The Math Behind Your Best Body Ever

If your trying to lose weight, put on muscle, or simply maintain your body the way it is, it’s critical that you know precisely how many calories your body needs. Although body weight isn’t solely controlled by calories in vs. calories out (hormones play a major role), calorie levels are still the main driver of weight fluctuations.

The Formula

The most accurate formula to calculate your maintenance calorie level, or the number of calories you should eat to maintain the same weight, is called the Katch-McArdle Formula. This is the most ideal formula because it takes into account a persons body composition when calculating calorie needs. Because muscle burns more calories at rest than fat, a person with a higher % of muscle mass will typically need more calories than someone who weighs the same but has more fat mass. A 170 lb male with 10% body fat will burn more calories than a 170 lb male with 20% body fat (more on this in a moment).

Before we do the calculation, you must find your body fat %, or estimate it if you do not have access to a caliper/way of measuring it.

Once you have found your approximate body fat %, calculate your lean mass. Multiply your weight in pounds x (1-body fat %).

I weigh 170 pounds with 8% body fat. My lean mass is 170 x (1-.08 or .92) = 156.4lbs of lean mass

Convert your lean mass in pounds to kilograms. Divide your lean mass by 2.2.
For me, I have 156.4 lb/2.2 = 71.09kg of lean mass.

Next, you must calculate your BMR or Basal Metabolic Rate. This is the number of calories that your body uses to sustain basic life functions such as breathing, regulating temperature, keeping your heart beating, liver and kidney function, etc.

BMR = 370 + (21.6 x lean body weight in kilograms)
My BMR = 370 + (21.6 x 71.09kg) = 1905.56 calories
My body burns 1905.56 calories per day just to keep me alive, thus I should never eat less than this number.

The final step is to multiply your BMR by your TDEE multiplier or Total Daily Energy Expenditure multiplier, which gives you your Total Daily Energy Expenditure.
Refer to the chart below and assess which category you fall into. WARNING: Be honest with yourself. If you exaggerate your activity level you’re going to end up with a calorie level that’s above your true maintenance level, thus you would gain weight. So don’t! My advice is to underestimate if you’re unsure. If you feel that you’re in between two categories than you can use a multiplier that lies in between.

I workout 6x/week for at least one hour so my TDEE multiplier is 1.725.
My Total Daily Energy Expenditure is 1.725 x 1905.56 = 3287
The calorie requirement to maintain my current weight is 3287 per day.
Calculating Total Daily Energy Expenditure (TDEE)
Amount of Exercise Total Daily Energy Expenditure (TDEE)
Little or No Exercise, Desk Job, Student TDEE = 1.2 x BMR
Light Exercise, Sports/Workout for ~1 hour, 1-3x/week TDEE = 1.375 x BMR
Moderate Exercise, Sports/Workout for ~1 hour, 3-5x/week TDEE = 1.55 x BMR
Heavy Exercise, Sports/Workout for ~1 hour, 6-7x/week TDEE = 1.725 x BMR
Very heavy exercise, Physical Job/Athlete or Training 2x/day ~1 hour each, 6-7x/week TDEE = 1.9 x BMR

To show you the impact muscle mass can have on daily calorie expenditure, consider my calorie needs if I instead had 18% body fat (I still weigh 170 lbs).
My lean mass in pounds would be 170 x .82 = 139.4
Converted to kilograms = 63.36kg
BMR = 370 + (21.6 x 63.36) = 1738.65
My TDEE = 1738.65 x 1.725 = ~ 3000
My daily caloric allowance to maintain my weight is ~ 3000 calories.
This is ~ 300 calories less than my daily caloric allowance with 8% body fat!

Losing Weight

If I wanted to lose 1 pound/week I would just need to subtract 500 from my TDEE. My daily caloric allowance to lose 1 pound per week is 3287-500 = 2787 calories/day.
If I wanted to lose 2 pounds/week I would subtract 1000 from my TDEE. My daily caloric allowance to lose 2 pounds/week is 3287-1000=2287 calories/day.

Gaining Weight

If you’re trying to put on weight I’m assuming you’re trying to build muscle, not fat. Contrary to popular belief muscle doesn’t have the same amount of calories as fat. Many people trying to gain muscle simply eat as much food as possible. Yeah you’ll gain muscle doing that, but you’ll also put on a boatload of fat.

You should aim to eat 200-300 calories/day above maintenance when trying to build muscle.
If I want to build muscle I should consume 3287 + 200 = ~3500 calories/day