Basal Metabolic Rate (BMR): What It Is and How to Calculate It

Basal metabolic rate (BMR) is the energy your body uses at complete rest to keep you alive: breathing, circulation, organ function and keeping cells running. Resting energy makes up about two-thirds of the calories you burn in a day (Hall et al., 2012). You can estimate it with the Mifflin-St Jeor equation, and the TDEE calculator does it for you and adds your activity on top.

BMR, RMR and TDEE

You will see three terms used almost interchangeably.

• BMR (basal metabolic rate) is measured under strict lab conditions: lying down, after an overnight fast, rested and at a comfortable temperature.
• RMR or REE (resting metabolic rate, resting energy expenditure) is measured at rest under looser conditions. The prediction equations people use online, including Mifflin-St Jeor, were built from resting energy measurements, so calculators that say "BMR" are giving you this estimate.
• TDEE (total daily energy expenditure) adds the energy used to digest food and every bit of movement. It is what you need to eat to keep your weight stable, covered in maintenance calories.

BMR formulas

Mifflin-St Jeor

This equation comes from resting energy measured by indirect calorimetry in 498 healthy adults aged 19 to 78, both normal weight and obese (Mifflin et al., 1990):

• 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

If you use pounds and inches, convert first: kg = lb ÷ 2.205 and cm = inches × 2.54.

The fat-free mass version

In the same study, fat-free mass was the best single predictor of resting energy: BMR = 19.7 × fat-free mass (kg) + 413. You need a body fat estimate to use it. Fat-free mass = weight × (1 − body fat %), and the body fat calculator gives you a tape-measure estimate.

Harris-Benedict

The oldest equation still in wide use dates from 1918. In the Mifflin study it overestimated measured resting energy by about 5% on average.

A worked example

Two people, with made-up numbers:

Here both formulas agree, because these two people have average body composition for their size. They split apart for someone with much more muscle or much more fat than average. A lifter of 90 kg at 10% body fat would get about 2,010 kcal from the fat-free mass formula, while Mifflin-St Jeor would still say 1,924.

Age moves the formula less than people expect. Mifflin-St Jeor takes off 5 kcal for every year, which is 50 kcal a decade. Same height and weight, different ages:

Forty years cost 200 kcal in the formula. A bigger drop in real life usually comes from changes in weight, muscle and activity, which the next section covers.

To turn BMR into a daily target, multiply by an activity factor. The TDEE calculator uses factors from 1.2 for a desk job with no exercise to 1.9 for hard training plus a physical job.

What makes BMR higher or lower

A study of 150 adults split the differences in BMR between people into their causes. Fat-free mass explained 63%, fat mass 6% and age 2%. Once those were accounted for, sex had no significant effect, and 26% of the variation remained unexplained (Johnstone et al., 2005).

In practice:

• Body size and muscle: bigger bodies, and bodies with more lean tissue, burn more at rest. This is most of the reason men usually have a higher BMR than women.
• Age: in a database of energy expenditure measured in people aged 8 days to 95 years, expenditure adjusted for body size stayed stable from about 20 to 60 years and declined in older adults (Pontzer et al., 2021). Much of the slow drop people notice in their 30s and 40s comes from losing muscle and moving less.
• Dieting: resting energy falls when you lose weight, partly because there is less of you, and there is also evidence of an active reduction during calorie restriction that grows with its severity (Hall et al., 2012).
• Unexplained individual differences: about a quarter of the variation in the Johnstone study had no known cause. Two people with identical stats can differ.

Want to raise your BMR with more muscle?

Get a muscle-building routine that fits your body with a simple scan.

Get

How much does muscle raise your BMR?

Fat-free mass explains most of the difference between people, so it is tempting to think a few kilos of muscle will transform your metabolism. The tissue numbers say otherwise. A study that combined resting energy measurements with MRI scans of organs and tissues in 131 healthy adults confirmed these approximate resting rates for adults up to 50 (Wang et al., 2010):

Per kilo, muscle burns about three times as much as fat at rest, but the organs do most of the work. Gaining 2 kg of muscle adds roughly 26 kcal a day to your BMR. Muscle still matters for strength, for how you look and for how much you can move, and moving is where the bigger calorie differences are.

BMR and the menstrual cycle

A 2020 meta-analysis of 26 studies with 318 women found that resting metabolism was slightly higher in the luteal phase, the second half of the cycle, than in the follicular phase. In the studies published since 2000, the difference was smaller and no longer statistically significant, and the authors flagged small samples and weak control of other factors (Benton et al., 2020). Whatever the size of the effect, it is small next to the error of any BMR formula, so it is not worth changing your targets week by week.

How accurate are BMR calculators?

A systematic review compared the four most common equations. Mifflin-St Jeor predicted resting energy within 10% of the measured value in more people, both nonobese and obese, than the other equations. The authors also noted that errors can be large for individuals and that older adults and US ethnic minorities were underrepresented (Frankenfield et al., 2005).

Athletes are another group where formulas struggle. A meta-analysis of 29 studies in athletes found that Mifflin-St Jeor significantly under- or overestimated measured resting energy, while equations such as Cunningham and Harris-Benedict did not differ significantly on average. Even the most precise equation in that review got 80% of participants within 10% (O'Neill et al., 2023). If you are very lean and muscular, the fat-free mass formula above is worth trying.

The only way to measure resting energy directly is indirect calorimetry in a clinic or lab. For most people, an estimate plus a few weeks of tracking weight is accurate enough.

Common mistakes

• Don't eat your BMR as a diet target. BMR covers rest only. Your deficit should start from your TDEE, and the calorie deficit to lose weight article covers how big it should be.
• Don't trust all four digits. "1,847 kcal" looks precise, but the real value may be 150 kcal either way.
• Don't ignore body composition. If you carry a lot of muscle or a lot of fat for your height, the standard formula is less reliable.
• Don't blame a "slow metabolism" too early. Check your logging and your daily step count before assuming your BMR is unusually low. If you suspect a thyroid problem or another medical cause, see a doctor.

This article is general information. If you have a medical condition, are pregnant or breastfeeding, or have had an eating disorder, talk to a doctor or a registered dietitian before you change how much you eat.