BMR Calculator: Basal Metabolic Rate Formula & What It Means

What Is Basal Metabolic Rate?

Your basal metabolic rate (BMR) is the number of calories your body burns in a state of complete rest — no movement, no digestion, no stress. It represents the minimum energy required to keep you alive: heart beating, lungs breathing, kidneys filtering, cells dividing.

BMR accounts for 60–75% of total daily energy expenditure for most sedentary adults, according to research published in the American Journal of Clinical Nutrition. That means even if you never moved, your body would still burn the majority of your daily calories just to exist.

Understanding your BMR matters because it sets the floor for any nutrition or weight management plan. Eat well below it for extended periods and you risk muscle loss, hormonal disruption, and nutrient deficiencies. Eat slightly above your total needs and you gain weight. BMR is the anchor number everything else is calculated from.

The Mifflin-St Jeor Formula (Gold Standard)

Developed in 1990, the Mifflin-St Jeor equation is the most accurate BMR formula for general use. A landmark 2005 meta-analysis in the Journal of the American Dietetic Associationtested five common equations and found Mifflin-St Jeor was accurate within 10% for approximately 82% of healthy adults — better than every other formula tested.

For men:

BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5

For women:

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

Example — 35-year-old man, 185 lbs (84 kg), 5'10" (178 cm):

BMR = (10 × 84) + (6.25 × 178) − (5 × 35) + 5
BMR = 840 + 1,112.5 − 175 + 5
BMR = 1,782.5 calories/day

Example — 30-year-old woman, 140 lbs (63.5 kg), 5'5" (165 cm):

BMR = (10 × 63.5) + (6.25 × 165) − (5 × 30) − 161
BMR = 635 + 1,031.25 − 150 − 161
BMR = 1,355.25 calories/day

You do not need to do this by hand. Our BMR Calculator does it instantly.

The Harris-Benedict Equation (Original and Revised)

The Harris-Benedict equation was the standard for decades after its introduction in 1918. It was revised by Roza and Shizgal in 1984. Both versions are still in use, though Mifflin-St Jeor has largely replaced them.

Original Harris-Benedict (1919):

• Men: BMR = 66.5 + (13.75 × kg) + (5.003 × cm) − (6.775 × age)
• Women: BMR = 655.1 + (9.563 × kg) + (1.850 × cm) − (4.676 × age)

Revised Harris-Benedict (Roza & Shizgal, 1984):

• Men: BMR = 88.362 + (13.397 × kg) + (4.799 × cm) − (5.677 × age)
• Women: BMR = 447.593 + (9.247 × kg) + (3.098 × cm) − (4.330 × age)

For the same 35-year-old man above, the revised Harris-Benedict gives a BMR of roughly 1,900 calories — about 6% higher than Mifflin-St Jeor. The original Harris-Benedict tends to overestimate BMR by 5–15% in people with obesity, which is why Mifflin-St Jeor is preferred for clinical and practical use.

BMR Formula Comparison: When to Use Each

For most people without a DEXA scan or hydrostatic weighing to measure lean body mass, Mifflin-St Jeor is the right choice. If you're an athlete with low body fat and known lean mass, Katch-McArdle can be more precise.

Average BMR Ranges by Age and Sex

These estimates are based on average height and weight data from the CDC and NHANES survey results, using the Mifflin-St Jeor equation.

These are population averages. Your individual BMR depends heavily on your muscle mass, body composition, and health status. Two people the same age, height, and weight can have BMRs that differ by 200–300 calories if one has significantly more muscle.

5 Factors That Affect Your Basal Metabolic Rate

BMR is not fixed. Multiple variables push it up or down, often significantly. Here are the five biggest drivers, ranked by impact.

1. Lean Body Mass (Biggest Factor)

Muscle tissue burns roughly three times more calories at rest than fat tissue. A 2011 study in the Journal of Nutritionfound that each kilogram of skeletal muscle burns approximately 13 calories per day at rest, while fat burns only about 4.5 calories per kg. This is why two people with the same weight but different body compositions can have dramatically different BMRs. It's also why resistance training is so effective for long-term weight management — it raises your metabolic floor.

2. Age

BMR declines roughly 1–2% per decade after age 20. The primary driver is age-related muscle loss (sarcopenia), not aging itself. NIH research shows that people who maintain muscle mass through strength training experience far smaller metabolic declines than sedentary individuals of the same age. Staying active slows the clock.

3. Sex

Men typically have 10–20% higher BMRs than women of the same age, height, and weight. The main reason is body composition: men naturally carry more skeletal muscle and less body fat than women, and muscle is more metabolically active. Hormonal differences (testosterone vs. estrogen) play a secondary role.

4. Thyroid Function

The thyroid gland regulates metabolism at the cellular level. Hypothyroidism (underactive thyroid) can lower BMR by 15–40%, while hyperthyroidism can raise it by 25–80%, according to endocrinology research published by the NIH. If your calculated BMR seems wildly off from your real-world experience, thyroid function is worth discussing with a doctor.

