BMR Calculator
Estimate your basal metabolic rate — the calories your body burns at complete rest — using the Mifflin-St Jeor equation, the modern reference standard for healthy adults.
BMR — calories burned at rest (kcal/day)
1,699
- Per hour at rest (kcal/hr)
- 70.8
- Harris-Benedict (revised) — for comparison (kcal/day)
- 1,763
BMR is the energy your body burns at complete rest just to keep organs running — heart, lungs, brain, kidneys, body temperature. Mifflin-St Jeor (1990) is the modern reference equation, endorsed by the Academy of Nutrition and Dietetics as the most accurate predictor for healthy adults. The Harris-Benedict revised value (Roza & Shizgal 1984) is shown for comparison; older calculators still use it. To estimate the calories you actually need each day, multiply BMR by an activity factor — that is TDEE, not BMR.
How to use this calculator
Pick your sex, enter your age in years, choose metric or imperial units, and type in your weight and height. The calculator returns your BMR in kilocalories per day using the Mifflin-St Jeor equation, plus a per-hour figure and the Harris-Benedict revised value for comparison. BMR is the energy you would burn lying still in bed for 24 hours — to estimate the calories you actually need each day, multiply BMR by an activity factor (the TDEE calculator does this automatically).
How the calculation works
The Mifflin-St Jeor equation predicts BMR in kcal/day from weight in kilograms, height in centimetres, age in years and a sex offset: 10 × kg + 6.25 × cm − 5 × age + 5 for men, or − 161 for women. The Academy of Nutrition and Dietetics 2014 evidence analysis identified it as the most accurate of the major predictive equations, with a typical error of about ±10% versus indirect calorimetry. Imperial inputs convert via the NIST-exact factors of 1 lb = 0.45359237 kg and 1 inch = 2.54 cm before the metric formula is applied, so the answer is identical whichever system you use. Harris-Benedict revised (Roza & Shizgal 1984) is shown alongside because many older calculators and apps still rely on it — it tends to overestimate BMR by 50–100 kcal/day relative to Mifflin-St Jeor.
Worked example
A 30-year-old man, 80 kg, 180 cm. BMR = 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 800 + 1125 − 150 + 5 = 1,780 kcal/day at complete rest. Per hour that is 1,780 ÷ 24 ≈ 74 kcal — roughly the energy in a slice of bread, every hour, just to keep his body running. The Harris-Benedict revised value for the same person is 88.362 + 13.397 × 80 + 4.799 × 180 − 5.677 × 30 = 1,853 kcal/day, about 73 kcal higher than Mifflin-St Jeor.
Frequently asked questions
What is BMR and how is it different from TDEE?
BMR (basal metabolic rate) is the calories your body would burn lying perfectly still in a thermoneutral room for 24 hours, with nothing in your stomach — pure organ function, no digestion, no movement. TDEE (total daily energy expenditure) is BMR multiplied by an activity factor, which adds in the calories you burn from movement, exercise and digesting food. BMR is roughly 60–70% of TDEE for most people; a sedentary adult sits around BMR × 1.2, an active one around BMR × 1.55.
Which BMR equation is most accurate?
For healthy adults, the Mifflin-St Jeor equation (1990) is the modern reference. The Academy of Nutrition and Dietetics 2014 evidence analysis compared the major predictive equations against indirect calorimetry and found Mifflin-St Jeor was correct within ±10% for about 70% of people, beating both Harris-Benedict (1919, revised 1984) and the WHO/FAO/UNU equations. For people with very high lean body mass — competitive athletes, bodybuilders — the Katch-McArdle equation (which uses lean mass directly instead of total weight) is usually more accurate.
Why do men have higher BMR than women at the same weight?
Men typically carry more lean muscle mass and less essential body fat than women at the same total weight, and lean tissue is metabolically more expensive than fat — roughly 13 kcal/day per kg of muscle versus 4 kcal/day per kg of fat at rest. Mifflin-St Jeor captures this with a flat sex offset (+5 for men, −161 for women) on top of the weight and height terms. The 166 kcal gap is an average across the population, not a per-person guarantee — a lean, muscular woman can have a higher true BMR than a sedentary man of the same weight.
Does BMR change with age?
Yes — BMR drops gradually after early adulthood, mostly because lean muscle mass declines and the proportion of metabolically slower fat tissue rises. Mifflin-St Jeor models this with a flat 5 kcal/day per year of age. A 2021 paper in Science (Pontzer et al.) found the decline is steeper than the equation predicts after about age 60, so older adults using Mifflin-St Jeor may see a small overestimate. Resistance training partially offsets the decline by preserving lean mass.
Can I lose weight by eating below my BMR?
You can, but it is not recommended. Eating below BMR for more than a few days can trigger metabolic adaptation — the body slows non-essential processes (immune function, reproductive hormones, thyroid output) to conserve energy, which makes further loss harder and risks muscle loss, fatigue and nutrient deficiencies. Standard guidance is to eat at a moderate deficit below your TDEE (typically 300–500 kcal/day below), which produces about 0.25–0.5 kg of fat loss per week without dipping under BMR.
How accurate are predictive BMR equations versus measurement?
Predictive equations like Mifflin-St Jeor are within ±10% of measured BMR for roughly 70% of healthy adults. The remaining 30% sit further off, usually because of unusual body composition, thyroid disease, or recent dieting which lowers measured BMR below the predicted value. The gold standard is indirect calorimetry — breathing into a hood or chamber while gas exchange is measured — typically only available in research labs or specialist clinics. For everyday calorie planning, the predictive value is close enough to set a starting point and adjust based on how your weight actually moves.