A question I often get asked by some of my fellow sportsmen (I suppose because I am a scientist) is “What would my perfect weight be?”. This question is not really easy to answer, more so given that the society is plagued by the “Body Mass Index (BMI)” myth. Let me explain a bit…
The BMI is an index made up sometime in the seventies to approximate the healthy weight range for individuals. It is a formula which takes your weight (in kg) and divides is by your height (in m) twice. For example, if you weigh 100kg and are 200cm tall, then your BMI would be:
BMI = (100kg/2m)/2m = 25 kg/m^2
Now, according to health guidelines from those times, this person would already fall in the overweight range. I think that from the viewpoint of modern society, if you saw someone like this, you might even call him slim!
A much better “index” one should look at is the percentage of fat in the body. This can be measured by modern scales (which will be approximate at best), or even better, by a fat caliper (buy one for a few bucks on amazon). Here is a much better way to calculate your ideal weight:
Lean Body Mass/(1- Desired Body Fat Percentage)
where the lean body mass = Your Body Weight – (Your Body Weight x Your Current Body Fat Percentage). Just to be clear, your LBM is your “fat free” mass, in other words, everything in your body that’s not fat: your bones, blood, muscle, and organs.
Let me give you an example of this ideal body weight formula in action so you can see why it’s so useful. Let’s take Jake who is 90 kg and has 22% body fat. Using this information, we know that his LBM is 70.2 kg and the amount of body fat he has is 19.8 kg. So what should Jake’s ideal weight be? Well that’s really up to him. For most men, a mid double digit body fat percentage of say 15% is considered pretty good. This depends on the gender and age. Here’s a chart for your reference:
To get a better idea of how this actually looks, here are some (motivational) images as well:
OK, so now that you have determined your lean body mass and decided on what body fat you are aiming at, you can calculate your new “ideal” weight. Let’s continue with our example. Let Jake be 30 and have LBM of 70.2 kg. Further, let’s say he decided he wants to have 13% body fat. This means his ideal weight would be:
Lean Body Mass/(1- Desired Body Fat Percentage) = 70.2 / (1-0.13) = 80.7 kg
Well now, you might still ask – but what if he is really tall, say 220cm? Wouldn’t this weight make him underweight? Indeed, if we calculated his BMI at this point, it would be:
80.7/2.2/2.2 = 16.7
which is too low, and indeed, he would likely be underweight. Why, then, doesn’t this fat formula work? The reason is simple – fat is bad for your health and the doctors will only tell how much (in fact, little) fat you should have. No one will ever tell you how much muscle you should have, because it can’t hurt to have too much (well, in extreme cases, it can, but no normal person ever gets there). So, I thought about this a little and concluded this – it is likely best to be built like an athlete. An athlete build depends heavily on the sport one is active in. To make a good estimate, I took the averages of (about) 10000 athletes that were at the Olympics this year. I found data for both men and women, which you can see here:
For men, the median weight and height are 78kg and 182cm. For women, 62kg and 170cm. The calculated BMI for men this way is 23.55 (almost overweight!) and for women 21.45. So if Jake wants to have this BMI, he should weigh:
Weight = BMI * height * height = 23.55 * 2.2 * 2.2 = 114kg
So, Jake, at this point, you are not overweight, but you are fat! Jake should reduce his fat to 13% of body mass and build some muscle (to reach the “ideal” weight).
A summary of all of this – to find your ideal body weight:
1. Write down your body mass and measure your body fat percentage
2. Calculate your LBM = body mass * (1-body fat percentage)
3. Look into the chart and find your ideal body fat percentage (or decide on one)
4. Calculate your “new” body mass BM=LBM / (1-desired fat percentage)
5. See if your “new” body mass is close to the ideal BMI for athletes
6. Try to increase/decrease your mass (muscle AND fat) to reach the ideal BMI
Any questions, just ask!