What Is BMR? Your Basal Metabolic Rate, Explained Simply

What is BMR?

BMR, or basal metabolic rate, is the number of calories the body burns in a day at complete rest, just to breathe, pump blood, and stay warm. Most calculators estimate it with the Mifflin-St Jeor equation. For a 30-year-old woman who is 5 ft 6 in (167.6 cm) and weighs 140 lb (63.5 kg), that is about 1,372 calories.

You finish setting up a fitness app, and there it is: "BMR: 1,412." No explanation. Just a number that sounds important. Here's something most people don't realize: in the math our calculators use, your resting burn makes up more than half of your daily calories at every activity level. Want yours now? Try our BMR calculator.

What does BMR actually measure?

BMR measures the cost of keeping the lights on. That means breathing, pumping blood, keeping your brain working, and holding your body temperature steady.

You will also see the term RMR (resting metabolic rate) or REE (resting energy expenditure). These are close cousins of BMR. The research papers behind most calculators actually measured resting burn. For everyday use, you can treat them as the same idea.

How do scientists measure it? With a lab test called indirect calorimetry. You lie still while a machine measures the oxygen you breathe in and the carbon dioxide you breathe out. From that, researchers work out how much energy you are burning.

Most of us will never do that test. So we use formulas instead.

What is the difference between BMR and TDEE?

BMR is your burn at rest. TDEE is your BMR plus everything you do. This is where a lot of people get mixed up.

• BMR is your burn at rest.
• TDEE (total daily energy expenditure) adds walking, working, training, even fidgeting.

To go from BMR to TDEE, you multiply by an activity factor. Our calculators use:

• 1.2 (sedentary)
• 1.375 (lightly active)
• 1.55 (moderately active)
• 1.725 (very active)
• 1.9 (extremely active)

Your TDEE is your maintenance calories, the amount that keeps your weight steady. Our TDEE calculator does this step for you.

How is BMR calculated?

BMR is calculated with a formula that uses your weight, height, age, and sex, or your lean mass. The three you will see most often are Mifflin-St Jeor, revised Harris-Benedict, and Katch-McArdle.

Use weight in kilograms (1 lb = 0.4536 kg), height in centimeters (1 inch = 2.54 cm), and age in years.

1. Mifflin-St Jeor (1990)

• Women: (10 × weight) + (6.25 × height) - (5 × age) - 161
• Men: (10 × weight) + (6.25 × height) - (5 × age) + 5

Researchers at the University of Nevada built this from lab measurements of 498 healthy adults aged 19 to 78, both with and without obesity.

A 2005 systematic review by Frankenfield and colleagues for the American Dietetic Association found it was the most reliable of the common equations. It predicted resting burn within 10% of the measured value in more people than any other equation tested. That is why our calculators use it by default.

2. Revised Harris-Benedict (1984)

• Women: 447.593 + (9.247 × weight) + (3.098 × height) - (4.330 × age)
• Men: 88.362 + (13.397 × weight) + (4.799 × height) - (5.677 × age)

The original Harris-Benedict equation dates back to 1919. Roza and Shizgal updated it in 1984. They reported it estimates resting burn in healthy people with a precision of about 14%.

In the 1990 Mifflin study, the original 1919 version overestimated measured resting burn by 5%.

3. Katch-McArdle

• Everyone: 370 + (21.6 × lean body mass in kg)

This one skips weight, height, age, and sex. It only uses your lean body mass, meaning everything that is not fat: muscle, bone, organs, and water. You need to know your body fat percentage to use it.

Do the three BMR formulas give the same answer?

For an average woman, they land close together. Take a 30-year-old woman who is 5 ft 6 in (167.6 cm), weighs 140 lb (63.5 kg), and has 28% body fat. All three formulas sit within about 70 calories of each other.

Mifflin-St Jeor:

• 10 × 63.5 = 635
• 6.25 × 167.6 = 1,047.5
• 5 × 30 = 150
• 635 + 1,047.5 - 150 - 161 = about 1,372 calories

Revised Harris-Benedict:

• 9.247 × 63.5 = 587.2
• 3.098 × 167.6 = 519.2
• 4.330 × 30 = 129.9
• 447.6 + 587.2 + 519.2 - 129.9 = about 1,424 calories

Katch-McArdle:

• Lean mass = 63.5 × (1 - 0.28) = 45.7 kg (100.8 lb)
• 370 + (21.6 × 45.7) = about 1,357 calories

For her, the formulas mostly agree.

Why does lean mass matter for BMR?

Because fat-free mass was the best single predictor of resting burn in the original 1990 Mifflin study. It beat body weight alone. In plain words, what your body is made of matters, not just how much it weighs.

Here is how that plays out for a lifter. Take a 28-year-old man who is 5 ft 10 in (177.8 cm), weighs 180 lb (81.6 kg), and has 15% body fat:

For him, the lean-mass formula lands about 76 calories higher than Mifflin-St Jeor.

If you train hard and know your body fat percentage fairly well, Katch-McArdle may suit you better. Our body fat calculator and lean body mass calculator can help you get the inputs.

One honest caveat: Katch-McArdle is only as good as your body fat number. A rough guess can throw it off.

What changes your BMR?

Weight, height, age, sex, and lean mass all change the estimate. Look at the formulas and you can see what they treat as important:

• Weight: a bigger body costs more to run.
• Height: taller people get a higher estimate at the same weight.
• Age: in Mifflin-St Jeor, each extra year lowers the estimate by 5 calories.
• Sex: at the same weight, height, and age, the Mifflin-St Jeor estimate for men is 166 calories higher than for women. That is the gap between the +5 and the -161.
• Lean mass: as the Mifflin study found, fat-free mass tracks closely with resting burn.

How accurate is a BMR formula?

It is a good estimate, not a measurement. Every formula gives an answer for an average person with your stats, and you are not an average.

The Frankenfield review said this clearly: even the best equation has noteworthy errors for individuals. It also pointed out that older adults and US ethnic minorities were underrepresented in the research behind these equations.

So treat your BMR as a smart starting point, not a fact carved in stone.

If you have a health condition or take medicines that you think may affect your weight, talk to your doctor or a registered dietitian. They can tell you whether a lab measurement makes sense for you.

How do you use your BMR?

Use it as the first step toward a daily calorie target. You don't need to obsess over it. You just need to use it well:

1. Get your BMR from our BMR calculator.
2. Multiply by your honest activity level to get maintenance.
3. Adjust for your goal. Our guide on how many calories to eat to lose weight walks through that step.
4. Watch your weekly weight trend for a few weeks and fine-tune.

My take

What I find interesting is how close the three formulas landed in both examples here. My hunch is that the activity level we pick often matters more than which BMR formula we use, since the multipliers range from 1.2 to 1.9.

That isn't something the studies here tested, though. It is my opinion. So if your numbers feel off, check your activity setting first.

And if you lift, the lean-mass finding is good news. Lean mass is something you can work on in the gym, and the Mifflin study found it tracks closely with resting burn.

The bottom line

BMR is the energy your body burns at rest, and it is the foundation of every calorie target. Mifflin-St Jeor is the most reliable everyday formula. Katch-McArdle is a great option if you know your body fat.

Find your number with our BMR calculator, then build your plan on top of it with confidence.