The Math Behind Weight Loss: Caloric Deficit explained

Losing weight comes down to creating a “caloric deficit.” That is, burning more calories a day than you eat.This takes into account more than your BMR.

BMR = basal metabolic rate. This is how many calories you burn from existing. If you sat around all day and did absolutely NOTHING your body would burn this many calories just to exist.

TDEE = BMR + activity level, or “total daily energy expenditure”. Since the average person doesn’t sit around on the couch and not move at all, this takes into account other things. For example, a sedentary person (who works a desk job and doesn’t exercise) may have a TDEE slightly above their BMR. Someone who is very active (their job is manual labor, or they lift very heavy multiple times a week) may have a TDEE almost twice their BMR.

How do you calculate your BMR or TDEE? Let’s cut the math – here is my favorite calculator. You input your gender, age, weight, body fat % and “activity level” and it will tell you how many calories you need to maintain, lose weight, etc.

So what is a caloric deficit?

Let’s use a real life example. My BMR is 1550 calories a day, my maintenance is 2,100 calories a day. Based on how often I exercise, how I spend my day, etc. I need to eat 2,100 calories to maintain my weight. If I want to lose weight I need to eat less than 2,100 calories a day. What’s important to note about this number, my maintenance number, is that it takes my TDEE into consideration. I don’t subtract the amount of calories I burn from exercise, that’s already taken into consideration. That’s the definition of TDEE – total daily energy expenditure. It’s already taking into account that I exercise as much as I do, or I’m as active as I am at my job.

If one pound = 3,500 calories, to lose one pound a week I need to have a deficit of 3,500 calories that week. Divided over 7 days, that’s a 500 calorie deficit a day. That means eating 1,600 calories a day for me. It’s also important to remember that your body takes an average. 24 hours is a human invention, as long as you have -3,500 calories a week it doesn’t matter what your day to day intake is. It doesn’t matter if these calories are eaten at 5pm, 6am or 12am.

What is NOT a caloric deficit?

When I was sick I used to think that a caloric deficit meant my total numbers for the day were a negative. Basically, I would eat 600 calories a day and then exercise off 1,000 calories and think that I was in a 400 calorie deficit. What I didn’t realize is that my body requires a certain amount of calories to exist that’s independent of how many calories I eat a day – that’s my BMR.

The BMR Trap

People who don’t understand what BMR is believe they need to eat less than their BMR. What they’re forgetting is that they do more than sit around all day. Yes, some people live very sedentary lives – they work at a computer desk, only get up to use the bathroom, go home and eat dinner in front of the TV. These people may need to eat at their BMR because they don’t move enough to create a TDEE much higher than their BMR. Others – people who walk from class to class, go to the gym a few times a week, have a job where they’re on their feet all day at a restaurant or as a nurse – these people have a TDEE higher than their BMR. If they eat at their BMR they will find themselves energy deprived, sluggish, and fall into the trap of binging to compensate for their low energy.

Comments (10)

  1. Jaime

    According to my bmr I’m supposs to eat 1600 Calories a day. I want to loose 10-15 kgs in like 2-3 months,
    I weigh 90 kgs
    160 Cms talk and an female.
    I don’t get how many calories I need to eat and how many I need to loose per day to loose this weight, can you help because I’m super confused 🙁

  2. Alannah Bloch

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  3. jackie

    The calculator is a read only page, that sucks ;(

    1. Ste

      Just unprotect the sheet 🙂

  4. Kris

    How does this work if you’ve had an eating disorder and your body just won’t lose weight like normal like that? (And believe me there is weight to lose, my body fat is very high and my BMI is well into the normal range and higher than it’s ever been – I have stopped growing so this does not explain it) When does your body start working again? I am worried because I am approaching my mid twenties already and I know it gets harder and harder to lose weight, and I want to be able to lose it while it’s still easier, but it seems like nothing works anymore…

  5. D Warren Rose

    Thank you so much for this in depth explanation of BMR and TDEE! This is the best explanation that I’ve seen! And the calculator is excellent!

    1. Jane Whitaker

      The calorie deficit model “explains” NOTHING, fool. You are mixing up E=MC2 in a way Einstein would NOT forgive. Humans are NOT nuclear powered.

    2. Jane Whitaker

      There is NO caloric energy being turned into matter in a human. Humans cannot convert fat matter into energy or heat. That matter is still present.

      1. Megan

        True it doesn’t turn into energy or heat it is breathed out via carbon dioxide.

      2. Don

        How does you heat beat? Then energy from the sugar in the blood stream. How does your brain get energy to think? From the sugar in your bloodstream.
        Sugar is converted to ATP inside your cells which is “burned” to provide energy
        This process also produces heat. As your body uses up the sugar in your blood, it begins to turn fat into sugar which is then used up.
        A Calorie is the measurement of how much energy a substance produces when it is burned up.
        So in a sense you are right, no energy is beign converted to mass. However, the mass (food) is measured by its caloric content (how much energy it can produce when burned.) or potential energy. A Calorie is not a measurement of energy, but of potential chemical energy.


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