Every weight plan — losing fat, gaining muscle, or simply maintaining — starts with one number: how many calories your body burns just keeping you alive. That’s your Basal Metabolic Rate (BMR), the energy your brain, heart, liver, and other organs consume at complete rest. Everything else you eat beyond BMR goes to movement, digestion, and activity. Our free BMR calculator uses the Mifflin-St Jeor equation — the formula the American Dietetic Association endorses as the most accurate for healthy adults — to give you that number in seconds.
BMR Calculator
What Is the Mifflin-St Jeor Equation?
Published in 1990 by Mifflin and colleagues after testing it against measured metabolic rates in hundreds of subjects, this equation beat older formulas (including the famous 1919 Harris-Benedict) for accuracy in modern populations:
- Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
- Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
The constants reflect real physiology: each kilogram of body weight costs roughly 10 kcal/day to maintain, each centimetre of height about 6.25, and BMR drops roughly 5 kcal per year of age — the “metabolism slows with age” effect, which is mostly loss of muscle mass.
How to Use
- Select your gender (the formulas differ by a 166 kcal constant).
- Enter your age, weight in kg, and height in cm.
- Click Calculate — your BMR in kcal/day appears.
- Use it as the base for your TDEE: multiply by your activity factor (our TDEE calculator does this automatically).
Worked Example
Ali is 30, weighs 70 kg, and is 175 cm tall. BMR = 10 × 70 + 6.25 × 175 − 5 × 30 + 5 = 700 + 1093.75 − 150 + 5 = 1,649 kcal/day. Even if Ali lay in bed all day doing nothing, his body would burn about 1,649 calories. Any diet plan that ignores this number is guessing.
Tips & Limitations
- BMR is not your total calorie need — it excludes all movement. Multiply by 1.2–1.9 depending on activity to get TDEE (total daily energy expenditure).
- Crash diets below BMR backfire: the body adapts by burning less and losing muscle. Most dietitians set the floor near BMR.
- Muscle raises BMR slightly (muscle burns ~6 kcal/lb/day vs fat’s ~2), which is why strength training helps long-term weight control.
- Illness, fever, and extreme cold/heat raise BMR temporarily; the calculator assumes a healthy resting state.
FAQs
What is a normal BMR?
Most adults land between 1,200 and 2,000 kcal/day. Taller, heavier, younger men sit higher; smaller, older women lower. Your own number is what matters, not the average.
Mifflin-St Jeor vs Harris-Benedict — which is better?
Mifflin-St Jeor (1990) tested more accurate than Harris-Benedict (1919, revised 1984) in modern adults, and is the one dietitians and the ADA recommend. Both are estimates — Mifflin just estimates better.
Does metabolism really slow with age?
About 1–2% per decade after 40, driven mostly by muscle loss (sarcopenia) and lower activity — not some broken “metabolic switch.” Resistance training and protein blunt most of it.
Can I eat below my BMR?
Short-term, small dips are survivable, but chronically eating far below BMR risks muscle loss, hormonal disruption, and the weight rebounding. Build your deficit from TDEE, not below BMR.
BMR vs TDEE — what’s the difference?
BMR is rest-only burn. TDEE = BMR × activity factor (1.2 sedentary to 1.9 very active) = what you actually burn in a day. Weight plans always work from TDEE.