Is this a Mifflin-St Jeor calculator?

Yes. This BMR tool uses the Mifflin-St Jeor equation, a widely used method for estimating resting calorie needs in adults.

BMR (Basal Metabolic Rate) is the number of calories your body burns at complete rest to support vital functions like breathing, blood circulation, and cell repair. It does not include calories burned through movement or exercise.

How is BMR calculated?

This tool uses the Mifflin-St Jeor equation. For men: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age + 5. For women: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age - 161.

What is the difference between BMR and TDEE?

BMR is your resting calorie requirement. TDEE includes BMR plus calories burned from activity and digestion. Use a TDEE Calculator when you want a maintenance calorie target.

Can I use BMR for weight loss planning?

BMR is a baseline, not a full daily target. Most people should plan calories using total daily energy expenditure. A Calorie Calculator can help convert your BMR into practical daily targets for loss, maintenance, or gain.

What is the difference between BMR and RMR?

BMR is measured under strict conditions: 12+ hours fasting, complete rest, thermoneutral environment. RMR (Resting Metabolic Rate) is measured under less strict conditions and is typically 3–10% higher. Clinically, they're often used interchangeably.

Why do men have higher BMR than women?

Men typically have more muscle mass and less body fat than women of the same weight. Muscle is metabolically active tissue, so more muscle means higher resting calorie burn. Testosterone also supports higher metabolic rates.

How does age affect BMR?

BMR declines 1–2% per decade after age 20, primarily due to loss of muscle mass (sarcopenia). A 50-year-old burns roughly 150–200 fewer calories at rest than they did at 20. Resistance training helps slow this decline.

What percentage of daily calories does BMR represent?

For most adults, BMR accounts for 60–75% of total daily energy expenditure. The remaining 15–30% comes from physical activity, and ~10% from the thermic effect of food (digestion).

Can I measure my actual BMR?

Yes, through indirect calorimetry — measuring oxygen consumption and CO2 production at rest. This requires specialized equipment and is typically available at hospitals or metabolic testing labs. Cost ranges from $100–300.

Why might my actual BMR differ from calculated BMR?

Genetics, thyroid function, body composition (muscle vs. fat), recent calorie restriction, sleep quality, and hormonal status all affect actual metabolic rate. BMR equations are population averages, not individual predictions.

What is metabolic adaptation?

When you eat below maintenance long-term, your body reduces BMR to conserve energy — reducing thyroid hormone, increasing efficiency of metabolic processes, and decreasing non-essential functions. This makes continued weight loss harder.

Can crash dieting permanently damage metabolism?

Not permanently, but adaptations can persist for months to years after severe calorie restriction. The body 'remembers' the deficit and may maintain a lower BMR. Gradual weight loss with adequate protein minimizes this effect.

Does eating more meals boost BMR?

No. Total calorie intake, not meal frequency, determines metabolic rate. The 'eat 6 small meals to boost metabolism' advice is a myth. What matters is total daily calories and macronutrient composition.

Do certain foods boost BMR?

Protein has a higher thermic effect (20–30% of calories burned during digestion) compared to carbs (5–10%) and fat (0–3%). High-protein diets modestly increase total energy expenditure. Caffeine and spicy foods have small, temporary effects.

How does thyroid function affect BMR?

Thyroid hormones regulate metabolic rate. Hypothyroidism can reduce BMR by 10–20%, causing weight gain and fatigue. Hyperthyroidism increases BMR, causing weight loss despite normal eating. Unexplained metabolic changes warrant thyroid testing.

Online Medical Tools — BMR Calculator

Printed on 8/14/2026

For informational purposes only. This is not medical advice.

Body Metrics

BMR Calculator

Basal Metabolic Rate (BMR) is the number of calories your body needs over 24 hours at complete rest to maintain essential functions such as breathing, circulation, and temperature regulation. This calculator uses the Mifflin-St Jeor equation, one of the most commonly used formulas for estimating resting energy expenditure in adults. To account for activity level, multiply BMR by activity factor using TDEE Calculator. Build a nutrition plan with Macro Calculator and Calorie Calculator. Monitor weight status with BMI Calculator and Body Fat Calculator.

Formula: Mifflin-St Jeor: BMR = 10 x weight + 6.25 x height - 5 x age + 5 (men) or -161 (women)

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Age

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Height

Weight

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How It Works

Enter your stats

Input your sex, age, height, and weight. These variables determine your baseline energy expenditure at complete rest.

Apply the Mifflin-St Jeor equation

The calculator uses this validated formula to estimate how many calories your body burns over 24 hours just to maintain vital functions — no movement included.

