Your basal metabolic rate (BMR) is the number of calories your body burns just to keep you alive at complete rest, things like breathing, circulating blood, and regulating temperature. It typically accounts for 60 to 70 percent of the calories you burn each day.
Knowing your BMR is the starting point for any calorie or weight goal. Multiply it by an activity factor and you get your total daily energy expenditure (TDEE), the figure you'd use to plan a cut, bulk, or maintenance diet. This BMR calculator works in both metric and US units and updates instantly as you type.
How the bmr calculator works
This calculator uses the Mifflin-St Jeor equation, widely regarded as the most accurate BMR formula for the general population. The exact formula is:
BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age + sex constant
The sex constant is +5 for men and -161 for women. The result is your basal metabolic rate in calories per day (kcal/day).
You need four inputs: weight, height, age, and biological sex. If you enter US units (pounds and feet/inches), the calculator converts them to kilograms and centimeters first, then applies the formula. Heavier, taller, younger people have higher BMRs because they have more metabolically active tissue, while BMR naturally declines with age. The equation was published by Mifflin and St Jeor in 1990 and validated against indirect calorimetry, so it reflects real measured metabolism rather than a rough guess.
Worked example
Take a 30-year-old man who weighs 80 kg and is 180 cm tall:
BMR = (10 x 80) + (6.25 x 180) - (5 x 30) + 5 BMR = 800 + 1125 - 150 + 5 BMR = 1780 kcal/day
So his body burns about 1,780 calories per day at complete rest. A woman with the same stats would use -161 instead of +5: 800 + 1125 - 150 - 161 = 1,614 kcal/day. To estimate total daily calories burned, multiply BMR by an activity factor (typically 1.2 to 1.9).
Things to keep in mind
- BMR estimates from any formula are population averages and can be off by 5 to 10 percent for an individual, since they don't measure your actual metabolism.
- The Mifflin-St Jeor equation doesn't account for body composition, two people of the same weight can have different BMRs if one has more muscle. For lean or very muscular individuals, a lean-body-mass-based formula may be more accurate.
- BMR alone is not a calorie target. You need to multiply by an activity factor (giving TDEE) before using it to plan eating or weight goals.
- These numbers are general fitness estimates, not medical advice. Consult a doctor or registered dietitian before starting a significant calorie deficit or surplus.
Frequently asked questions
What is BMR and why does it matter?
BMR (basal metabolic rate) is the number of calories your body burns at complete rest just to stay alive, breathing, circulating blood, and maintaining organs. It usually makes up 60 to 70 percent of your daily calorie burn, so it's the foundation for any calorie or weight-management plan.
What formula does this BMR calculator use?
It uses the Mifflin-St Jeor equation: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age, plus 5 for men or minus 161 for women. This 1990 formula is considered the most accurate BMR estimate for most healthy adults.
What's the difference between BMR and TDEE?
BMR is the calories you burn at complete rest. TDEE (total daily energy expenditure) is your full daily burn, including movement, exercise, and digestion. You get TDEE by multiplying BMR by an activity factor, usually between 1.2 (sedentary) and 1.9 (very active).
How accurate is the Mifflin-St Jeor BMR formula?
It's accurate within about 10 percent for most healthy adults and is generally the most reliable equation for the general population. Accuracy drops for people with unusually high or low muscle mass, since the formula uses total body weight rather than body composition.
Should I eat my BMR number of calories?
No. Your BMR is the calories you'd burn lying in bed all day, so eating only that amount usually creates too large a deficit. Use TDEE instead, then adjust up or down for your goal. Eating below BMR long-term is not recommended without professional guidance.
Does BMR change as I get older?
Yes. BMR generally declines with age, the Mifflin-St Jeor formula subtracts 5 calories per year of age. This reflects gradual loss of muscle mass and metabolic activity, which is one reason strength training to preserve muscle becomes more valuable over time.