What Is BMR? Basal Metabolic Rate Explained

What Does BMR Measure?

Your Basal Metabolic Rate represents the minimum energy your body needs to sustain basic physiological functions while you are completely at rest. These functions include circulating blood, breathing, growing and repairing cells, regulating body temperature, and maintaining brain and nerve function.

If you spent an entire day lying motionless in bed without eating, your body would still burn your BMR worth of calories. For a 35-year-old man weighing 200 pounds at 6 feet tall, that baseline is approximately 1,882 calories per day. For a 35-year-old woman weighing 150 pounds at 5 feet 7 inches, it is around 1,409 calories per day, according to Cleveland Clinic BMR estimates.

The remaining 30 to 40 percent of your daily calorie burn comes from two other sources: physical activity (roughly 20 to 30 percent) and the thermic effect of food, or TEF (roughly 10 percent). TEF is the energy your body uses to digest, absorb, and metabolize the food you eat.

How to Calculate Your BMR

Several formulas exist for estimating BMR, but the Mifflin-St Jeor equation is the one recommended by the Academy of Nutrition and Dietetics as the most reliable for healthy adults. Published in 1990, it replaced the older Harris-Benedict equation (1919), which tends to overestimate calorie needs by 5 to 15 percent.

The Mifflin-St Jeor Formula

For men: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5

For women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) - 161

Step-by-Step Example

Consider a 28-year-old woman who weighs 145 pounds (65.8 kg) and is 5 feet 6 inches (167.6 cm) tall:

• (10 x 65.8) = 658
• (6.25 x 167.6) = 1,047.5
• (5 x 28) = 140
• 658 + 1,047.5 - 140 - 161 = 1,404.5 calories/day (BMR)

This means her body burns approximately 1,405 calories per day just to maintain basic life functions. Any movement, exercise, or digestion adds to this baseline number.

Other BMR Formulas

While Mifflin-St Jeor is the standard, two other formulas are worth knowing:

• Harris-Benedict (revised 1984): Older and slightly less accurate, but still widely used. It tends to overestimate BMR, especially in overweight individuals.
• Katch-McArdle: Uses lean body mass instead of total body weight, making it more accurate for people who know their body fat percentage. Formula: BMR = 370 + (21.6 x lean mass in kg).

What Factors Affect Your BMR?

Your BMR is not a fixed number. It changes based on several factors, some within your control and some not.

Body Composition

Muscle tissue is more metabolically active than fat tissue. One pound of muscle burns approximately 6 calories per day at rest, while one pound of fat burns only about 2 calories per day. This is why two people of the same weight can have very different BMRs: the person with more muscle burns more calories at rest.

Age

BMR declines by approximately 1 to 2 percent per decade after age 20. This decline is primarily driven by the gradual loss of lean muscle mass, a process called sarcopenia. After age 30, adults lose roughly 3 to 8 percent of their muscle mass per decade, and this rate accelerates after age 60.

Sex

Men generally have a higher BMR than women of the same age and weight because men tend to carry more muscle mass and less body fat. On average, a man's BMR is about 5 to 10 percent higherthan a woman's of comparable size.

Genetics and Hormones

Thyroid hormones (T3 and T4) are the primary regulators of metabolic rate. Hypothyroidism can reduce BMR by 15 to 40 percent, while hyperthyroidism can increase it by a similar margin. Genetic variation in metabolic rate between individuals of the same size, age, and sex can account for a difference of up to 200 to 300 calories per day.

Diet and Caloric Restriction

Severe caloric restriction (crash dieting) can lower your BMR by 20 percent or more as your body enters a conservation mode. This metabolic adaptation is one reason why extreme diets often lead to weight regain. A study published in the journal Obesity tracked contestants from "The Biggest Loser" and found their metabolic rates were significantly suppressed even six years after the show.

BMR vs. TDEE: What Is the Difference?

