BMR Explained: What Your Basal Metabolic Rate Means

Your basal metabolic rate (BMR) is the number of calories your body burns at complete rest, just to keep your heart, lungs, brain and other organs working. For most adults it's about 1,200-1,900 calories a day, and it makes up roughly 60-70% of everything you burn.

The exact number depends mostly on how big you are, how much of that is lean tissue, your age and your sex. You can estimate it with an equation, but only a lab test with indirect calorimetry measures it.

Key takeaways

• Organs do most of the work. Per kilogram, your heart and kidneys burn about 34 times more than skeletal muscle at rest.
• Mifflin-St Jeor is the most reliable formula for most adults. Harris-Benedict tends to run about 5% high.
• Age matters less than you've heard. Size-adjusted energy use stays stable from 20 to 60.
• Muscle helps, modestly. About 6 calories per pound per day at rest.
• Hard dieting can lower resting burn beyond what weight loss alone explains.

What is basal metabolic rate?

BMR is the energy cost of staying alive while doing nothing. In a lab it's measured lying still, awake, after an overnight fast, in a room that is neither warm nor cold. Those conditions remove digestion, movement and temperature control from the picture.

Resting metabolic rate (RMR) is the same idea with looser rules, such as a shorter fast or less rest before the test, so it usually reads a little higher. The difference rarely matters for you, because the equations most calculators use, Mifflin-St Jeor included, were built from resting measurements anyway. You'll see the two terms used interchangeably.

Your total daily energy expenditure (TDEE) is BMR plus everything else: walking, exercise, fidgeting and digesting food. For a typical activity level, BMR is roughly 60-70% of the total. Our BMR vs TDEE guide covers which of the two to eat around.

What burns calories at rest?

Not all tissue costs the same to run. These are the resting rates first proposed by Elia and confirmed against MRI scans and calorimetry in 131 healthy adults (Wang 2010).

Your organs weigh only a few kilograms in total, but they burn at a far higher rate than anything else. Muscle burns little per pound, yet it adds up because you carry so much of it. Fat burns the least.

The same study found these rates run about 3% lower in adults over 50 (for example, 12.6 instead of 13 for muscle).

How BMR is calculated

Three equations show up in most calculators. W is weight in kg, H is height in cm and A is age in years.

Katch-McArdle needs your lean body mass, so it only helps if you have a decent body fat estimate. For a 70 kg (154 lb) person at 25% body fat, lean mass is 52.5 kg and the result is 1,504 calories.

Our BMR calculator runs these for you.

Which BMR formula is most accurate?

Mifflin-St Jeor came out of measurements on 498 healthy adults aged 19-78, both normal weight and obese. In that group, the old Harris-Benedict equation overestimated measured resting energy expenditure by about 5%.

A later systematic review for the American Dietetic Association compared Harris-Benedict, Mifflin-St Jeor, Owen and the WHO equation. Mifflin-St Jeor predicted resting metabolic rate within 10% of the measured value in more people than any other, in both nonobese and obese adults, and had the narrowest error range.

Here's what the formulas give for three people:

The gap is 60-100 calories a day, about 5-7%, close to the 5% Mifflin's team found. Even the best equation can be off for an individual, so treat the result as a starting point and adjust it with your real weight trend.

What is a normal BMR?

There's no single normal value, because BMR scales with body size. This table uses the Mifflin-St Jeor equation for a reference man (5 ft 10 in, 154 lb / 178 cm, 70 kg) and a reference woman (5 ft 4 in, 126 lb / 163 cm, 57 kg).

Taller, heavier or more muscular people sit higher. The formula drops 50 calories every 10 years for everyone, which is a population average baked into the math, not something your body has to follow.

What affects your BMR?

Body size and lean mass

This is the big one. In Mifflin's data, fat-free mass alone explained about 64% of the differences in resting energy between people. More lean tissue, especially larger organs and more muscle, means a higher BMR.

Sex

Men usually have a higher BMR at the same height and weight because they tend to carry more lean mass and less fat. That's why the equations have separate versions.

Age

The usual line is that metabolism slows a little every decade from your 20s. A 2021 study in Science, which measured daily energy use with doubly labeled water in people aged 8 days to 95 years, found something different: once you adjust for body size and fat-free mass, energy use stays stable from age 20 to 60, then declines in older adults. Much of the "slowing" people notice in midlife comes from losing muscle and moving less.

Dieting

When you lose weight, BMR falls because there is less of you to run. It can also fall further than the lost tissue explains. Contestants from "The Biggest Loser" had a resting metabolic rate 610 calories a day lower at the end of the 30-week show. Six years later it was still 704 calories below where they started, and about 499 calories of that gap couldn't be explained by changes in body composition or age.

That's an extreme case of very fast weight loss, but it shows why crash diets backfire. Our article on metabolic adaptation covers what you can do about it.

Food

Digesting food burns calories too, about 5-15% of daily energy on a mixed diet. Protein and alcohol cost the most to process, then carbs, then fat. Strictly speaking this sits on top of BMR, but it's another reason protein helps during a diet.

Can you increase your BMR?

Only a little, and mostly by changing what you're made of.

• Build muscle. At 13 calories per kg per day, 10 lb (4.5 kg) of new muscle adds about 59 calories a day at rest. Not much on its own, but you also burn more during the training that builds it.
• Avoid crash diets. A moderate deficit with enough protein protects lean mass, which protects your BMR.
• Keep moving. Activity doesn't change BMR much, but it's where most of the controllable part of your daily burn lives. The active vs resting calories guide breaks this down.

How to use your BMR

1. Estimate it with the BMR calculator, or the RMR calculator if you prefer that term.
2. Turn it into a daily target by multiplying for activity. The TDEE calculator and our step-by-step TDEE for weight loss guide do this.
3. Set a moderate deficit from your TDEE, not your BMR. Resting calories still count toward your deficit, as explained in do resting calories count.
4. Check it against reality. After 2-3 weeks of logging, compare your average intake with your weight trend and adjust.

If you have a thyroid condition or another medical issue that affects metabolism, talk to your doctor before setting a target.

Nutrify sets a calorie target from your profile and lets you log meals by photo, barcode or voice, so you can see how your intake compares with your estimate. It has a 7-day free trial, then a paid subscription.

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.
• Wang Z, et al. Specific metabolic rates of major organs and tissues across adulthood. Am J Clin Nutr, 2010.
• Pontzer H, et al. Daily energy expenditure through the human life course. Science, 2021.
• Fothergill E, et al. Persistent metabolic adaptation 6 years after "The Biggest Loser" competition. Obesity, 2016.
• Westerterp KR. Diet induced thermogenesis. Nutr Metab, 2004.