What is BMR?
Basal metabolic rate (BMR) is the number of calories your body burns in a day at complete rest, just to keep your heart beating, lungs breathing, brain working and body temperature steady. It's usually the largest part of your daily energy use, and it's the starting point for working out how much you need to eat.
This calculator estimates BMR from four inputs: sex, weight in kilograms, height in centimeters and age in years. The result is rounded to the nearest whole calorie (kcal) per day.
The Mifflin-St Jeor equation
The calculator uses the equation published by Mifflin, St Jeor and colleagues in the American Journal of Clinical Nutrition in 1990. They derived it from measured resting energy expenditure in 498 healthy adults, both normal-weight and obese.
- Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
- Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
Each extra kilogram adds 10 kcal a day, each extra centimeter adds 6.25 kcal, and each year of age takes away 5 kcal. The only difference between the two versions is the constant at the end.
Mifflin-St Jeor replaced the older Harris-Benedict equation (1919) as the usual default. A 2005 systematic review in the Journal of the American Dietetic Association compared the common prediction equations against measured values and found Mifflin-St Jeor was the most likely to land within 10% of a person's measured resting metabolic rate.
Worked example
A 35-year-old man who weighs 80 kg and is 180 cm tall:
- Weight term: 10 × 80 = 800
- Height term: 6.25 × 180 = 1,125
- Age term: 5 × 35 = 175
- Add them up and add the male constant: 800 + 1,125 − 175 + 5 = 1,755 kcal/day
For a 30-year-old woman who weighs 65 kg and is 165 cm tall: 650 + 1,031.25 − 150 − 161 = 1,370.25, which the calculator shows as 1,370 kcal/day.
If your measurements are in pounds or feet and inches, convert them first with the weight converter and length converter. 1 lb is 0.4536 kg and 1 inch is 2.54 cm.
BMR vs. daily calorie needs
BMR is a resting figure. You burn more than this every day, because moving, exercising and even digesting food all cost energy. Total daily energy expenditure (TDEE) is usually estimated by multiplying BMR by an activity factor:
| Activity level | Typical multiplier | Example (BMR 1,755) |
|---|---|---|
| Sedentary | 1.2 | 2,106 |
| Light | 1.375 | 2,413 |
| Moderate | 1.55 | 2,720 |
| Active | 1.725 | 3,027 |
| Very active | 1.9 | 3,335 |
The calorie calculator applies these same multipliers to the same Mifflin-St Jeor BMR and adds targets for losing or gaining weight.
What affects your BMR
The equation captures the biggest factors, but not all of them:
- Body composition. Muscle uses more energy at rest than fat. Two people with the same weight can have different BMRs, and the equation can't tell them apart.
- Age. The formula subtracts 5 kcal for every year, which reflects the average decline in lean mass and resting energy use with age.
- Health and hormones. Thyroid conditions, some medications, illness, pregnancy and breastfeeding all change energy needs and aren't part of the equation.
Limits of this calculator
- It's designed for adults. Mifflin-St Jeor was developed in people aged roughly 19 to 78, so results for children and teens aren't reliable.
- It reports one number, not a range. Treat it as an estimate that could be a couple of hundred calories off in either direction for an individual.
- It doesn't measure anything. The only way to know your actual BMR is indirect calorimetry, which measures oxygen use and carbon dioxide output in a clinic.
Related calculators
- Calorie calculator: turns BMR into daily calories for your activity level.
- BMI calculator: checks your weight category from the same height and weight.
- Body fat calculator: estimates body fat percentage, the composition detail BMR formulas can't see.
- Ideal weight calculator: reference weights for your height.
This calculator is for general information and doesn't replace advice from a doctor or registered dietitian.
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