Day-to-Day Weight Fluctuations and Mental Stress

Note: This was originally published on the TrendWeight blog. Since that blog is no longer active, I am reposting the few useful articles from that site here for posterity.

A common question on various weight loss forums I see is, "How often should I weigh myself?" There are lots of reasonable answers to that question, but I tend to favor "Every Day" as the best answer, but there is a catch! If you don't want to go insane, you absolutely must ignore what the scale says each day and only look at the trend over time. What does that mean?

Normal Daily Weight Fluctuations

Diagram titled "Typical Human Mass Throughput Pounds/Day" showing a human figure with arrows indicating mass intake and output. Inputs: Food 2.5, Water 9.2, O2 1.8. Outputs: Solids 0.3, Water 11, CO2 2.2. Total intake and output are both 13.5 pounds per day.

John Walker talks about this in much more detail in his book, The Hacker's Diet, but I'll summarize the key point here. A lot goes into your body every day and a lot comes out. The food you eat, the liquids you drink, the air you breath, etc all come in. And in general, an equal amount of those things come back out as well. For a typical person, these add up to 13.5 pounds of "stuff" coming in and out of your body each day:

But keep in mind that that balance of 13.5 pounds in and 13.5 pounds out is only an average over time. On any given day, at any given time of the day, your intake may be more or less than your outputs often by as much as 1-2 pounds. Keep this in mind: Even if you are not gaining or losing weight (i.e. fat or muscle), if you weigh yourself every day, the scale will still show your weight going up and down each day, often by as much as 1-2 pounds. That's just the way it is. What matters is that, in that example, the fluctuations will be consistently around a particular weight.

Mental Stress

Losing weight is hard. Really hard. And a good portion of it is mental. It takes real mental effort to really change your lifestyle or behavior in order to lose weight. I you are trying to lose weight and you step on the scale tomorrow morning and it says you gained half a pound, that may psychologically crush you and make you question your resolve, not to mention how it would feel to see that happen 5 out of every 10 days. But as I just explained, if you weigh yourself every day, the scale is going to go up on some days.

The solution is to focus on the weight trend and not any individual day's weight. By focusing on the weight trend, the random day-to-day fluctuations will fade into the background and you can focus on the fact that you're making consistent progress towards your goal. Figuring out your weight trend is pretty easy to do, but I'll come back to that in a minute.

Wait, if weighing yourself every day has all these issues, why do it? Why not just weigh your self less often? That is an alternative option, but remember that if random day-to-day changes may often be as much as 1-2 pounds and you want to make sure that real weight loss isn't hidden by these, they you may have to weigh yourself really infrequently.

If you are trying to lose 1 pound per week, you'd have to weigh yourself only once every 2-3 weeks in order to make sure that your real weight loss always shows through the "noise" of random daily fluctuation. That's a long time to go without any positive feedback. By weighing yourself every day and focusing on the trend instead of the number on scale, I get daily reinforcement that I am slowly but surely making progress. That helps. A lot.

Focusing on the Trend

Let me show you in pictures what this really means. I have been weighing myself nearly every day for the past year. First let's look at what my day-to-day weight changes have been over the past year. In this picture, the green arrows are days where the scale showed a lower number than the day before (yay! I lost weight), and the red arrows are days where the scale showed a higher number than the day before (oops! I gained weight):

So how did I do? Over the long term, if you looked at the actual numbers, it would be obvious that I lost weight, but in the trenches, day-to-day, if I was focusing on the number on the scale, I would have felt like I was on a crazy roller-coaster and likely I would have been highly frustrated:

Ok, so that sucks. But what kind of difference does it make if I look at the weight trend each day instead of the number on the scale? Let's say each day I figure out what my average weight was over the past 10 days (let's call that my "trend weight"). Because this trend weight is an average of many days, the random day-to-day fluctuations will mostly cancel each other out, and the trend weight will slowly go down each day if my weight is decreasing over time. You may commonly hear this idea of a trend weight referred to as a "moving average".

But let's not worry about the math right now, and let's just see what this does to my day to day mental stress. If I take my daily weight data for the last year and look at trend weights instead of what the scale said on any given day, here is the result:

What a difference! For the first two thirds of that time range, I was pretty happy almost every day because almost every day, the trend weight got a little smaller day after day. I can tell you that this reinforcement was a huge help, especially at the beginning when I was not sure I had it in me to lose the weight I needed to lose.

In the last third of this time range, I had some hiccups. I really was gaining weight, so the trend weight wasn't lying. If you are curious, the first long stretch of weight gain was the result of a change to my blood pressure medications that affected how much water I retained and as a result, I gained 10 pounds over the course of two weeks. And the second long stretch was a family vacation where I fell off the wagon, so to speak. The good news is that I'm back on track and have re-lost all the pounds I gained over the summer.

Ignore the specifics of my situation for a moment and look at the two pictures again. I hope you can see what a difference looking at the weight trend makes. I think the daily, positive reinforcement was key to my really getting over the initial mental hurdle at the start.


Ok, so I think you get my point that I am a huge fan of daily weight measurement combined with the use of a moving average to look at the weight trend instead of any individual scale reading. Doing this is not as hard as you might imagine and doesn't require you to be a math wiz either.

In my opinion, the best actual technique to monitor your weight trend over time is the one that John Walker describes in The Hacker's Diet in the "Signal and Noise" chapter. There are lots of free tools available that handle all the math that John talks about so that you don't have to worry about it.

First, is the web application I built, initially just for myself, to do this kind of tracking for people that have Fitbit or Withings WiFi scales. If you have one of those scales, TrendWeight is free and will automatically pull your weight readings each day and show you your weight trend. Note, you actually don't need a WiFi scale to use TrendWeight. It works just as well even if you just have a account and are using their iPhone app to manually enter your weight each day.

But there are lots of other options if you don't have a fancy electronic scale as well. There are both iPhone apps and Android apps that have this approach baked in. You manually enter your weight each day, and they do all the math for you.

If you don't want to use your smartphone, there are websites that will let you manually enter your weight each day and will also do all the math. John Walker created the Hacker's Diet Online to do this, and Physics Diet is another popular choice.

And last but not least, you don't even need a computer to do this. John Walker also explains how you can do this yourself with a sheet of paper and a pencil you keep next to your bathroom scale and all you need is basic addition and subtraction (no calculator needed). You can read about it in the Paper and Pencil chapter of his book.

If you are trying to lose weight, first, good for you! Second, I hope I have convinced you to at least try weighing yourself daily and calculating your trend weight. I honestly believe this approach will help you see your daily successes without getting distracted by random day-to-day fluctuations.