How to Calculate BMR (Basal Metabolic Rate)

Basal Metabolic Rate (BMR) is the number of calories your body burns per day to perform basic life-sustaining functions while at complete rest. Breathing, pumping blood, regulating temperature, repairing cells, and maintaining organ function all require energy. For most people, BMR accounts for 60-75% of their total daily calorie expenditure, making it the single largest component of energy use.

Understanding your BMR gives you the baseline for building a calorie plan. This guide compares the two most popular formulas, explains what makes your metabolic rate rise or fall, and shows how BMR connects to the number you actually use for meal planning: TDEE. For a quick calculation, use our BMR Calculator.

Medical Disclaimer: BMR calculations are estimates based on population averages. Individual metabolic rates vary due to genetics, medical conditions, hormonal status, and medications. Do not use these formulas to self-diagnose metabolic disorders. Consult a healthcare provider or registered dietitian for personalized guidance.

What BMR Actually Measures

BMR is measured under strict lab conditions: the person must be in a post-absorptive state (12 hours without food), lying down in a thermoneutral environment, fully awake but completely still. In clinical settings, indirect calorimetry measures oxygen consumption and carbon dioxide production to determine the exact energy cost of these resting processes.

Since most people can’t access a metabolic lab, predictive equations estimate BMR from easily measured variables: weight, height, age, and sex. These equations were derived from studies where researchers measured actual BMR in hundreds or thousands of subjects and then built regression formulas to predict it.

Resting Metabolic Rate (RMR) is a closely related term. RMR is measured under slightly less strict conditions (shorter fasting period, less rigid body position) and tends to run 3-10% higher than BMR. Many online calculators use BMR and RMR interchangeably, which is close enough for practical purposes.

The Mifflin-St Jeor Equation

Published in 1990 by Mifflin, St Jeor, and colleagues, this formula has consistently outperformed older equations in validation studies. The American Dietetic Association recommended it as the most reliable predictive equation in a 2005 evidence analysis.

For men:

$BMR = (10 \times weight_{kg}) + (6.25 \times height_{cm}) - (5 \times age) + 5$

For women:

$BMR = (10 \times weight_{kg}) + (6.25 \times height_{cm}) - (5 \times age) - 161$

Worked Example (Male): A 40-year-old man, 178 cm tall, weighing 88 kg.

• Weight: 10 x 88 = 880
• Height: 6.25 x 178 = 1,112.5
• Age: 5 x 40 = 200
• BMR = 880 + 1,112.5 - 200 + 5 = 1,797.5 calories/day

Worked Example (Female): A 32-year-old woman, 163 cm tall, weighing 60 kg.

• Weight: 10 x 60 = 600
• Height: 6.25 x 163 = 1,018.75
• Age: 5 x 32 = 160
• BMR = 600 + 1,018.75 - 160 - 161 = 1,297.75 calories/day

The Harris-Benedict Equation

Originally published in 1919 by James Arthur Harris and Francis Gano Benedict, this equation was the gold standard for decades. It was revised in 1984 by Roza and Shizgal to improve accuracy. The revised version is the one most commonly used today.

Revised Harris-Benedict (1984):

For men:

$BMR = (13.397 \times weight_{kg}) + (4.799 \times height_{cm}) - (5.677 \times age) + 88.362$

For women:

$BMR = (9.247 \times weight_{kg}) + (3.098 \times height_{cm}) - (4.330 \times age) + 447.593$

Same male example (40 years, 178 cm, 88 kg):

• Weight: 13.397 x 88 = 1,178.9
• Height: 4.799 x 178 = 854.2
• Age: 5.677 x 40 = 227.1
• BMR = 1,178.9 + 854.2 - 227.1 + 88.362 = 1,894.4 calories/day

Same female example (32 years, 163 cm, 60 kg):

• Weight: 9.247 x 60 = 554.8
• Height: 3.098 x 163 = 505.0
• Age: 4.330 x 32 = 138.6
• BMR = 554.8 + 505.0 - 138.6 + 447.593 = 1,368.8 calories/day

Mifflin-St Jeor vs. Harris-Benedict: Which Is Better?

Factor	Mifflin-St Jeor	Harris-Benedict (Revised)
Year published	1990	1919 (revised 1984)
Accuracy	Within 10% for ~82% of people	Within 10% for ~75% of people
Tendency	Slightly underestimates	Tends to overestimate by 5-15%
Best for	General adult population	Historical reference, larger body sizes
Recommended by	American Dietetic Association	Still widely used in clinical tools

For the male example above, Mifflin-St Jeor predicted 1,797 calories while Harris-Benedict predicted 1,894 calories, a difference of about 97 calories or 5.4%. For the female example, the gap was about 71 calories or 5.5%.

In practice, both formulas get you in the right neighborhood. Mifflin-St Jeor edges ahead in accuracy studies, particularly for people who are overweight or obese, where Harris-Benedict tends to overestimate more significantly.

Factors That Affect Your BMR

Body Composition

Muscle tissue is metabolically expensive. Each pound of muscle burns roughly 6 calories per day at rest, compared to about 2 calories per pound of fat. This is why two people of the same weight, height, age, and sex can have different BMRs: the one with more muscle mass burns more calories doing nothing.

This is also why strength training supports long-term weight management. Adding 10 pounds of muscle increases resting metabolism by roughly 60 calories per day, or about 22,000 calories per year.

