEE Calculator — Estimate your total daily energy expenditure based on activity.
• Calorie Calculator — Plan your daily calorie intake.

External Authority Resources

• Harris-Benedict Equation History on Wikipedia — Overview of the foundational metabolic rate equation.
• Journal of the Academy of Nutrition and Dietetics00327-0/fulltext) — Validation of BMR estimation equations.

Sources

• Mifflin MD et al. Am J Clin Nutr. 1990;51(2):241-7.
• Roza AM, Shizgal HM. The Harris Benedict equation reevaluated. Am J Clin Nutr. 1984;40(1):168-82.
• Katch VL, McArdle WD. Introduction to Nutrition, Exercise, and Health. 1993.
• Rosenbaum M, Leibel RL. Adaptive thermogenesis in humans. Int J Obes. 2010;34(Suppl 1):S47-55.