The Quick Answer
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 represents the minimum energy your body needs just to stay alive.
The most widely recommended formula is the Mifflin-St Jeor equation:
- Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
Example: A 30-year-old male, 175 cm tall, weighing 75 kg: BMR = (10 × 75) + (6.25 × 175) − (5 × 30) + 5 = 750 + 1,093.75 − 150 + 5 = 1,698.75 ≈ 1,699 calories/day
This is the energy cost of doing absolutely nothing. Your actual daily calorie burn (TDEE) is higher because it includes physical activity.
What BMR Actually Measures
BMR measures the energy your body consumes while at complete physical rest in a temperature-neutral environment, after 12 hours of fasting and 8 hours of sleep. Specifically, BMR accounts for:
- Organ function: Brain, heart, liver, kidneys, and lungs alone consume roughly 60–70% of BMR
- Cellular processes: Cell division, protein synthesis, ion transport across membranes
- Breathing: The mechanical work of expanding and contracting your lungs
- Circulation: Keeping blood moving through approximately 100,000 km of blood vessels
- Temperature regulation: Maintaining core body temperature at approximately 37°C (98.6°F)
For most people, BMR accounts for 60–75% of total daily energy expenditure. The remaining calories come from physical activity (15–30%) and the thermic effect of food (roughly 10%).
How to Calculate BMR
Two equations are commonly used. Both require the same inputs: weight, height, age, and sex.
Mifflin-St Jeor Equation (recommended)
Published in 1990, this is currently considered the most accurate BMR estimation for the general population.
Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Harris-Benedict Equation (original)
Published in 1919 and revised in 1984, this was the standard for decades. It tends to overestimate BMR by 5–15% compared to measured values.
Men: BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) − (5.677 × age)
Women: BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) − (4.330 × age)
Which formula should you use?
The Mifflin-St Jeor equation has been validated in multiple studies as the more accurate predictor for people of typical body composition. The American Dietetic Association recommended it as the preferred method in 2005.
The Harris-Benedict equation is still acceptable but consistently produces slightly higher estimates.
Worked Examples
Example 1: 25-year-old woman, 165 cm, 60 kg
Mifflin-St Jeor: BMR = (10 × 60) + (6.25 × 165) − (5 × 25) − 161 BMR = 600 + 1,031.25 − 125 − 161 = 1,345 calories/day
Harris-Benedict: BMR = 447.593 + (9.247 × 60) + (3.098 × 165) − (4.330 × 25) BMR = 447.593 + 554.82 + 511.17 − 108.25 = 1,405 calories/day
Difference: Harris-Benedict gives 60 more calories (about 4.5% higher).
Example 2: 40-year-old man, 180 cm, 90 kg
Mifflin-St Jeor: BMR = (10 × 90) + (6.25 × 180) − (5 × 40) + 5 BMR = 900 + 1,125 − 200 + 5 = 1,830 calories/day
Harris-Benedict: BMR = 88.362 + (13.397 × 90) + (4.799 × 180) − (5.677 × 40) BMR = 88.362 + 1,205.73 + 863.82 − 227.08 = 1,931 calories/day
Difference: Harris-Benedict gives 101 more calories (about 5.5% higher).
Imperial unit conversion
If you measure in pounds and feet/inches, convert before applying the formula:
- Weight: pounds × 0.4536 = kilograms
- Height: (feet × 12 + inches) × 2.54 = centimeters
Example: 154 lbs, 5′9″ (69 inches)
- Weight: 154 × 0.4536 = 69.85 kg
- Height: 69 × 2.54 = 175.26 cm
Then apply the standard equation with these metric values.
BMR vs. TDEE: What's the Difference?
