Last Updated: March 21, 2015
I am often asked how many calories one should eat in a day in order to lose weight. While the answer is more complex, understanding the basal metabolic rate is an important first step. The human body is an incredibly complex machine, and most of the calories we burn in a day are spent on keeping us alive (with the exception of people who engage in very intense and/or prolonged exercise).
This article and the calculator below are designed to help you find out how many calories you burn to stay alive - i.e. your basal metabolic rate. In addition, by inputting your activity level, you can find out what your total daily energy expenditure is.
|Sedentary:||little or no exercise|
|Lightly Active:||light exercise or sports 1-3 days/week|
|Moderately Active:||moderate exercise or sports 3-5 days/week|
|Very Active:||intense exercise or sports 6-7 days/week|
|Extra Active:||very intense daily exercise or sports & physical job or twice daily training|
The basal metabolic rate (or bmr) is defined as the basal metabolism of an animal. BMR is the minimal rate of energy expenditure compatible with life. In other words, it is the minimum amount of energy (expressed in number of calories) our body needs to stay alive at rest.
There are several ways of calculating these numbers. One of the best known formulas is the Harris-Benedict equation for estimating the resting energy expenditure or REE (which is the same thing as basal metabolic rate or BMR).
For men: BMR (REE) = 66 + 13.75(weight) + 5.0(height) - 6.76(age)
For women: BMR (REE) = 655 + 9.56(weight) + 1.85(height) - 4.68(age)
The results are measured in kcal/day.
Although the Harris-Benedict equation has been used for a long time (almost 100 years, to be more precise, since Harris JA, Benedict FG. published their study "A Biometric Study of Basal Metabolism in Man." in 1919) by health professionals and nutritionists to estimate people's basal metabolic rate, new studies have shown that in overweight or obese persons, this equation is not accurate, as it tends to overestimate the resting energy expenditure in overweight or obese persons by at least 5 percent.
As almost two thirds of Americans are overweight, and the situation in the rest of the developed world is not much different, we decided to use another equation, called the Mifflin equation, published in the February 1990 issue of the American Journal of Clinical Nutrition, under the title A new predictive equation for resting energy expenditure in healthy individuals.
For men: BMR (REE) = 5 + 10(wt) + 6.25 (ht) - 5 (age)
For women: BMR (REE) = -161 + 10 (wt) + 6.25 (ht) - 5 (age)
The results are measured in kcal/day.
As far as measuring your daily energy requirements is concerned, we used a formula that factors in the so-called activity factor. This is essentially a number based on the level of physical activity you selected in the calculator above. We then multiply the BMR by this activity factor, and the result is your daily energy requirements.
For more details you can visit this page from Cornell dealing with BMR and Energy Expenditure.
While these formulas are widely used to predict the basal metabolic rate, please keep in mind they only provide an estimate, as there is significant variability in basal metabolic rates between similar individuals, and even in the same individual, bmr can vary from day to day.
For the basal metabolic rate estimation to be accurate, several assumptions must be true at the time of measurement:
If you want to read more about BMR variability, J.V.G.A. Durnin from the University of Glasgow Glasgow, Scotland, has published a very interesting paper on this topic, entitled BASAL METABOLIC RATE IN MAN.
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