This is the most important question you must ask yourself when it comes to starting any health / fitness routine, and is an important number to know regardless of your lifestyle. How many calories do you need? Well, there are many different answers to this single question, and it all starts with two formulas that calculate you BMR. “What’s a BMR,” you ask? Well, BMR stands for “Basal Metabolic Rate,” and it is the amount of calories your body burns simply by existing over a 24 hour period. After you’ve figured out your BMR, you then multiply that number by an “Activity Multiplier,” which will tell you how many calories your body burns in a given day. More on this later, though. . . On with the formulas:
The Harris – Benedict Formula
This is the most widely used equation for calculating BMR, as it relies on information that people typically know. While it is very accurate, it can be completely inaccurate for the overly muscular, and the overly obese. The equation is:
Men: 66 + 13.7x + 5y -6.8z | Women: 655 + 9.6x +2.8y – 4.7z
Where “x” is your weight in kilograms, “y” is your height in centimeters, and “z” is your age. As an example, let’s say you’re a 24 year old, 6 foot tall male weighing 180 pounds; The equation would read as follows: 66 + 13.7(81.81) + 5(182.88) – 6.8(24) | After you’ve run the equation, you’re left with 1,938 calories. Remember that number– We’ll be back.
The Katch – McArdle Formula
This is a lesser known formula for calculating your BMR, but it also happens to be the most accurate, because it only relies on your lean body mass (LBM), which is what requires fuel in your body. This equation is not used as often because most people are completely unaware of their body fat percentage, and thus, they do not know their LBM. So, how do you find out your LBM? Buy some calipers, or ask a personal trainer to find out your body fat percentage (In the case of our hypothetical 180 lb. man, 11%); Multiply your weight by the percentage (180*.11), and you will know how many pounds of fat are on your body. Then, subtract that from your weight, and you have an LBM. Covert that to metric, and then plug it into the following formula:
Men & Women: 370 + 21.6w, where “w” is your LBM in kilograms. According to this, our 180 lb. man has a BMR of 1,943 calories.
Now, comparing the two, 1,938 to 1,943, you can see that they’re both similar, which is why the Harris – Benedict Formula is a perfectly acceptable for you to start– But now, we need an activity multiplier. . . After all, a BMR is the amount of calories your body burns existing, not moving.
By default, you have to multiply your BMR by 1.2 to get your “Total Daily Energy Expenditure” (TDEE), because getting out of bed, using the restroom, and eating all require calories. If you’re more active than a couch potato, though, you use the following guideline to determine your activity multiplier:
Default / Sedentary = BMR * 1.2 (Little to no exercise, nor activity)
Lightly Active = BMR * 1.375 (Light exercise 1 – 3 days per week; Active job)
Moderately Active = BMR * 1.55 (Moderate exercise / sports 3 – 5 days per week)
Very Active = BMR * 1.725 (Hard exercise / sports 6 – 7 days per week)
Extremely Active = BMR * 1.9 (You’ll know if this is you.)
Our 180 lb. man that we have been using in this experiment works out 6 days per week, doing 3 days of hard weight lifting and 3 days of light cardio– This would put him in the “Moderately Active” group, which brings his TDEE to 3,012 calories.
So why do we use an activity multiplier? This is done to take the guess work out of your calorie counting– Rather than guess how many calories you burned weight lifting, and the calories burned standing on your feet at work, this tried and true method does the work for you. From there, you determine how many calories you need in relation to your goals, which will come in part 2 of this blog!