First and Foremost
Let me say, that I’m so happy that there are much smarter, more passionate doctors/professionals in the field of physical health than myself. Scientists for decades and decades have monitored, revised and built out algorithms to be able to calculate how many calories are in a lb of fat, how much you weigh and how much you burn. The caveat is that you use them while understanding that these are base calculations which are more or less right… Meaning? They were built on averages. So its not perfect or as exact as if you were to use a buoyancy tank and find out how much your actual body fat percentage was, in addition to wearing a heart rate monitor and using that to calculate your calorie burn. But these calculations cost a lot less (free minus the time is takes to fill out the form on the websites linked below) and frankly for the nonOCD individual its close enough.
Also, I’m not a scientist and what I’ve written and borrowed below is more a reflection of my personal thoughts than anything else.
Let’s Start with the Basics
First we have to calculate your BMR or Basal Metabolic Rate. This is how many calories you would burn if you sat in bed all day and did nothing.
Now you can use this equation:
English BMR Formula 

Women: BMR = 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches ) – ( 4.7 x age in years )Men: BMR = 66 + ( 6.23 x weight in pounds ) + ( 12.7 x height in inches ) – ( 6.8 x age in year ) 
Metric BMR Formula 
Women: BMR = 655 + ( 9.6 x weight in kilos ) + ( 1.8 x height in cm ) – ( 4.7 x age in years )Men: BMR = 66 + ( 13.7 x weight in kilos ) + ( 5 x height in cm ) – ( 6.8 x age in years ) 
OR
You use a prebuilt, easy to use calculator found here or here.
For me the formula looks like this (as of May 5, 2015):
BMR = 66 + ( 6.23 x 280) + ( 12.7 x 77 ) – ( 6.8 x 31) = 2577.5 k cal
And that’s if I do nothing. But, we all move a bit in our day, don’t we?
Adding Activity
Here’s the fun part, because we walk and move all day. Because a fair number of us play a sport or go hiking or are just simply active? You get to increase this number. Increasing the number means that you have more calories to consume to maintain your current weight. So if you’re trying to lose weight, the higher the number, the easier it is.
For example: Lets say you have a BMR + Activity calorie count of 4000 calories. That’s ALOT of food. That’s about 7 big macs or 85.1 cups of strawberries (that’s 5.3 gallons). Cutting back on calories here, is super easy as you can imagine ( I myself can only consume 70 cups of strawberries kidding!)
So here’s how you calculate your Activity using the Harris Benedict Equation.
Harris Benedict Equation 

To determine your total daily calorie needs, multiply your BMR by the appropriate activity factor, as follows:

For me? I’m a Light exerciser. Usually I workout 36 times a week with a goal of getting to 7 sometime soon. (As of May 5, 2015)
My BMR x Light Active = 2577.5 x 1.375 = 3544 calories
That’s a lot of food.
Losing the Fat
A pound of fat is 3500 calories.
Basically you must burn 3500 calories on top of the calories you’re burning daily (remember, you eat food… this adds calories) in order to burn a pound of fat. Now 3500 calories is a tall sum for one day, which is why you should try and do something simpler, like 500 calories a day (or in my case, a goal of 1000).
Its really easy to break down.
 3500cal = 1 lb of fat
 3500 cal / 7 days (a week) = 500 calories a day
 1 week = 1lb of fat burned
 52 weeks a year X 1lb of fat = 52 lbs of fat burned
Pretty awesome huh?
 My BMR + Activity = 3757 cal
That means that to lose one lb of fat a week? I need to eat only 3257 cal a day. Do you know how many calories that is? McDonalds here I come right?
Wrong! To maintain my weight loss I have to consistently come in under the calories goals I’ve set for myself. I have to take care in what and how much I eat
Remember
All these take into consideration your age, weight, gender, height and actual activity level. You can say that you worked out, but when its only 15 minutes on a treadmill? The only one you’re hurting is yourself. You’ve got to keep yourself accountable and take into consideration your bodies needs. From nutrition, to recovery and healing time. No one has gone from sitting on the couch constantly to marathon runner in a week.
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