What is basal metabolic rate (BMR)?

Basal metabolic rate is the minimal amount of energy the body spends on homeostatic processes, the background work that keeps you alive, such as breathing, circulating blood, keeping body temperature stable, and maintaining cells and organs. It is usually expressed in calories (kilocalories) per day.

A true BMR is measured under strict conditions: lying completely at rest after a night of sleep and about 12 hours without food, in a quiet room at a comfortable temperature. Daily life adds eating, moving, and changes in temperature, so the energy you use even on a very quiet day is higher than your BMR.

How BMR differs from RMR and TDEE

• BMR is the lowest level of energy expenditure, measured under the strict conditions above.
• RMR (resting metabolic rate, also called resting energy expenditure) is measured under less restrictive resting conditions. The terms are often used interchangeably, but BMR can be up to about 10% lower than RMR.
• TDEE (total daily energy expenditure) is the energy used across a whole day. It combines basal metabolism, diet-induced thermogenesis, and the energy cost of physical activity. Diet-induced thermogenesis, also called the thermic effect of food, is the energy used to digest and process what you eat.

Resting energy expenditure is typically the largest contribution to daily energy expenditure, but it is only one part of it. That difference matters as soon as you use BMR to think about how much to eat.

How does the BodyMetrics.Pro calculator estimate BMR?

Most people never have their BMR measured. Calculators estimate it instead, using prediction equations based on body size, age, and sex. The BodyMetrics.Pro BMR calculator uses the Mifflin-St Jeor equation:

• Men: BMR = 10 x weight (kg) + 6.25 x height (cm) - 5 x age (years) + 5
• Women: BMR = 10 x weight (kg) + 6.25 x height (cm) - 5 x age (years) - 161

If you enter pounds and inches, the calculator converts them to kilograms and centimeters first. The gender field selects which of the two versions is used.

Mifflin, St Jeor, and colleagues published the equation in 1990. They derived it from resting energy expenditure measured in 498 healthy adults aged 19 to 78, using indirect calorimetry, and included people of normal weight and people with obesity. Strictly speaking, it predicts resting energy expenditure, even though calculators commonly label the result BMR.

A worked example

Take a 35-year-old woman who is 165 cm tall and weighs 65 kg:

• 10 x 65 = 650
• 6.25 x 165 = 1,031.25
• 5 x 35 = 175
• 650 + 1,031.25 - 175 - 161 = 1,345.25, or about 1,345 calories per day

That number estimates her baseline energy use at complete rest. It is not a measurement, and it is not a recommended daily intake.

Why other calculators may show a different number

Several prediction equations exist, and they do not give identical results for the same person. Many websites still use the Harris-Benedict equation, which dates from the early 20th century. In the study that produced Mifflin-St Jeor, the 1919 Harris-Benedict equations overestimated measured resting energy expenditure by 5%.

A 2005 systematic review in the Journal of the American Dietetic Association compared the four equations most often used in clinical practice: Harris-Benedict, Mifflin-St Jeor, Owen, and WHO/FAO/UNU. Mifflin-St Jeor was the most reliable. It predicted resting metabolic rate within 10% of the measured value in more people, with and without obesity, than any other equation, and it had the narrowest error range. If another calculator gives you a different result, it may simply be using a different equation.

How accurate is a BMR estimate?

"Most reliable" does not mean exact. The same review found noteworthy errors and limitations when the equation is applied to individuals, and older adults and US ethnic minorities were underrepresented in the studies used to develop and validate prediction equations. Body size and composition explain much, but not all, of the differences between people: fat-free mass and fat mass explain only about 70% of the variation in resting energy expenditure, so two people with the same body composition can still differ by a few hundred calories a day.

Mifflin-St Jeor also uses total weight rather than body composition, so it cannot tell whether a given weight is mostly muscle or mostly fat. When an accurate value matters, for example in clinical care, resting metabolic rate can be measured with indirect calorimetry instead of estimated.

What affects your BMR?

Body size and composition

Larger bodies use more energy at rest, and fat-free mass, meaning everything in the body other than fat, is the strongest determinant of differences in BMR between people. Not all fat-free tissue is equally active, though. Estimates of tissue-specific metabolic rates, validated across adulthood, put skeletal muscle at about 13 calories per kilogram per day at rest and fat tissue at about 4.5, compared with roughly 200 to 440 for the liver, brain, heart, and kidneys.

Age

Age is one of the equation's inputs. In an analysis of measured energy expenditure in more than 6,000 people aged 8 days to 95 years, total and basal expenditure were stable from about age 20 to 60. After about 60, they declined along with fat-free mass and fat mass, and the decline was not explained by those changes in body composition alone.

Sex

For the same weight, height, and age, Mifflin-St Jeor gives men an estimate 166 calories a day higher than women. That gap reflects average differences in body composition: when resting energy use, measured as oxygen consumption, was expressed relative to body cell mass, the metabolically active part of the body, it did not differ between the sexes.