BMR and TDEE are related but answer different questions:
| BMR | TDEE | |
|---|---|---|
| Measures | Calories burned at complete rest | Total calories burned per day |
| Includes activity? | No | Yes |
| Use case | Understanding baseline metabolism | Planning calorie intake |
| Typical range | 1,200–2,000 for most adults | 1,600–3,200+ depending on activity |
TDEE = BMR × Activity Multiplier
Standard activity multipliers:
- Sedentary (desk job, little exercise): BMR × 1.2
- Lightly active (light exercise 1–3 days/week): BMR × 1.375
- Moderately active (moderate exercise 3–5 days/week): BMR × 1.55
- Very active (hard exercise 6–7 days/week): BMR × 1.725
- Extremely active (physical job + intense training): BMR × 1.9
Example: BMR of 1,699 calories with moderate activity: TDEE = 1,699 × 1.55 = 2,633 calories/day
This is the number most relevant for meal planning and calorie goals.
BMR vs. RMR: Are They the Same?
BMR and RMR (Resting Metabolic Rate) are often used interchangeably, but they are measured under slightly different conditions:
- BMR is measured after a full night of sleep, 12 hours of fasting, and in a darkened, temperature-controlled room. The person must be lying completely still.
- RMR is measured under less restrictive conditions — typically after a few hours of fasting, without the strict sleep/rest requirements.
In practice, RMR tends to be about 3–10% higher than BMR because the measurement conditions are less controlled. For everyday calorie planning, the difference is small enough that either number works.
Most online "BMR calculators" (including ours) technically estimate RMR-like values, since they use equations derived from population studies rather than laboratory measurements.
What Affects Your BMR
BMR is not a fixed number. Several factors influence it:
Body composition
Muscle tissue burns more energy at rest than fat tissue. Two people of identical weight, height, age, and sex can have meaningfully different BMRs if one carries significantly more muscle mass.
Age
BMR decreases with age — roughly 1–2% per decade after age 20. This decline is partly due to muscle loss (sarcopenia) and partly due to hormonal changes.
Sex
Males generally have higher BMRs than females of the same height and weight, primarily due to differences in average muscle mass and hormonal profiles.
Body size
Larger bodies require more energy to maintain. Both height and weight contribute — a taller person with more tissue has a higher BMR even at the same body fat percentage.
Genetics
Metabolic rate has a heritable component. Studies on identical twins suggest genetics may account for roughly 40% of the variation in BMR between individuals.
Hormones
Thyroid hormones (T3 and T4) are the primary regulators of metabolic rate. Hypothyroidism can reduce BMR, while hyperthyroidism can increase it. Other hormones (insulin, cortisol, growth hormone) also play roles.
Environmental temperature
Exposure to cold temperatures increases BMR as the body generates heat to maintain core temperature. Extreme heat can also increase metabolic rate slightly.
Diet and fasting
Prolonged calorie restriction can reduce BMR by 10–20% — a phenomenon sometimes called "metabolic adaptation" or "adaptive thermogenesis." This is one reason aggressive dieting often leads to plateaus.
Common Mistakes When Using BMR
Mistake 1: Eating at or below your BMR
BMR represents the energy your body needs for basic survival. Eating at or below this level chronically can trigger metabolic adaptation, muscle loss, and nutrient deficiencies. Calorie targets should be based on TDEE, not BMR.
Mistake 2: Overestimating activity level
Choosing "very active" when you exercise 3 times a week inflates your TDEE calculation significantly. Most people with office jobs who exercise regularly are "lightly active" or "moderately active." When in doubt, choose the lower activity level.
Mistake 3: Treating the number as exact
BMR equations are statistical estimates with a standard error of approximately ±200 calories. Two people with identical inputs can have genuinely different BMRs due to genetics, body composition, and other factors the formula doesn't capture.
Mistake 4: Ignoring the thermic effect of food
Digesting food itself costs energy — roughly 10% of total calorie intake. Protein has a higher thermic effect (20–30%) than carbohydrates (5–10%) or fats (0–3%). This isn't captured in BMR but does affect total energy expenditure.
Mistake 5: Recalculating too often
BMR doesn't change day to day. Recalculating weekly or monthly as your weight changes is sufficient. Daily fluctuations in body weight are mostly water, not metabolic shifts.
