Basal Metabolic Rate (BMR): What It Is and How It’s Estimated

BMR, total daily energy expenditure, and what they mean

Basal metabolic rate (BMR) is the energy your body uses at complete rest to maintain vital functions, including breathing, heartbeat, body temperature, and brain and organ activity. For adults, it usually makes up the largest share of daily energy expenditure.

Total daily energy expenditure adds the energy used to digest food (the thermic effect, about 10% of the total) and all the energy used for movement: work, commuting, household tasks, and exercise. That’s why two people with the same BMR can have very different total energy expenditure.

How equations estimate BMR

Measuring your actual BMR requires indirect calorimetry, a lab test that measures how much oxygen you use. In everyday use, BMR is estimated with equations developed from measurements of groups of people. The two best-known equations are:

• Mifflin-St Jeor (1990): 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years), + 5 for men or − 161 for women. This is the equation used in the Calorie Calculator.
• Harris-Benedict (1919, revised in 1984): uses different coefficients for weight, height, and age for men and women.

Worked example

A 30-year-old woman who weighs 65 kg and is 165 cm tall:

• Mifflin-St Jeor: 650 + 1,031.25 − 150 − 161 = 1,370 kcal/day;
• Revised Harris-Benedict: 447.593 + 9.247 × 65 + 3.098 × 165 − 4.330 × 30 ≈ 1,430 kcal/day.

The difference of about 60 kcal highlights the key point: these are estimates. Studies comparing the equations with lab measurements find individual errors of 10% or more for some people.

The activity factor adds more uncertainty

To estimate total daily energy expenditure, multiply BMR by an activity factor:

Choosing “moderately active” instead of “lightly active” changes the result by almost 250 kcal. That’s why people who track their diet with a dietitian often use the estimate as a starting point, then adjust based on actual weight changes over several weeks.

A man and a woman with the same height

Compare two 40-year-olds who are both 178 cm tall: a man who weighs 80 kg and a woman who weighs 80 kg. Using the Mifflin-St Jeor equation:

• Man: 800 + 1,112.5 − 200 + 5 = 1,717.5 kcal;
• Woman: 800 + 1,112.5 − 200 − 161 = 1,551.5 kcal.

The 166-calorie difference comes from the equation’s constant, which aims to reflect average differences in body composition between sexes (men tend to have more muscle mass). For a specific person, that average may not apply: a female athlete may have a higher BMR than a sedentary man of the same weight.

That’s also why some approaches use equations based on lean body mass, measured with tests such as a DEXA scan, rather than total weight for people with a high percentage of body fat. A healthcare professional can determine whether this more detailed approach is appropriate.

What affects BMR

• Muscle mass: Muscle burns more energy at rest than fat. Equations don’t account for how much muscle you have, so athletes tend to have a higher actual BMR than the estimate.
• Age: BMR gradually declines through adulthood, partly due to muscle loss.
• Body size: Larger people burn more energy.
• Hormones and health: Thyroid conditions, fever, and some medications can affect energy expenditure.
• Prolonged calorie restriction: The body may lower its energy expenditure in response to very restrictive diets.

One important note: Don’t use BMR as an “absolute minimum” calorie target without professional guidance, and avoid very restrictive diets. Pregnant or breastfeeding people, teens, and people with chronic conditions need personalized guidance. For a quick estimate based on your weight, try the BMI Calculator.

Sources: Mifflin MD, St Jeor ST et al., American Journal of Clinical Nutrition, 1990; Roza AM, Shizgal HM, American Journal of Clinical Nutrition, 1984; Dietary Guidelines for the Brazilian Population (Ministry of Health). Updated October 2026. This content is for informational purposes only and is not a substitute for advice from a doctor, dietitian, or other healthcare professional.

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