Nutrition

How to Calculate BMR (Basal Metabolic Rate)

By Harun Karahan · July 6, 2026 · Updated September 24, 2026 · 6 min

How to calculate BMR using the Mifflin St Jeor formula. Understand your basal metabolic rate and use it for accurate calorie targets.

Your basal metabolic rate (BMR) is the energy your body uses at complete rest: breathing, circulating blood, keeping your organs working and maintaining body temperature. For most adults it is the largest part of daily calorie burn, usually 60–70% of the total. That makes it the foundation of every calorie target.

This guide shows you how to calculate BMR by hand, compares the main equations, explains what makes your BMR higher or lower, and shows how to use the result. The BMR calculator does the same calculation instantly, and compares it with the Harris-Benedict result.

BMR vs RMR

Strictly, BMR is measured in a lab after an overnight fast and a night's sleep, lying still in a warm room. Resting metabolic rate (RMR) is measured under less strict conditions and usually comes out slightly higher. The equations used by calorie calculators, including Mifflin-St Jeor, were built from resting measurements. So in practice, "BMR" on a calculator means your estimated resting burn. For setting a calorie target, the difference doesn't matter.

The Mifflin-St Jeor equation

Mifflin-St Jeor was published in 1990 from measurements of about 500 healthy adults aged 19 to 78, including people with normal weight and with obesity. A 2005 systematic review found that it predicted measured resting metabolic rate within 10% for more people than the other common equations, so dietitians use it as the default for healthy adults.

• Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
• Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161

The result is in kilocalories (kcal) per day, the unit food labels call "calories".

Worked example (metric)

A 35-year-old woman, 165 cm tall, weighing 75 kg:

1. Weight: 10 × 75 = 750
2. Height: 6.25 × 165 = 1,031.25
3. Age: 5 × 35 = 175
4. BMR = 750 + 1,031.25 − 175 − 161 = 1,445 kcal a day

Worked example (imperial)

Convert first: kilograms = pounds × 0.4536, and centimetres = inches × 2.54.

A 40-year-old man, 6 ft 0 in (72 in) tall, weighing 200 lb:

1. Weight: 200 lb × 0.4536 = 90.7 kg, so 10 × 90.7 = 907
2. Height: 72 in × 2.54 = 182.9 cm, so 6.25 × 182.9 = 1,143
3. Age: 5 × 40 = 200
4. BMR = 907 + 1,143 − 200 + 5 = 1,855 kcal a day

Other BMR equations

Mifflin-St Jeor isn't the only option. Here is how the three most common equations compare for the same two people:

• Harris-Benedict dates from 1919 and was revised in 1984. It tends to give slightly higher estimates than Mifflin-St Jeor, which the 2005 review found to be more accurate.
• Katch-McArdle (BMR = 370 + 21.6 × lean body mass in kg) ignores age and sex and uses your fat-free mass instead. It can be better for very muscular people, but only if you know your body fat percentage accurately. Estimates from bathroom scales and most smart devices are often several percentage points off.

For most people, the equations agree within about 100 kcal. Picking the "right" equation matters far less than how accurately you estimate your activity level and how honestly you log food.

What makes your BMR higher or lower

• Fat-free mass. Muscle, organs and bone use more energy than fat. In a 2005 study of 150 adults, fat-free mass explained about 63% of the differences in BMR between people, far more than any other factor. This is the main reason men, taller people and more muscular people have higher BMRs.
• Body size. A larger body, including more fat mass, needs more energy to maintain. That's why BMR drops as you lose weight.
• Age. In the equations, each extra year lowers BMR by 5 kcal. Much of the real-world decline comes from losing muscle. A 2021 study of more than 6,000 people found that, once body size is accounted for, energy expenditure stays fairly stable from about age 20 to 60 and declines only after that.
• Hormones and health. Thyroid hormones affect metabolic rate, and so do fever, illness and some medications. If your weight changes a lot without a change in diet or activity, see a doctor rather than a calculator.
• Individual variation. About a quarter of the differences between people in that 2005 study couldn't be explained by any measured factor. Two people with identical stats can differ by 100–200 kcal.

Here is how age alone changes the estimate for the 165 cm, 75 kg woman in the example:

Staying strong as you age, through strength training and enough protein, is the most effective way to limit that decline.

How to use your BMR

BMR isn't your calorie target. It leaves out everything you do beyond lying still, and even a desk-bound day burns about 20% more. To turn BMR into a daily number:

1. Multiply it by an activity factor to get your total daily energy expenditure (TDEE). A sedentary factor of 1.2 turns the woman's 1,445 kcal into 1,734 kcal. The full table is in how to calculate TDEE.
2. For weight loss, subtract a moderate deficit from TDEE. How many calories to eat to lose weight shows how to choose one.
3. Track your weight trend for 2–3 weeks and adjust, because every equation is an estimate for an average person.

BMR is also a useful sanity check. If a diet plan sets your intake well below your BMR, it is asking for a very large deficit. That is hard to sustain, makes muscle loss more likely, and makes it harder to meet your protein and micronutrient needs. Eating below BMR isn't dangerous in itself for a short time, but without medical supervision it rarely makes sense.

FAQ

Is a higher BMR better?

Not in itself. A higher BMR means you can eat more at maintenance, which some people find easier. The healthy way to raise it is to build or keep muscle through strength training.

Can I speed up my metabolism?

Only modestly. Building muscle raises BMR a little, and daily movement raises total burn much more. Foods and supplements marketed as "metabolism boosters" have negligible effects.

How accurate are online BMR calculators?

For most healthy adults, Mifflin-St Jeor estimates fall within about 10% of lab measurements. In the 2005 review, errors were larger in people with obesity, and there was little data on older adults and some ethnic groups. Treat the result as a starting point and correct it with your real weight trend.

Does a fitness watch measure my BMR?

No. Watches and smart scales estimate BMR with the same kinds of equations, sometimes adjusted with heart rate or body-fat readings. Only indirect calorimetry (a breathing test) actually measures it.

Should I use my current weight or my goal weight?

Your current weight. Using your goal weight underestimates your needs and creates a much bigger deficit than you intended. Recalculate as your weight changes.

Sources

• Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990.
• 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.
• Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr. 1984.
• Johnstone AM, et al. Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. Am J Clin Nutr. 2005.
• Pontzer H, et al. Daily energy expenditure through the human life course. Science. 2021.