Your basal metabolic rate (BMR) is the number of calories your body burns while doing absolutely nothing — lying awake in bed, fasting, warm and completely still. It is the energy required just to keep your heart beating, your lungs breathing, your liver processing, and your brain signaling. Roughly 60–70% of the calories you burn in a typical day go to this quiet baseline, which is why knowing your BMR is the foundation of any sensible eating, weight, or training plan.
This calculator uses the Mifflin-St Jeor equation, the formula that the Academy of Nutrition and Dietetics and most registered dietitians recommend because it predicts measured resting metabolism more accurately than the older Harris-Benedict equations. It is not a BMI score — BMI only compares weight to height and says nothing about calories — and it is not your TDEE either. Your BMR is the resting floor; your TDEE (total daily energy expenditure) is that floor multiplied by an activity factor. Start with the floor and you can never confuse the two.
How to Use
- Choose your Sex — male or female. The equation uses a sex-specific constant (+5 for men, −161 for women) because, on average, men carry more metabolically active lean mass at the same height and weight.
- Enter your Age in completed years. BMR declines naturally by about 1–2% per decade once adulthood is reached, mostly because lean mass slowly gives way to fat mass if activity is not maintained.
- Type your Height in centimeters. If you normally measure yourself in inches, multiply by 2.54 first (for example, 70 inches × 2.54 = 177.8 cm).
- Enter your Weight in kilograms. Pounds convert by multiplying by 0.4536 (for example, 154 lb × 0.4536 = 69.85 kg). Weigh yourself in the morning, before breakfast, for the most repeatable figure.
- Read the result card: your BMR in kcal per day, the same value in kilojoules, your resting burn per hour, and your resting energy over a full week.
Because the Mifflin-St Jeor equation is linear in weight, height, and age, every input you change moves the result instantly. A 1 kg change in weight shifts BMR by roughly 10 kcal per day — small on any single day, but about 70 kcal per week, which is why gradual weight change almost always sticks better than crash diets.
The Mifflin-St Jeor Equation
Published by Mifflin, St Jeor and colleagues in 1990, the equation estimates resting energy expenditure from four easy-to-measure variables:
- 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
Weight is the dominant lever: every kilogram of body costs about 10 kcal per day just to maintain. Height contributes about 6.25 kcal per centimeter, and each decade of life shaves off roughly 50 kcal per year of age × 5 = 250 kcal per decade. The sex constant (−161 for women) reflects the average difference in lean-to-fat composition at the same body size.
The result is an estimate, not a laboratory measurement. True resting metabolism is measured in a hood or a whole-room calorimeter, typically within ±10% of this equation for most adults. Highly muscular athletes often score above the estimate, and very sedentary adults below it.
How BMR Differs from BMI and TDEE
It is worth being explicit, because these three numbers get confused constantly:
- BMI (body mass index) = weight ÷ height². It is a population-level weight-category screening tool. It contains no calorie information at all.
- BMR = the Mifflin-St Jeor result on this page. It is your resting calorie floor, measured in kcal/day.
- TDEE = BMR × an activity multiplier (about 1.2 sedentary up to 1.725 very active). It is the total you actually burn in a day, and the number you compare against your food intake.
If your goal is to lose weight, the practical recipe is: estimate BMR here, multiply by your activity level to get TDEE, then eat a moderate 10–20% deficit below TDEE. Cutting below your BMR on purpose is unnecessary and counterproductive — your body already burns that much while you sleep.
Examples
Example 1 — A 30-year-old man, 175 cm, 70 kg. BMR = 10×70 + 6.25×175 − 5×30 + 5 = 700 + 1093.75 − 150 + 5 = 1,648.75 kcal/day (about 6,898 kJ, or 68.7 kcal per hour). With a desk job he multiplies by roughly 1.2 for a sedentary TDEE of about 1,978 kcal/day.
Example 2 — A 25-year-old woman, 163 cm, 55 kg. BMR = 10×55 + 6.25×163 − 5×25 − 161 = 550 + 1018.75 − 125 − 161 = 1,282.75 kcal/day (about 5,367 kJ, or 53.5 kcal per hour). A weekly resting energy budget of roughly 8,979 kcal means her body burns nearly 9,000 kcal at rest before any walking, cooking, or workout.
Example 3 — The effect of lean mass. Two 40-year-old people, 170 cm and 75 kg, compute to the same Mifflin-St Jeor BMR — about 1,677 kcal/day. In practice, the one who lifts weights carries more muscle and may actually rest-burn 5–10% more, while the one who sits all day may rest-burn a little less. The equation cannot see body composition, which is why trends matter more than a single digit.
FAQ
What is a good BMR for my age and weight?
There is no single "good" BMR. It is simply your resting calorie demand: roughly 1,300–1,500 kcal/day for an average adult woman and 1,600–1,800 kcal/day for an average adult man. What matters is whether your BMR lines up with your observed weight trend, not whether it falls in an arbitrary band.
How accurate is the Mifflin-St Jeor equation?
In validation studies it predicts measured resting metabolism within about ±10% for most adults, which is better than the original 1918 Harris-Benedict equations and close to the revised versions. Errors grow for athletes, people with very high or very low body fat, and anyone on medications that affect metabolism.
Why do men and women have different BMR constants?
The equation uses +5 for men and −161 for women because, at the same height, weight, and age, women on average carry a slightly higher percentage of body fat and less lean mass — and fat tissue burns fewer calories at rest than muscle tissue does. The constant is a population average, not a biological rule for any individual.
Does BMR change with weight loss?
Yes. Because weight is multiplied by 10, losing 5 kg lowers BMR by about 50 kcal/day. This is one reason weight loss slows down over time: your new body genuinely needs fewer calories to maintain itself. Rebuilding or preserving lean mass with resistance training and adequate protein is the best way to keep BMR from dropping.
How do I raise my BMR naturally?
Three evidence-backed levers: build and keep muscle (about 6 kcal per pound of muscle per day at rest), avoid crash diets that drop BMR by 10–20% through adaptive thermogenesis, and stay reasonably active — even walking adds up outside the BMR number itself. Sleep deprivation and large calorie deficits are the fastest ways to push resting metabolism down.
What is the difference between BMR and RMR?
BMR is measured under strict laboratory conditions after waking and a full overnight fast. RMR (resting metabolic rate) is measured more casually and is usually 10–20% higher. This calculator estimates BMR; treat the result as a clean, conservative resting floor.
Disclaimer
This BMR calculator is provided for informational purposes only and does not constitute medical, nutritional, or dietary advice. Results are estimates from the Mifflin-St Jeor equation and may differ from your actual measured resting metabolism by roughly ±10%. Do not use this tool to design a medical weight-loss plan, manage pregnancy, eating-disorder recovery, thyroid disease, diabetes, or any other clinical condition without supervision from a licensed physician or registered dietitian.