5. Genetics and Hormones

Genetic variation accounts for 40–70% of individual differences in BMR after controlling for age, sex, and body composition, according to twin studies cited in the American Journal of Clinical Nutrition. Leptin, insulin, cortisol, and growth hormone also influence resting metabolic rate. These factors are harder to control but explain why some people seem to burn calories effortlessly while others struggle despite similar habits.

From BMR to TDEE: How Activity Changes Everything

BMR is only the starting point. Your total daily energy expenditure (TDEE) is what you actually burn across an entire day, including movement, digestion, and exercise. To estimate TDEE, multiply your BMR by an activity factor.

Using the 35-year-old man example (BMR ≈ 1,782): if he works a desk job and exercises lightly three days a week (lightly active), his TDEE is 1,782 × 1.375 = 2,450 calories/day. That's his maintenance number — the calories he needs to stay the same weight.

To lose about one pound per week, he'd eat 500 calories below TDEE, or roughly 1,950 calories. For a deep dive on this, see our guide on how to calculate TDEE and macros. You can also use our calorie and TDEE calculator to get the full picture in seconds.

BMR, Calories, and Weight Loss: What the Research Says

A 2021 systematic review in Obesity Reviews confirmed that calorie deficit size, not diet composition, is the primary driver of fat loss. But the relationship between BMR and weight loss is more nuanced than a simple math problem.

When you lose weight, your BMR drops — both because you weigh less (carrying less mass requires fewer calories) and because of metabolic adaptation, where the body downregulates energy expenditure beyond what weight loss alone explains. This is sometimes called “adaptive thermogenesis,” and it's part of why long-term weight loss plateaus are common.

The practical implication: as you lose weight, recalculate your BMR and TDEE every 10–15 pounds. What worked at 220 lbs may not work at 190 lbs. Our macro calculator can help you set updated calorie targets as your composition changes. For protein targets specifically, see our protein intake guide.

BMR vs. RMR: What's the Difference?

BMR and RMR (resting metabolic rate) are often used interchangeably, but they're technically different measurements.

• BMR is measured under strict laboratory conditions — lying still, completely fasted for 12–18 hours, at a thermoneutral temperature, after a full night's sleep.
• RMR is measured at rest but without the strict fasting and environmental controls. It's easier to test and more commonly used in clinical settings.

RMR is typically 10–20% higher than true BMR because digestion (the thermic effect of food) and slight movement add to calorie burn. Most online calculators, including ours, estimate what is technically closer to RMR — but the terms are used interchangeably in practice because the difference rarely matters for everyday nutrition planning.

Frequently Asked Questions

What is basal metabolic rate (BMR)?

Basal metabolic rate (BMR) is the number of calories your body burns at complete rest to maintain vital functions — breathing, circulation, cell production, and temperature regulation. It accounts for roughly 60–75% of total daily energy expenditure and is calculated using your age, sex, height, and weight.

What is the most accurate BMR formula?

The Mifflin-St Jeor equation (1990) is the gold standard for most healthy adults. A 2005 meta-analysis in the Journal of the American Dietetic Associationfound it accurate within 10% for about 82% of people — outperforming the older Harris-Benedict equation, especially for people with overweight or obesity.

What is a normal BMR for a woman?

For adult women, a typical BMR ranges from about 1,200 to 1,600 calories per day, depending on age, height, and weight. A 30-year-old woman who is 5'5" and 140 lbs has a BMR of roughly 1,440 calories using the Mifflin-St Jeor equation.

What is a normal BMR for a man?

For adult men, a typical BMR ranges from about 1,500 to 2,100 calories per day. A 35-year-old man who is 5'10" and 185 lbs has a BMR of approximately 1,870 calories using the Mifflin-St Jeor equation. Men burn more calories at rest due to higher average muscle mass.

Does BMR decrease with age?

Yes. BMR declines roughly 1–2% per decade after age 20, primarily due to age-related muscle loss (sarcopenia). Research in the American Journal of Clinical Nutrition found resting metabolic rate drops about 150 calories per day between ages 20 and 70 for sedentary individuals. Resistance training slows this decline.

How do I use BMR to lose weight?

Multiply your BMR by an activity factor to get your TDEE (total daily energy expenditure). To lose weight, eat 300–500 calories below your TDEE. Never eat below your BMR for extended periods — doing so triggers metabolic adaptation, muscle loss, and nutrient deficiencies, per NIH guidance.

What is the difference between BMR and RMR?

BMR is measured under strict lab conditions — lying still, fasted for 12 hours, at a neutral temperature. RMR is measured at rest but without strict fasting. RMR is typically 10–20% higher than BMR. In practice, the terms are often used interchangeably in nutrition calculators.