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.
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.
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:
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.
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:
|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.
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:
The Harris-Benedict equation for BMR:
The Mufflin equation for RMR:
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.
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.