*A useful measure, but not definitive*

We’ve all heard of the Body Mass Index, and how it tells us whether we’re underweight, normal weight or overweight (or even obese) for our height. News headlines these days focus particularly on obesity, specifically the estimated proportion of people in a country who (according to their BMI) are deemed to be obese. For example, a 2016 compilation estimated that 99 million Americans are overweight, 70 million of those being obese (which is about 22% of the 2016 population). You think that’s a lot? Obesity is estimated at 26% in the UK and 27% in Canada.

In this blog post I’ll explain what BMI is, how it’s interpreted, and what are its shortcomings (useful though it undoubtedly is as a measure for each of us).

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First, what is BMI? It’s a number. It’s calculated as your weight (In kilograms) divided by the square of your height (in meters). (OK, I’ll convert from metric to inches and pounds shortly).

For example, if you weigh 75 kg and your height is 1.75m, your BMI is 75/(1.75×1.75) = 24.5.

Convert that to pounds and inches. Your weight is 165 pounds and your height is 68.9 inches (or 5 feet 8.9 inches). Your BMI, in those units, is calculated by the formula: 703 times your weight (in pounds), divided by the square of your height (in inches). And that comes to 24.4, essentially the same as before. (There’s rounding involved in those height and weight conversions, as you can guess.)

So, how do you interpret that measure? (At any rate, if you’re older than 20. It’s different for children.)

You’re deemed to be underweight if your BMI is below 18.5; normal weight if your BMI is in the range 18.5 – 24.9; overweight in the range 25.0 – 29.9; and obese if at least 30.0. So the person in this example is deemed to be of normal weight (though if they had approximately an additional 1.5 kg, or 3.4 pounds, they would hit the overweight category).

Simple.

But … it’s a strangely defined measure, and ignores some important things. Let’s get to those aspects next.

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It’s strange because height is one of three physical dimensions. So it would be more logical to divide our weight by the cube of our height. If we compare two people with the same overall shape (in other words, having the same frame) but of different heights, the formula gives a higher BMI measure to the taller person. So, people of above average height would get a higher measure for that reason alone (other things being equal).

Another shortcoming is that the overweight and obese interpretations assume that the excess weight comes from body fat, which of course is something we don’t want to have. But muscular people may not have excess body fat: their weight may be higher because of their muscle. The formula ignores this, and doesn’t adjust for the proportion of muscle to body fat.

Finally, body frames have gender and racial differences, so the interpretations of over- and under-weight should vary by gender and race. In fact, in different countries the numbers are interpreted differently (though not, as far as I can tell, by gender). In fact, the measure was originally designed (around 1830) for European men.

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With those limitations, it’s still a very useful measure. The US National Institutes of Health (NIH) say: “The higher your BMI, the higher your risk for certain diseases such as heart disease, high blood pressure, type 2 diabetes, gallstones, breathing problems, and certain cancers.”

Because a high amount of body fat isn’t good for us, and because abdominal fat poses more health risks than fat elsewhere, another measure may be useful. It’s your waist circumference. According to the NIH, waist circumference exceeding 40 inches for men or 35 inches for non-pregnant women is considered to represent a high risk for most of those diseases mentioned earlier. (Again, racial variations.) In fact, waist circumference (according to Wikipedia) can be a better indicator of obesity-related disease than BMI.

Not surprisingly, considering how BMI is an overstated measure for tall people, the same may be true if you simply measure your waist circumference. Hence the proposal to measure the waist-to-height ratio rather than simply the waist. That’s a test endorsed recently by the UK’s National Institute for Health and Care Excellence (or NICE, as it’s colloquially referred to), in addition to BMI. In 2022 NICE suggested that a waist-to-height ratio is healthy in the range 0.40 – 0.49, it’s in the “take care” range at 0.50 – 0.59, and in the “take action” range if it’s at least 0.60. NICE says (according to Wikipedia) that those ranges apply to both genders and all ethnicities, for people with a BMI under 35.

That 0.50 limit, which of course simply means a half, leads to a simple way to check your own measure without a tape. Measure your height with a piece of string. Fold or cut it in half, and see if the half goes around your waist. Yes is good, no means you need to lose inches to get to the healthy range.

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**Takeaway**

*Start measuring your height, waist and weight, to see if you need to take action.*

### 2 Comments

I have written about retirement planning before and some of that material also relates to topics or issues that are being discussed here. Where relevant I draw on material from three sources: The Retirement Plan Solution (co-authored with Bob Collie and Matt Smith, published by John Wiley & Sons, Inc., 2009), my foreword to Someday Rich (by Timothy Noonan and Matt Smith, also published by Wiley, 2012), and my occasional column The Art of Investment in the FT Money supplement of The Financial Times, published in the UK. I am grateful to the other authors and to The Financial Times for permission to use the material here.

Don, I thought this was very good and I especially liked the pragmatic way you ended it.

I had one question about the statement: “If we compare two people with the same overall shape (in other words, having the same frame) but of different heights, the formula gives a higher BMI measure to the taller person. So, people of above average height would get a higher measure for that reason alone (other things being equal).”

I don’t quite get that. If the person is taller, won’t the denominator squared be larger? So, and assuming the numerator is the same, the BMO for the taller person should be smaller? Apologies if I am missing something.

Thanks for this. A nice way to enter the weekend.

Sorry, I didn’t explain the math assumptions clearly. Consider two people with the same frame (shape), one being taller than the other (and hence larger in all dimensions). Weight is a function of all three dimensions. If the denominator only takes two dimensions into account while the numerator reflects all three dimensions, the measure will reflect differences in one dimension, so (the shape being the same) the taller one will have a higher BMI.