How to use the BMR calculator
- Choose your units — US (feet, inches, pounds) or metric (centimeters, kilograms).
- Enter your sex, age, height and weight. The equations are built for adults, so the calculator accepts ages 18 to 100.
- Add your body fat percentage if you know it from a scan, calipers or the body fat calculator. It is optional and adds a third estimate based on lean mass.
The headline is your BMR from the Mifflin-St Jeor equation, in calories (kcal) per day. Next to it you see the revised Harris-Benedict result, and below it the same BMR in kilojoules, per hour, and for a sedentary day. The table shows each equation worked out with your numbers.
What BMR means
Basal metabolic rate is the energy your body needs to stay alive while doing nothing at all: breathing, pumping blood, keeping your brain and organs working, repairing cells and holding your temperature steady. For most people it is the largest part of the calories they burn in a day. It depends mainly on body size and especially on lean mass — muscle and organs use more energy than fat — which is why it is usually higher in men and in taller, heavier or more muscular people.
BMR vs RMR vs TDEE
| Term | What it measures |
|---|---|
| BMR (basal metabolic rate) | Energy at complete rest, measured in a lab after an overnight fast, lying still and awake in a comfortable room |
| RMR (resting metabolic rate) | Energy at rest under less strict conditions; usually a little higher than BMR |
| TDEE (total daily energy expenditure) | Everything you burn in a day: BMR plus digesting food, exercise and everyday movement |
Strictly speaking, Mifflin-St Jeor and the revised Harris-Benedict equation predict resting energy expenditure, but in everyday use BMR and RMR are treated as the same number because the difference is small. TDEE is the figure for diet planning: BMR multiplied by an activity factor from 1.2 for a desk job with no exercise up to about 1.9 for hard daily training. The row "Sedentary day (BMR × 1.2)" gives the lowest of those levels; for your own activity level and calorie targets, use the TDEE calculator.
The three BMR equations
- Mifflin-St Jeor (1990) — men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5; women: the same with − 161 instead of + 5.
- Harris-Benedict, revised by Roza and Shizgal (1984) — men: 88.362 + 13.397 × kg + 4.799 × cm − 5.677 × age; women: 447.593 + 9.247 × kg + 3.098 × cm − 4.330 × age.
- Katch-McArdle — 370 + 21.6 × lean body mass (kg), where lean mass = weight × (1 − body fat %). It ignores sex and age and works from body composition instead.
In US units the calculator first converts pounds to kilograms (× 0.4536) and inches to centimeters (× 2.54), because all three equations were published in metric units. A 2005 systematic review in the Journal of the American Dietetic Association compared the common equations with measured metabolism in healthy adults of normal weight and with obesity and found Mifflin-St Jeor the most reliable; the Harris-Benedict equation tends to come out somewhat higher. Katch-McArdle can suit very lean or very muscular people better, but only if the body fat figure is accurate.
Worked example
A 35-year-old woman who is 165 cm (about 5 ft 5 in) tall and weighs 68 kg (about 150 lb):
- Mifflin-St Jeor: 10 × 68 + 6.25 × 165 − 5 × 35 − 161 = 680 + 1,031.25 − 175 − 161 = 1,375 kcal a day (about 5,754 kJ, or 57 kcal an hour).
- Revised Harris-Benedict: 447.593 + 628.80 + 511.17 − 151.55 = 1,436 kcal.
- Katch-McArdle at 32% body fat: lean mass 68 × 0.68 = 46.24 kg, so 370 + 21.6 × 46.24 = 1,369 kcal.
The three answers fall within about 70 kcal of each other, which is typical. On a sedentary day she would burn roughly 1,375 × 1.2 ≈ 1,650 kcal. For a 30-year-old man of 180 cm and 80 kg, the same equations give 1,780, 1,854 and — at 15% body fat — 1,839 kcal.
What changes your BMR
- Body size and muscle. More lean mass means a higher BMR. Strength training that adds muscle raises it modestly.
- Age. The equations subtract a few calories per year, mostly reflecting the loss of muscle with age. A large 2021 study in Science found that, adjusted for body size and composition, metabolism stays fairly stable from about 20 to 60 and declines after that.
- Weight loss. A lighter body burns less, so recalculate after every 5 kg (10 lb) lost.
- Health and hormones. Thyroid disorders, fever, some medications, pregnancy and breastfeeding all change energy use in ways no equation captures.
Limits of the estimate
Every equation predicts the average person with your measurements. Even the best one misses the measured value by more than 10% for a sizable minority of people, more often in people with obesity, older adults and athletes. The only exact BMR comes from indirect calorimetry, a breathing test some clinics and sports labs offer. Do not use your BMR as an eating target: eating that little leaves nothing for daily activity. Talk to your doctor or a registered dietitian before a large change in diet, especially if you are pregnant, breastfeeding, under 18 or living with a chronic condition.
Important: results are estimates for general information only and are not medical advice. They cannot replace an assessment by a doctor, midwife, registered dietitian or other qualified health professional.
Sources
- Mifflin MD, St Jeor ST et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr, 1990.
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated. Am J Clin Nutr, 1984.
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc, 2005.
- Mayo Clinic, Metabolism and weight loss: How you burn calories.
Frequently asked questions
What is a normal BMR?
There is no single normal value, because BMR scales with body size, sex and age. As examples, the Mifflin-St Jeor equation gives about 1,375 kcal a day for a 35-year-old woman of 165 cm and 68 kg and about 1,780 kcal for a 30-year-old man of 180 cm and 80 kg.
Which BMR equation is the most accurate?
For most healthy adults, Mifflin-St Jeor: a 2005 systematic review found it came within 10% of measured resting metabolism more often than the other common equations. Katch-McArdle can fit very lean or muscular people better if their body fat percentage is reliable.
Should I eat my BMR to lose weight?
No. BMR is what you burn lying still all day. Eating only that much leaves nothing for movement and is often below commonly cited minimums. A deficit of 250–500 kcal below your total daily energy expenditure is a more common starting point; ask a professional before going lower.
Is BMR the same as RMR?
Almost. BMR is measured under strict lab conditions (overnight fast, lying still, comfortable temperature); RMR is measured under looser conditions and usually comes out slightly higher. The equations on this page are used for both.
How do I convert BMR from calories to kilojoules?
Multiply by 4.184. A BMR of 1,500 kcal a day is about 6,276 kJ. The calculator shows the kilojoule figure in the second results row.
Does BMR drop with age?
The equations take off about 5 kcal per year of age (Mifflin-St Jeor), mostly reflecting lost muscle. Keeping active and doing strength training helps preserve lean mass and, with it, your BMR.
Last updated: October 19, 2026