Understanding Resting Metabolic Rate

I often tell people: here’s what the evidence shows, but remember that you are a study of one.

Research helps us understand what is generally true across populations, but every population is made up of individuals. Each person brings their own nuances: genetics, lifestyle, training history, environment, body composition, sleep, stress, and even the circumstances under which they were tested.

Those differences matter when we apply broad research findings to a specific person.

Resting metabolic rate is a good example. There are well-established ways to estimate or measure it, but the number you get can vary depending on the method used, the assumptions built into the equation, and what was happening in your body on the day of testing.

Increasingly, I am being asked: What is resting metabolic rate? Is it the same as basal metabolic rate? How do you calculate it? And if two methods give you two different numbers, which one should you trust?

While simple online calculators make it easy to estimate resting (or basal) metabolic rate, interpretation of the results is not straightforward.

First: RMR vs BMR

Resting metabolic rate, or RMR, is the amount of energy your body uses at rest. It reflects the calories required to keep basic functions running: breathing, circulation, temperature regulation, brain activity, organ function, and cellular maintenance.

Basal metabolic rate, or BMR, is similar, but technically stricter. BMR is measured under more controlled “basal” conditions, usually after an overnight fast, complete rest, and no recent activity. RMR is also measured at rest, but the protocol is usually a little less rigid.

In everyday conversation, people often use RMR and BMR interchangeably. For practical nutrition planning, RMR is usually the more relevant term.

Why RMR matters

RMR is usually the largest part of total daily energy expenditure. In sedentary people, it can account for roughly 60–80% of total energy burned per day. In athletes or very active people, the proportion may be lower because exercise and daily movement contribute more.

Total daily energy expenditure, or TDEE, includes:

• Resting metabolic rate
• The thermic effect of food, which is the energy used to digest and process food
• Exercise activity
• Non-exercise movement, such as walking, standing, fidgeting, housework, and occupational movement

So RMR is only one part of the full picture.

How RMR is calculated or measured

There are three common ways people get an RMR number.

1. The mask: indirect calorimetry

Indirect calorimetry is the reference method for measuring RMR (also called resting energy expenditure) in clinical and research settings.

This is the test where you sit or lie quietly while wearing a mask or using a mouthpiece. The device measures oxygen consumption and carbon dioxide production. From those values, resting energy expenditure is calculated.

This is a measured value under test conditions, not a prediction equation. That makes it more accurate than a calculator, but not immune to error.

Testing protocol is important here.

Indirect calorimetry results are most reliable when the test is done under standardized conditions:

• Fasted for at least 7 hours
• Rested quietly before the test
• No recent vigorous exercise, ideally for about 14–24 hours
• No caffeine or nicotine beforehand
• No major acute stress
• Comfortable, quiet testing conditions
• Steady breathing during the measurement

If those conditions were not met, the result may be biased.

For example, recent exercise, caffeine, nicotine, stress, poor sleep, a warm testing environment, or not being fully rested can all push the number away from a true resting value.

2. DXA-based estimate using fat-free mass

A DXA scan measures body composition, including fat mass, lean tissue, and bone mineral content. Fat-free mass is determined by subtracting fat mass from total mass.

Fat-free mass can then be entered into an RMR prediction equation. 

This is a calculated estimate, not a measured RMR.

That distinction matters. The DXA scan is measuring body composition, not metabolism. The metabolic rate is then inferred from the amount of fat-free mass.

This method is different from standard weight-based equations like the Harris-Benedict equation and the weight-based Mifflin-St Jeor, which use weight, height, age, and sex rather than body composition.

Because fat-free mass includes metabolically active tissues–muscle in particularly–a body-composition-based equation may be more  accurate than a weight-only equation for people with higher muscle mass, lower muscle mass, or body composition that differs from the population average.

But it is still built from population data, which means that it does not know you perfectly.

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3. Weight-based prediction equations

The most common calculator-style methods use weight, height, age, and sex.

Examples include weight-based Mifflin-St Jeor and Harris-Benedict.

These equations are easy to use and can be helpful for rough planning. Their biggest limitation is that they do not directly account for body composition.

Two people can have the same age, sex, height, and weight but very different amounts of muscle and fat. A weight-based equation may give them the same estimated RMR, even though their actual energy needs may differ.

This is why these equations can be less accurate for athletic, muscular, older, or low-muscle individuals.

Why the numbers might differ

Different calculators (Harris-Benedict, Mifflin-St Jeor weight-based and FFM-based) will produce different results. 

A standard calculator estimates RMR from body size, age, and sex. A DXA-based equation estimates RMR from fat-free mass. 

Indirect calorimetry measures oxygen consumption and carbon dioxide production under resting test conditions. When done under the correct conditions this is the most accurate, but it is also not feasible for most people. 

Which number should you trust?

If indirect calorimetry was done properly, it is generally the number to trust most. However, this isn’t feasible for most people. You need access to specific equipment and it needs to be done under the right conditions. 

If you did the test after coffee, after a workout, while stressed, sleep deprived, not fasted, in a warm environment, or without resting beforehand, the result may not reflect your true resting metabolic rate.

In that case, compare the indirect calorimetry result with the DXA-based estimate and the standard equation estimate. If the numbers are meaningfully different and you are using the result to make nutrition decisions, consider repeating indirect calorimetry under better conditions.

This is where the study-of-one mindset becomes useful.

Different calculators can perform better or worse for different people. Body composition, age, activity level, and ethnicity all influence which equation is most accurate for any given individual. For example, weight-based equations tend to underestimate RMR in people with high muscle mass, while FFM-based equations tend to be less accurate in people with overweight or obesity. Older adults tend to get more accurate results from the Harris-Benedict equation, while younger active adults and athletes tend to get better estimates from FFM-based equations like Cunningham or the FFM-based Mifflin-St Jeor. No single equation is consistently accurate for more than about 70% of people at the individual level, and even the best equations can be off by 300 kcal/day or more.

Then observe what happens over the next few weeks. If your weight, body composition, performance, energy levels, and hunger patterns are moving in the direction you want, the estimate is probably close enough to be useful. If they are not, adjust and reassess.

Population-based equations can give you a starting point. Your own response tells you whether that starting point works for you.

What to do with the number

RMR is not your daily calorie target.

It is the starting point for estimating total daily energy expenditure.

A simple way to estimate TDEE is to multiply RMR by an activity factor:

For example, if your RMR is 1,600 kcal/day and you are moderately active:

1,600 × 1.6 = 2,560 kcal/day

That gives you an estimated TDEE of about 2,560 kcal/day.

From there, the target depends on the goal.

The math gives you a starting point. The next step is observing whether that starting point behaves the way you expected.

If your estimated TDEE says 2,500 kcal/day but your weight is increasing over several weeks, your actual intake is probably above your actual expenditure. If your weight is dropping quickly and performance is suffering, the deficit may be too aggressive.

Use 2–4 weeks of body weight trends, training performance, hunger, energy, and body composition goals to adjust.

The bottom line

RMR is the amount of energy your body uses at rest. BMR is a stricter version of the same general concept, but RMR is the more commonly used term in practical testing.

There are three common ways to get an RMR number:

• A standard calculator estimates it from weight, height, age, and sex.
• A DXA-based equation estimates it from fat-free mass.
• Indirect calorimetry measures it from oxygen consumption and carbon dioxide production.

If done under proper conditions, indirect calorimetry is generally the best number to trust. But no method is perfect, and no single number should dictate your nutrition plan forever.

Use RMR as a starting point. Estimate TDEE. Choose a calorie target based on the goal. Then adjust based on real-world trends over 2–4 weeks.