Calculating your caloric needs

[Reincarnate]

This post is not meant to be rigorous, but rather a starting point for further discussion.

Weight change = Caloric input - caloric output

Calculating caloric output:

You have a BMR (basal metabolic rate) = how many calories your body uses just to survive every day. This figure is typically dependent on factors such as your sex, weight, lean mass, age, etc.

But you do more than just survive -- you move around, too. As you move your body throughout the day, you burn additional calories (which I'll call DAE = Daily Activity Expenditure) on top of your BMR. This sum gives you your TDEE, or Total Daily Energy Expenditure.

One popular method is to estimate your BMR using some regression formulas and then apply an activity multiplier (which I'll denote with m) to estimate how much you expend in terms of activity. Note that these are "ballpark" approaches that simply give you a realistic starting point.

So TDEE = BMR + DAE = BMR + m*BMR

BMR formulas:

Weight is in kg, height is in cm, age in years:

Mifflin-St Jeor Formula:
For males: BMR = 9.99*Weight + 6.25*Height – 4.92*Age + 5
For females: BMR = 9.99*Weight + 6.25*Height – 4.92*Age -161

Katch-McArdle Formula:
BMR = 370 + 21.6*LBM (LBM is your lean body mass -- i.e. not fat, in kg)

(In general, people say Katch-McArdle is the best one if you know your bodyfat percentage, but if not, then Mifflin St. Jeor is probably better, but it tends to overestimate your BMR if you're overweight).

DAE Multipliers:

Then to estimate your DAE, apply the most relevant multiplier (m) to your BMR:

0.2x = Sedentary (Desk job, little exercise, etc. If you spend most of your time on your ass, really)
0.3x-0.4x = Lightly Active (Light daily activity and light exercise 1-3 days a week / someone on their feet most of the day like a teacher or retail worker)
0.5x-0.6x = Moderately Active (Moderately daily activity & moderate exercise 3-5 days a week / someone moving around a lot like a busy waiter)
0.7x-0.8x = Very Active (Physically demanding lifestyle & hard exercise 6-7 days a week / perhaps working in hard labor all day)
0.9x-1.2x = Extremely Active (Athlete in endurance training or very hard physical job)

From here, you just do TDEE = BMR + DAE and that's your daily output measured in calories.

Calculating caloric input:

Add up the number of calories from all the food you eat.