Use BMR for nutrition planning

Multiply your BMR by an activity factor to get total daily energy expenditure (TDEE). Use the TDEE Calculator for complete daily calorie needs.

Who Uses the BMR Calculator

Clinical Nutrition Assessment

Dietitians, physicians

Estimate resting energy expenditure as the foundation for calculating total calorie needs in hospital, outpatient, and weight management settings.

Weight Loss Planning

Dieters, nutrition coaches

Understand your metabolic baseline. Never eat below BMR for extended periods — doing so risks metabolic adaptation and muscle loss.

Muscle Building

Bodybuilders, athletes

Know your BMR to calculate appropriate surplus calories for lean mass gains. BMR increases as you add muscle, creating a positive metabolic feedback loop.

Metabolism Tracking

Fitness enthusiasts

Track how BMR changes as your weight and age change. Recalculate periodically to ensure your calorie targets remain appropriate.

Metabolic Adaptation Awareness

Chronic dieters

Understand why weight loss slows over time. BMR decreases as you lose weight — fewer calories are needed to maintain a smaller body.

Hospital Nutrition Support

Critical care teams

Estimate calorie needs for tube-fed or TPN patients. BMR-based calculations help prevent overfeeding and underfeeding in clinical settings.

Pro Tips

BMR is just the foundation

BMR represents calories burned at complete rest. Add activity calories using an activity factor (1.2–1.9) to get TDEE — your actual daily calorie needs.

Don't eat below your BMR long-term

Sustained calorie intake below BMR triggers adaptive thermogenesis — your body reduces metabolic rate to conserve energy. This makes further weight loss harder.

Muscle increases BMR

Muscle tissue burns more calories at rest than fat tissue. Building muscle through resistance training raises your BMR, making weight maintenance easier.

BMR decreases with age

Metabolic rate drops about 1–2% per decade after age 20, largely due to muscle loss. Maintaining muscle through exercise helps preserve BMR with age.

Men typically have higher BMR than women

At the same height and weight, men have higher BMR due to greater muscle mass and different hormonal profiles. The Mifflin-St Jeor equation accounts for this.

Accuracy varies by individual

BMR equations predict averages, not individuals. Your actual BMR may be 10–15% higher or lower than predicted. Track weight trends to calibrate.

Recalculate as weight changes

Every 10 kg of weight loss reduces BMR by roughly 100 kcal/day. Recalculate after significant weight changes to update your calorie targets.

Thyroid status affects BMR significantly

Hypothyroidism reduces BMR; hyperthyroidism increases it. If your actual metabolism seems very different from calculated, consider thyroid testing.

Sleep and stress affect metabolism

Chronic sleep deprivation and high cortisol can reduce metabolic rate. Optimizing sleep and managing stress supports healthy metabolism.

Mifflin-St Jeor is the gold standard

The Academy of Nutrition and Dietetics recommends Mifflin-St Jeor over older equations (Harris-Benedict) because it's more accurate for modern populations.

Common Questions About Your Results

Evidence-Based Methodology

The Mifflin-St Jeor equation (1990) is recommended by the Academy of Nutrition and Dietetics as the most accurate predictive equation for estimating resting metabolic rate in both non-obese and obese adults. It predicts RMR within ±10% for approximately 80% of adults, outperforming the older Harris-Benedict equation.

Clinical Content Trust

Last reviewed:: April 21, 2026
Guideline version:: General evidence framework v2026.04
Source set version:: Primary-source set v1

How to Interpret Your Result

Your BMR is your estimated resting calorie burn for 24 hours. It reflects the energy your body would use with no planned activity. Most adults have BMR values that account for the majority of daily calorie expenditure, but BMR alone is not a maintenance calorie target.

To plan nutrition in real life, convert BMR to total daily needs using activity. The TDEE Calculator applies activity multipliers to your BMR. If your goal is weight change, the Calorie Calculator and Macro Calculator are better next steps for creating practical daily targets.

When to Use This Tool

Use this tool when you want a baseline estimate of metabolism before setting diet targets. It is useful at the start of a nutrition plan, during body composition tracking, or when comparing changes over time as weight and age change.

In clinical and coaching settings, BMR is commonly used as an intermediate value before calculating total daily energy expenditure. It helps frame whether calorie intake is likely above or below baseline energy needs.

Limitations

BMR equations provide estimates, not direct measurements. Individual metabolism can differ due to genetics, thyroid status, body composition, medications, and adaptive changes from dieting. For some people, the true resting metabolic rate may differ meaningfully from the predicted value.

The equation does not account for lean body mass directly and is less precise at extremes of body composition or in certain medical conditions. Treat the result as a starting point and adjust using real weight trends over 2 to 3 weeks.

Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.

Changelog

April 21, 2026 · trust-baseline
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