BMR and TDEE are related but measure different things. BMR is your calorie burn at complete rest. TDEE (Total Daily Energy Expenditure) is your total calorie burn including all movement, exercise, and food digestion throughout the day.

To convert BMR to TDEE, you multiply by an activity factor:

Using the example above: if the 28-year-old woman exercises moderately (3-5 days per week), her TDEE is 1,405 x 1.55 = approximately 2,178 calories per day. This is the number she should use for weight management, not her BMR alone.

For a more detailed breakdown of TDEE and macronutrient targets, see our guide on how to calculate TDEE and macros.

Why BMR Matters for Weight Management

Understanding your BMR is essential for setting realistic calorie targets. Here is how BMR applies to common goals:

For Weight Loss

To lose weight, you need to eat fewer calories than your TDEE. A deficit of 500 calories per day produces roughly 1 pound of fat loss per week. However, you should generally not eat below your BMR for extended periods. Going below BMR can trigger metabolic slowdown, muscle loss, nutrient deficiencies, and hormonal disruption. The National Institutes of Health recommends that women eat at least 1,200 calories per day and men at least 1,500 calories per day without medical supervision.

For Muscle Gain

Building muscle requires eating above your TDEE (a caloric surplus). A surplus of 250 to 500 calories per day is typical for a lean bulk. Knowing your BMR helps you set the floor: your surplus should be above TDEE, and your TDEE is always above your BMR.

For Maintenance

Eating at your TDEE maintains your current weight. If your weight is stable for several weeks while eating a consistent number of calories, that number is a good estimate of your true TDEE, regardless of what any formula predicts.

How to Measure BMR Accurately

BMR formulas provide estimates. For a precise measurement, the gold standard is indirect calorimetry, a clinical test that measures the oxygen you consume and the carbon dioxide you produce over a period of 15 to 30 minutes. This test is available at hospitals, university labs, and some gyms. It typically costs between $75 and $250.

For most people, a formula-based estimate is accurate enough to set useful calorie targets. Use it as a starting point, track your weight for 2 to 4 weeks, and adjust based on real-world results.

Average BMR by Age and Sex

The following table shows approximate BMR values for average-sized adults based on the Mifflin-St Jeor equation. These assume a height of 5 feet 9 inches (175 cm) and 176 pounds (80 kg) for men, and 5 feet 4 inches (163 cm) and 143 pounds (65 kg) for women.

Notice the steady decline: roughly 50 calories per decade for both men and women. This is why calorie needs decrease with age and why strength training becomes increasingly important for maintaining metabolic rate.

Common Myths About BMR

Myth: Eating small, frequent meals boosts your metabolism

Research consistently shows that meal frequency has no significant effect on BMR or total daily calorie burn. A 2010 study in the British Journal of Nutrition found no metabolic advantage to eating six small meals versus three larger meals with the same total calories. What matters is total calorie intake, not how you divide it up.

Myth: Certain foods dramatically increase BMR

While some foods (like spicy peppers and green tea) have been shown to slightly increase metabolic rate, the effect is minimal, typically 50 to 80 extra calories per day at most. No food or supplement can replace the impact of building lean muscle mass and staying physically active.

Myth: A slow metabolism makes weight loss impossible

While metabolic rates do vary between individuals, the difference is usually 200 to 300 calories per day for people of similar size. True metabolic disorders (like hypothyroidism) are medical conditions that can be diagnosed and treated. For most people, the challenge is not a slow metabolism but rather underestimating calorie intake and overestimating calorie burn.

The Bottom Line

Your Basal Metabolic Rate is the foundation of your energy balance. It determines the minimum calories your body needs to function, and it serves as the starting point for calculating your TDEE and setting calorie targets for any goal. Use the Mifflin-St Jeor equation as a reliable estimate, track your results for a few weeks, and adjust based on what actually happens on the scale.

Want to skip the manual math? Our free BMR calculator does the Mifflin-St Jeor calculation instantly, and our TDEE calculator converts that into actionable calorie and macro targets.

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