Calculating BMR and RMR

Diet and Weight Loss Tutorial

BMR and RMR are estimates of how many calories you would burn if you were to do nothing but rest for 24 hours. They represent the minimum amount of energy required to keep your body functioning, including your heart beating, lungs breathing, and body temperature normal.

  • BMR stands for Basal Metabolic Rate, and is synonymous with Basal Energy Expenditure or BEE. BMR measurements are typically taken in a darkened room upon waking after 8 hours of sleep; 12 hours of fasting to ensure that the digestive system is inactive; and with the subject resting in a reclining position.
  • RMR stands for Resting Metabolic Rate, and is synonymous with Resting Energy Expenditure or REE. RMR measurements are typically taken under less restricted conditions than BMR, and do not require that the subject spend the night sleeping in the test facility prior to testing.

Most people searching the internet for information on their metabolic rate search for BMR because the term is so widely used. In fact, as explained in the Technical Notes, below, RMR is likely to be more appropriate for your needs and is the more accurate estimation.

However, if you are looking for an estimate of how many calories you need or burn in a day, we suggest that you not use BMR or RMR at all. We suggest that you calculate the actual activities that you perform in a 24 hour period as described in Calculating Daily Calorie Needs.

We Do the Math

The BMR & RMR Calculator will calculate your BMR and RMR for you. And don't worry if you measure yourself in pounds, feet or centimeters, or even stones, we'll convert the numbers to fit the equations.

We explain the equations in detail in the Technical Notes, below. But whether or not such details interest you, you might find a few observations about them interesting:

  • When your age goes up, your BMR and RMR go down.
  • When your height goes down, your BMR and RMR go down.
  • When your weight goes down, your BMR and RMR go down.

This means that as you get older, shorter, and lose weight, your BMR and RMR will go down and you will need to eat less or exercise more to maintain your current weight. Oh my, it's tough getting old. At least as we get older we get wiser. Well, hopefully.

Using the Calculations

As BMR and RMR only represent resting energy expenditure, an adjustment must be made to reflect your activity level. This is done by multiplying your BMR or RMR by an activity factor (McArdle et al 1996). Note that the following activity factors also take into account The Thermic Effect of Food:

Activity Factor Category Definition
1.2 Sedentary Little or no exercise and desk job
1.375 Lightly Active Light exercise or sports 1-3 days a week
1.55 Moderately Active Moderate exercise or sports 3-5 days a week
1.725 Very Active Hard exercise or sports 6-7 days a week4
1.9 Extremely Active Hard daily exercise or sports and physical job

Use of these activity factors produces a very rough estimate, and there are many different opinions on what these activity factors should be. So again, we suggest that you calculate the actual activities that you perform as described in Calculating Daily Calorie Needs.

And when you do, note that calculations made with the Activity Calculator reflect the total number of calories burned during the period of time calculated. Therefore when calculating how many calories you need or burn in a day, do not add your BMR or RMR.

Technical Notes

BMR and RMR Equations

Equations have been developed to estimate BMR and RMR when testing is not practical. We use the Harris-Benedict equation for BMR, and the Mifflin equation for RMR.

The Harris-Benedict equation has been the standard for decades and is still the most widely used for estimating BMR. This is why we offer it to our users. However, numerous studies have shown it to be inaccurate for a number of reasons:

  • According to today's test standards the Harris-Benedict equation does not estimate BMR, but rather RMR. This is because the test subjects did not spend the night at the test facility.
  • The test subjects used to develop the Harris-Benedict equation did not include an adequate representation of obese people, nor of younger and older people. These omissions continue to become more significant as populations become older and heavier.
  • While all equations for predicting energy expenditure only make estimates, the Harris-Benedict equation typically overestimates by 5% or more.
  • Since the Harris-Benedict equation was first published in 1919, a number of studies have attempted to improve it. Of these, none has been shown to produce more accurate results than the Mifflin equation we use for calculating RMR.
  • For further details please refer to Validation of several established equations for resting metabolic rate in obese and nonobese people, Journal of the American Dietetic Association, September 2003, David C. Frankenfield, et al.

The Harris-Benedict equation for BMR:

  • For men: (13.75 x w) + (5 x h) - (6.76 x a) + 66
  • For women: (9.56 x w) + (1.85 x h) - (4.68 x a) + 655

The Mufflin equation for RMR:

  • For men: (10 x w) + (6.25 x h) - (5 x a) + 5
  • For women: (10 x w) + (6.25 x h) - (5 x a) - 161

Where:

w = weight in kg
h = height in cm
a = age

Body Composition

The equations do not take into account body composition, a measure of the percentages of muscle and fat composing your body. It is therefore less accurate if you have a non-typical amount of muscle. This is because muscle burns calories, while fat does not.

A person with an above average amount of muscle will have a higher BMR or RMR than calculated; a person with a below average amount of muscle will have a lower BMR or RMR than calculated.

A Final Note

The vast majority of our users will never read this far into these notes, and they will do just fine using the calculators so long as they remember that the calculations are only estimates. But for those who want to fully understand how the calculators work, we would like to clarify one last point.

As explained in How the Activity Calculator Works, the calculator does not make use of your BMR or RMR calculations in calculating calories burned. Instead, it uses a constant based on your weight alone. Thus the parameters for sex, age and height are ignored in calculating calories burned.

It is therefore somewhat of an "apples and oranges" disparity to compare your BMR or RMR calculation with your calories burned calculations. You can see the discrepancy by calculating Sitting - quietly for 24 hours. Ideally, this calculation would equal your RMR.

Perhaps it is beginning to sound as if the calculators are flawed. While it is true that they only make estimates, we believe that they are the most accurate you will find. Something we do that is unique is to explain how they work, inaccuracies and all.