Age

BMR declines approximately 1-2% per decade after your twenties. This decline is primarily driven by the gradual loss of lean tissue (sarcopenia) rather than some inherent metabolic slowdown. People who maintain their muscle mass through resistance training experience a much smaller age-related BMR decline.

Thyroid Function

Your thyroid gland produces hormones (T3 and T4) that directly regulate metabolic rate. Hypothyroidism (underactive thyroid) can reduce BMR by 15-40%, causing unexplained weight gain, fatigue, and cold sensitivity. Hyperthyroidism (overactive thyroid) raises BMR, often causing weight loss despite increased appetite. If your actual calorie needs seem drastically different from calculated BMR, thyroid function is worth investigating with your doctor.

Genetics

Twin studies suggest that BMR has a genetic component of 40-70%. Some people genuinely run hotter metabolically than others, independent of body size and composition. However, the practical variation is usually 200-300 calories per day, not the “fast metabolism” myths that claim thousands of calories of difference.

Dieting History

Extended caloric restriction can reduce BMR beyond what weight loss alone would predict, a phenomenon called adaptive thermogenesis or metabolic adaptation. Research on contestants from weight loss shows suggests this reduction can persist for years. This is one reason why aggressive crash diets often backfire: they can suppress your resting metabolism, making it progressively harder to keep losing weight.

Other Factors

• Caffeine and stimulants temporarily increase BMR by 3-11%
• Ambient temperature affects BMR (cold exposure increases it)
• Pregnancy increases BMR by 15-20% to support fetal development
• Illness and fever raise BMR roughly 7% per degree Fahrenheit above normal body temperature

How BMR Connects to TDEE

BMR is the largest but not the only component of your daily calorie burn. Total Daily Energy Expenditure (TDEE) adds three more components:

Component	% of TDEE	Description
BMR	60-75%	Resting metabolism
Thermic Effect of Food (TEF)	8-15%	Energy cost of digesting food
Exercise Activity (EAT)	5-30%	Planned workouts
Non-Exercise Activity (NEAT)	5-20%	Fidgeting, walking, standing, chores

To estimate TDEE from BMR, multiply by an activity factor (1.2 for sedentary up to 1.9 for extremely active). TDEE is the number you use for actual meal planning. Our BMR Calculator computes both values for you.

Example: Our male with a BMR of 1,797 who exercises moderately (3-5 days/week):

• TDEE = 1,797 x 1.55 = 2,786 calories/day

This means his body uses about 1,797 calories just existing, and the remaining 989 calories go toward digestion, movement, and exercise.

Should You Ever Eat Below Your BMR?

As a general guideline, eating below your BMR for extended periods isn’t advisable without medical supervision. Your body needs that baseline energy to maintain organ function, immune response, and hormonal balance. Chronic under-eating relative to BMR can lead to muscle loss, menstrual irregularities, weakened bones, impaired immune function, and nutrient deficiencies.

Short-term very-low-calorie diets (800-1,000 calories) exist in clinical settings for specific medical situations, but they are supervised by physicians and include supplementation. They aren’t something to attempt based on a calculator result.

Skip the Math

Our BMR Calculator runs both the Mifflin-St Jeor and Harris-Benedict equations simultaneously, so you can compare results. Enter your stats and get your resting metabolic rate plus estimated TDEE for your activity level.

Frequently Asked Questions

Can I increase my BMR?

Yes, to a degree. The most effective method is building lean muscle through resistance training, since muscle tissue is more metabolically active than fat. Regular strength training 2-4 times per week, combined with adequate protein intake, can meaningfully raise your resting metabolic rate over months. Staying well-hydrated, sleeping 7-9 hours per night, and avoiding prolonged severe calorie restriction also help maintain a healthy BMR.

Why do men have a higher BMR than women?

On average, men have more lean body mass and less body fat than women at the same height and weight. Since muscle is more metabolically active than fat, this body composition difference drives the BMR gap. Hormonal differences (testosterone vs. estrogen balance) also play a role. When researchers control for lean body mass, the sex-based difference in BMR shrinks considerably.

Is BMR the same as metabolism?

Not exactly. “Metabolism” is a broad term that encompasses all chemical processes in your body, including BMR but also digestion, physical activity, and cellular repair. BMR is specifically the resting component of metabolism. When people say “I have a slow metabolism,” they usually mean their total calorie burn (TDEE) is lower than expected, which can be influenced by BMR, activity levels, and the thermic effect of food.

How do I know if my BMR calculation is accurate?

The most reliable way to validate your calculated BMR is to track your calorie intake and weight change over 3-4 weeks. If you eat at your calculated TDEE and your weight stays stable, your estimate is close. If you gain or lose unexpectedly, adjust your baseline by 100-200 calories and reassess. For clinical precision, indirect calorimetry testing at a medical facility or sports performance lab measures your actual resting metabolic rate through breath analysis.

Does fasting affect BMR?

Short-term fasting (16-24 hours) has minimal impact on BMR. Some studies suggest a very slight temporary increase in the first 24-48 hours due to norepinephrine release. However, prolonged fasting or sustained very-low-calorie diets over weeks to months can decrease BMR through adaptive thermogenesis as the body works to conserve energy. Intermittent fasting protocols that cycle between eating and fasting windows generally don’t cause significant BMR reduction, provided total weekly calorie intake remains adequate.