Thyroid hormones

Thyroid hormones control the way the body uses energy. When the thyroid makes too little hormone, many of the body's functions slow down, and when it makes too much, many of them speed up. A BMR calculator cannot detect or diagnose a thyroid problem.

Weight loss and metabolic adaptation

When you eat less and lose weight, energy expenditure tends to fall. Part of the drop comes from having a smaller body, and part from metabolic adaptation: reduced energy intake can lower energy expenditure by more than expected from changes in body composition. This is one reason an estimate that fits at the start of a weight change may not fit later.

How should you use your BMR?

BMR is a baseline, not a calorie target

BMR leaves out everything you do during the day and the energy used to process food, so it is lower than your total daily energy use. An intake equal to your BMR is therefore not a maintenance intake.

Maintenance starts from estimated TDEE

The BodyMetrics.Pro calculator also estimates TDEE by multiplying your BMR by a standard activity factor, from 1.2 for little or no exercise to 1.9 for very hard exercise combined with a physical job or twice-daily training. For the woman in the example, the calculator shows estimates from 1,614 calories a day at the lowest activity level to 2,556 at the highest.

Activity factors are broad averages, and your real needs can differ. The National Academies' guidance on energy requirements makes the same point: an individual's actual energy requirement may vary considerably from the estimate, so it is important to monitor body weight over time. Treat estimated TDEE as a starting point for maintenance, not an exact target.

Losing or gaining weight

• Losing weight requires taking in less energy than you expend over time. The comparison that matters is with your total expenditure, not with your BMR.
• Gaining weight requires taking in more energy than you expend over time. Eating more than your BMR is not enough, because an intake can be above BMR and still below TDEE. In the example, 1,500 calories a day would be above her estimated BMR but below every TDEE estimate in the calculator's table, so for her it would more likely create a deficit than a surplus.
• Gaining muscle takes more than extra calories, because energy intake alone does not decide whether added weight is muscle or fat. Resistance training also matters, as the next section explains.

No single deficit or surplus size suits everyone. Follow your weight trend over several weeks rather than day to day, and adjust as your weight, activity, and appetite change. If you have a medical condition, are pregnant or breastfeeding, or have a history of disordered eating, talk to a qualified professional before changing how much you eat.

Can you raise your BMR?

Much of what sets your BMR, such as height, age, and sex, is outside your control, and body composition changes slowly. A few points are worth knowing:

• Resistance training can add fat-free mass or help keep it. In a meta-analysis of trials in people with overweight or obesity, resistance training alone increased lean mass by about 0.8 kg compared with no training, and lean mass was maintained when resistance training was combined with calorie restriction. At roughly 13 calories per kilogram per day for muscle, a gain of that size raises BMR only modestly.
• Protein takes more energy to process than carbohydrate or fat: about 20 to 30% of its energy, compared with 5 to 10% for carbohydrate and 0 to 3% for fat. That raises the thermic effect of food, which is part of TDEE, not BMR itself.
• Physical activity, from planned exercise to everyday movement, adds to the activity part of TDEE rather than to BMR.

Common myths about BMR and metabolism

Myth: people with larger bodies have a slow metabolism

Not as a rule. People with obesity generally have a higher absolute resting energy expenditure, because they have more fat-free mass as well as more fat, and their resting energy use is generally in line with what is expected for their body size and composition.

Myth: eating less shuts down your metabolism

Energy expenditure does fall during calorie restriction, sometimes by more than changes in body composition would predict, and researchers still debate how long this metabolic adaptation lasts. It is a reduction in energy use that can slow weight loss, not a shutdown.

Myth: small, frequent meals speed up your metabolism

The main driver of the thermic effect of food is how much energy you eat, followed by the protein content of the diet. Studies that measured total 24-hour energy expenditure with whole-body calorimetry and doubly labeled water found no difference between eating many small meals and fewer larger ones.

When are BMR estimates less reliable?

• Teenagers and children. Mifflin-St Jeor was developed in adults aged 19 to 78, and growing bodies also need energy for growth: the estimated energy requirement for a growing child adds the energy cost of growth to total energy expenditure. Equations adapted for children, such as the WHO/FAO/UNU and Schofield equations, appear to give the best estimates of resting energy expenditure in children and adolescents.
• Older adults. Older adults were underrepresented in the studies used to develop and validate prediction equations, and energy expenditure declines after about age 60.
• Pregnancy and breastfeeding. Energy requirements include the energy cost of tissue growth during pregnancy and of milk production during breastfeeding, which BMR equations do not capture.
• Medical conditions. Conditions such as thyroid disorders can change energy use in ways a calculator cannot see.

In these situations, a doctor or registered dietitian can help interpret energy needs.

The bottom line

BMR estimates how much energy your body uses to stay alive at complete rest. The BodyMetrics.Pro calculator estimates it with the Mifflin-St Jeor equation and multiplies it by an activity factor to estimate TDEE. Use estimated TDEE, not BMR, as the starting point for thinking about maintenance calories, treat both numbers as estimates, and let your weight trend over several weeks guide any adjustments. You can try the BMR calculator to see your own estimates.