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What is BMR?
BMR (Basal Metabolic Rate) is the minimum amount of energy your body uses to keep its vital functions going — breathing, blood circulation, cell renewal, maintaining body temperature — while you do nothing at all: lying down, sleeping, at complete rest. In short, it's the calories you'd burn even if you stayed in bed all day, and BMR alone accounts for 60-70% of your total daily energy expenditure.
BMR isn't the same for everyone; three main things drive it: muscle mass (because muscle burns far more energy than fat, people with more muscle have a faster metabolism), sex (men tend to carry a higher proportion of muscle, so their BMR is usually higher than women's), and age (as muscle declines over the years, BMR drops). That's why two people at the same weight can have noticeably different daily calorie needs.
What is BMR used for?
BMR is the starting point of every nutrition plan; a dietitian setting a client's calorie target begins here too. Without knowing your metabolism's baseline expenditure, there's no accurate answer to "how many calories should I eat a day?" — because the same goal (say, losing 0.5 kg a week) means a different number of calories for different metabolisms. Depending on your goal:
- To lose weight: you eat fewer calories than you need (a calorie deficit).
- To maintain weight: you eat calories equal to your TDEE (your total daily expenditure).
- To gain weight/muscle: you eat more than your TDEE (a calorie surplus).
BMR is also a safety floor: dropping your calorie intake below your BMR slows your metabolism and leads to muscle loss. That's why weight-loss goals are always built on TDEE, never on BMR — BMR is the lowest your intake should ever go, not your target.
How is BMR calculated? — the Mifflin-St Jeor formula
This tool uses the Mifflin-St Jeor (1990) equation, considered the most accurate in the scientific community and the standard used by clinical dietitians:
Female: BMR = (10 × weight) + (6.25 × height) − (5 × age) − 161
Example: for a 30-year-old man, 175 cm, 75 kg:
BMR = (10 × 75) + (6.25 × 175) − (5 × 30) + 5 = 750 + 1,093.75 − 150 + 5 ≈ 1,699 kcal/day
Method: Basal metabolism is calculated with the Mifflin-St Jeor equation (Mifflin MD, St Jeor ST et al., American Journal of Clinical Nutrition, 1990); daily needs (TDEE) are found using standard activity factors.
The difference between BMR and TDEE
TDEE (Total Daily Energy Expenditure) is your real calorie need once you add your daily movement to your BMR. It's found by multiplying your BMR by a coefficient that matches your activity level:
Sedentary .............. × 1.2
Lightly active (1-3 days) .... × 1.375
Moderately active (3-5 days) ..... × 1.55
Very active (6-7 days) ...... × 1.725
Extremely active (athlete) ..... × 1.9
Frequently asked questions
The Mifflin-St Jeor equation is accurate to within ±10% in non-obese adults and is clinically the most reliable estimation method. A truly exact figure can only come from a metabolic test (indirect calorimetry — a lab test that measures the oxygen and carbon dioxide in your breath to find your real energy expenditure); this tool is more than enough for day-to-day planning.
Mifflin-St Jeor uses only weight, height, age and sex; it doesn't factor in body fat percentage. If your muscle mass is notably high (an athlete or bodybuilder), the Katch-McArdle formula — which is based on lean body mass (muscle, bone and organ weight, i.e. everything except fat) — gives a more accurate result.
A good rule of thumb: subtract 500 kcal from your TDEE — this yields roughly 0.45 kg of fat loss a week (1 kg of fat ≈ 7,700 kcal). The calculator above shows this figure automatically in the "Lose weight" box.
Yes. As you get older, muscle mass declines, so BMR falls — the "− 5 × age" term in the formula reflects this. Regular resistance training preserves muscle mass and keeps your metabolism high.
Most of your metabolism is determined by your muscle mass, because muscle burns far more energy than fat even at rest. The proven ways to raise it: <b>resistance training</b> (building muscle by lifting weights), getting enough <b>protein</b>, <b>avoiding very-low-calorie diets</b> (starving yourself slows your metabolism, doing the opposite of what you want) and staying active throughout the day (NEAT — non-exercise activity). Most claims about "metabolism-boosting" pills, teas or spices are exaggerated; the lasting difference comes from muscle and activity.
On average, women's bodies have a lower proportion of muscle and a higher proportion of fat; since muscle burns more energy, a woman at the same height and weight usually has a lower BMR than a man. The Mifflin-St Jeor formula accounts for this by subtracting 161 for women (while adding 5 for men). It isn't a flaw — it's the math reflecting a biological difference.