How BMR Changes Across Life Stages
BMR follows a predictable arc over a lifetime:
- Infants and children: Very high BMR per unit of body weight due to rapid growth
- Teenagers: BMR peaks during puberty alongside growth spurts
- 20s–30s: Gradual decline begins, approximately 1–2% per decade
- 40s–60s: Decline continues; accelerates somewhat due to age-related muscle loss
- 70s+: BMR may be 20–30% lower than at age 20, partly reversible with strength training
This decline is not entirely inevitable. Maintaining muscle mass through resistance training can slow the reduction in BMR significantly.
Frequently Asked Questions
What is a normal BMR?
For adult women, BMR typically falls between 1,100 and 1,700 calories per day. For adult men, between 1,400 and 2,000 calories per day. These ranges vary significantly with body size, age, and composition.
Why is my BMR so low?
Common reasons include smaller body size, older age, lower muscle mass, or being female (due to body composition differences). In some cases, medical conditions like hypothyroidism can lower BMR. If your BMR seems unusually low and you experience fatigue or unexplained weight gain, consult a healthcare provider.
Can I increase my BMR?
The most effective way to increase BMR is to build muscle through resistance training. Each kilogram of muscle burns roughly 13 calories per day at rest, compared to about 4.5 calories per kilogram of fat. Over time, a meaningful increase in muscle mass raises resting metabolism measurably.
Should I eat below my BMR to lose weight?
Consistently eating below your BMR is generally not recommended. It can lead to metabolic adaptation, muscle loss, fatigue, and nutritional deficiencies. A moderate calorie deficit of 300–500 calories below TDEE (not BMR) is a more sustainable approach.
Does exercise increase BMR?
Exercise increases total daily calorie burn (TDEE) directly. Intense exercise can also temporarily elevate metabolic rate for hours afterward (excess post-exercise oxygen consumption, or EPOC). Over the long term, the muscle gained from regular strength training raises BMR itself.
How accurate are BMR calculators?
BMR equations predict actual measured BMR within ±10% for most people. The Mifflin-St Jeor equation has been shown to be accurate within 10% for about 82% of individuals in validation studies. For more precise measurement, indirect calorimetry (a clinical test measuring oxygen consumption) is the gold standard.
What is the difference between BMR and calories burned?
BMR is calories burned at complete rest — just to stay alive. Total calories burned per day (TDEE) includes BMR plus activity and digestion. BMR typically represents 60–75% of your total daily calorie burn.
Does BMR change when you diet?
Yes. Prolonged calorie restriction can reduce BMR by 10–20% beyond what weight loss alone would predict. This metabolic adaptation is a survival mechanism that makes continued weight loss progressively harder. It can partially reverse when calorie intake returns to maintenance levels.
How is BMR measured in a lab?
Clinical BMR measurement uses indirect calorimetry: a hood or mask captures exhaled gases while the person lies completely still after fasting. By measuring oxygen consumed and carbon dioxide produced, the exact rate of energy expenditure is calculated. This test typically costs $150–$300 and takes 15–30 minutes.
Is BMR the same as metabolism?
BMR is one component of metabolism. "Metabolism" broadly refers to all chemical processes in the body, including energy production, tissue building, waste elimination, and more. BMR specifically quantifies the energy cost of basal metabolic processes at rest.
Does coffee or caffeine increase BMR?
Caffeine can temporarily increase metabolic rate by 3–11%, with effects lasting a few hours. However, regular caffeine users develop tolerance, reducing this effect. The increase is modest — roughly 50–150 extra calories per day at most, and it diminishes over time.
At what age does BMR start declining?
BMR begins declining gradually around age 20, at a rate of approximately 1–2% per decade. The decline accelerates after 60–70 due to increased loss of lean body mass. Regular strength training can significantly slow this decline.
Calculate Your BMR
Use the BMR calculator to calculate your Basal Metabolic Rate instantly. Enter your age, height, weight, and sex in metric or imperial units, select your activity level, and see your BMR, TDEE, and calorie targets for weight loss, maintenance, and weight gain.
For a broader view of nutritional needs, also try the calorie needs calculator or the macro calculator for protein, carbohydrate, and fat breakdowns based on your goals.