The Math

I think the weight comes flying back on after a loss because you get to a point where you want NOT to think about calories, or reducing, or limitations. You want to just enjoy food & drinks but your brain forgets that you are smaller and that it takes less calories per day to maintain your weight, meaning that when you indulge those extra calories are immediately being added to the piggy bank and the savings grows!!

BMR

Generally speaking it takes somewhere around 9-12 calories per day per pound just to support vital body functions, say if you were resting the entire day.  This is known as your Basal Metabolic Rate or "BMR".  So at 176 lbs I must have been eating about 2112 calories per day.

There is some actual science used to find BMR - see Mifflin St. Jeor Equation:

For men: BMR = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) + 5
For women: BMR = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) – 161

It is important to know your BMR so that you understand your basic calorie needs per day. (especially since that changes as you change)

As I lose weight, it takes less to maintain BMR. At 142 lbs (around my lowest weight in the past 10 years) I should have been eating 1700 calories. At a fairly sedentary lifestyle, anything over the 1700 started adding to the "piggy bank". This explains the math part of the rebound weight gain. (The hormonal and other factors we will discuss later).

TDEE

Total daily energy expenditure is the entire amount of calories per day that you need both for sustaining vital body function as well as the energy to perform daily activities (based on your level of activity).

The amount and rate at which you want to lose weight is translated into to a daily calorie deficit and then subtracted from your TDEE. This also changes over time.

Fortunately for us there are a variety of Apps out there that do all this crazy math for us. The key thing to keep in mind is to understand that calorie needs change throughout your weight loss journey.

Here is a summary of my calorie requirements on day one of my journey:

If I want to be 110 lbs I would need to become comfortable with 1320 calories per day. *This is what I am living on now in reduction mode. I'm guessing that as I get closer to 110 the 1320 calorie limit will get easier to live with