This BMR calculator estimates your basal metabolic rate — the number of calories your body burns per day at complete rest just to keep you alive: breathing, circulating blood, regulating temperature, and repairing cells. BMR typically accounts for 60–70% of all the calories most people burn, making it the single biggest component of daily energy expenditure.
The calculator uses the Mifflin-St Jeor equation, which research has shown to be the most reliable formula for estimating resting metabolism in the general population. It also multiplies your BMR by standard activity factors, so you can see your full daily calorie needs at every activity level in one table.
The Mifflin-St Jeor Equation
Published in 1990, the Mifflin-St Jeor equation estimates BMR from weight, height, age, and sex:
- Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
- Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161
Each term reflects real physiology: heavier and taller bodies have more tissue to maintain, metabolism slows roughly 5 calories per year of age, and men average more muscle mass than women at the same size, which is why the male constant is higher. To turn BMR into total daily needs, multiply by an activity factor from 1.2 (sedentary) up to 1.9 (very intense daily training or a physical job).
Mifflin-St Jeor vs. Harris-Benedict vs. Katch-McArdle
Three equations dominate BMR estimation, and they suit different situations:
- Mifflin-St Jeor (1990): the modern default. A 2005 American Dietetic Association review found it predicts measured resting metabolic rate within 10% more often than any other formula.
- Harris-Benedict (1919, revised 1984): the original standard. It tends to overestimate BMR by about 5%, partly because it was derived from an early-1900s population that was leaner and more active.
- Katch-McArdle: BMR = 370 + 21.6 × lean body mass (kg). It ignores age and sex because it works directly from lean mass, making it the best choice for very muscular or very lean people — but you need a reliable body fat percentage to use it.
If you do not know your body fat percentage, Mifflin-St Jeor is the safest pick.
Example: 30-Year-Old Male, 5'10", 165 lbs
Convert to metric: 165 lbs ÷ 2.2046 = 74.8 kg, and 70 inches × 2.54 = 177.8 cm.
BMR = 10 × 74.8 + 6.25 × 177.8 − 5 × 30 + 5 = 748 + 1,111 − 150 + 5 = 1,715 calories/day
That is about 71 calories per hour burned at complete rest. His full daily needs by activity level:
- Sedentary (× 1.2): 2,058 calories/day
- Light exercise (× 1.375): 2,358 calories/day
- Moderate exercise (× 1.465): 2,512 calories/day
- Very active (× 1.725): 2,958 calories/day
A 30-year-old woman with the same height and weight would have a BMR of 1,549 calories/day — 166 calories lower, reflecting the different constant in the formula.
Frequently Asked Questions
What is BMR?
BMR (basal metabolic rate) is the number of calories your body burns per day at complete rest to power essential functions like breathing, circulation, and cell repair. It makes up roughly 60–70% of total daily calorie burn for most people, with activity and digestion accounting for the rest.
What is the difference between BMR and RMR?
BMR is measured under strict laboratory conditions — after a full night's sleep, fasting for 12 hours, lying down in a temperature-controlled room. RMR (resting metabolic rate) uses more relaxed conditions and runs about 10% higher. In everyday use the terms are interchangeable, and calculator formulas approximate both.
Should I eat below my BMR to lose weight?
Generally no. You burn well above your BMR every day through movement and digestion, so a safe deficit almost always leaves you eating above BMR. Eating below it is not physically harmful in itself, but it usually means an overly aggressive deficit that risks muscle loss, fatigue, and nutrient shortfalls.
How can I increase my BMR?
Build muscle — each pound of muscle burns roughly 6 calories per day at rest versus about 2 for fat, so gaining 10 lbs of muscle adds around 40–60 calories daily. Adequate protein, resistance training, sufficient sleep, and avoiding prolonged crash diets all help maintain or raise resting metabolism.
Why does BMR decline with age?
The Mifflin-St Jeor equation subtracts 5 calories per year of age, reflecting gradual muscle loss and slowing cellular activity. Adults lose 3–8% of muscle mass per decade after 30 unless they strength train. Much of the "metabolic slowdown" of aging is preventable by preserving lean mass and staying active.