Today, let's talk about math: Fun with numbers!

Ok, kids, let’s talk about everybody’s favorite middle school subject, math! After all, that’s why we’re here, right? Fasting is, mathematically, the fastest way to lose weight. In terms of a deficit, there is no number with a better absolute value than zero. So that’s where we’re going to start, but what other numbers do we want to play with today? Follow along, if you like!

I think it’s safe to assume that most people here are familiar with the idea of TDEE, but for the sake of proper laziness I’ll go over that here very briefly. Total Daily Energy Expenditure (TDEE) is an estimation of how many calories you burn each day, including physical activity. More appropriately, your personal TDEE is how many calories your body uses in a day to function, whether you’re eating or not. If you haven’t figured out your own TDEE, you can do so here. In theory, if you have your exact TDEE value worth of calories in a day, you will neither gain nor lose any weight. If you go over you gain, and if you’re under you’ll lose. Pretty straightforward stuff, right?

“It is accurate that a pound of body fat contains approximately 3,500 calories…” was said by Gary Foster, PhD of the Center for Weight and Eating Disorders. This idea has been somewhat debated recently, but is generally accepted to be applicable to most folks. We’re going to use this number going forward.

So, once you know those two numbers, we can start playing with them. First, if you divide your TDEE by calories in a pound of fat, you’ll see how much fat you’ll lose for one whole day of fasting. I’ll do mine below:

2496 / 3500 = 0.713 <--- I should lose about .7 lbs per day of fasting.

I’m sure a lot of you have already thought of that, but it’s good to be on the same page. Now, let’s see about applying this to a simple plan. Let’s pick a reasonable date to work towards, allowing enough time to maintain control. Personally, I’m about 45 lbs over what I want to be. I think it is reasonable to lose this much by, let’s say, October 4th. I’ll enter this date into the calculator here, and it says that date is 256 days away from today. A brand new number to play with!

So now we know how much I lose in a day fasting, and how many days I have until my target. I think you all see where I’m going with this, but let’s go ahead and go there anyways. If I lose .713 lbs per day of fasting, let’s find out how many days it will take to lose the 45 lbs I’m no longer wanting to carry, again sticking with the 3500 calories per pound figure:

45 X 3500 = 157,500:  This is the total amount of calories I want to lose
157,500 / 2496 = 63.1:  This is total calories divided by my TDEE

Now we’re getting somewhere. By using this math, I can see I need to fast just over 63 total days to get down to where I want to be. Assuming that my non-fasting days don’t go over my TDEE, of course. I want to play with the numbers just a little bit more.

Using the date calculator from above, we found out that my target date is 256 days away from today. I’ll just divide that by the amount of days I need to fast, and it will give me a reasonable schedule to follow.

256 / 63.1 = 4.057:  This basically says that I fast every 4 days to be consistent

Alright. So now, if all of this is reasonably correct, then if I fast every 4th day until October 4th, I’ll be right around 45 lbs lighter than when I started. Obviously, if you use these sites and plug in your own numbers, you can do the same for yourself, as well. I’ll put it all together here just for those of you who appreciate brevity:

TDEE / 3500 = lbs lost per day
3500 X how much you want to lose = total calories to lose overall
total calories to lose / TDEE = fasting days required to lose your goal
days until target date / fasting days required = evenly spread fasting schedule

If you are a little more hardcore, you could fast for longer over smaller overall time frames, as well. In this example, we’ll calculate 3-day rolling fasts instead:

63 / 3 = 21:  I would need 21 rolling 3's to lose my target

If we’re really doing rolling fasts (fasting days into one non-fasting day followed by fasting days), then this is quick:

63 + 21 = 84:  Total days if I do 3-on-1-off consecutively

Just to throw another calculator into the mix, let’s use this one to find out when that date would be. According to that calculator, 84 days from today would be April 15th. That’s very tempting, but I know I don’t personally have the willpower to do rolling 3’s for the better part of 3 months.

So there we go. A mathematical approach to fasting for weight loss on a reasonable schedule. As mentioned, this won’t be accurate if your non-fasting days go over your TDEE or if you do a lot of extra activity above and beyond your TDEE, but you could always add or subtract a fasting day or two to cover the difference.

I hope this helps somebody get a good idea of their timeframes, and helps spark some conversation that could be interesting and educational. For those wondering, yes, I am using this method myself, but I just started with it yesterday. My target date is August 24th. I’ll do my best to stick with this schedule and, while late August seems like forever away, I will make sure to come back with the results to see how well “fasting by numbers” works out for me.