# Thread: Why is the BMI equation needlessly complex?

1. ## Why is the BMI equation needlessly complex?

srs...

If BMI is just a formula calculated based on height and weight... why can't you just use height/weight to get a ratio?

Why did they instead throw in a bunch of extra steps?

2. Isn't that a prevailing reason why this metric is falling out of favor, the fact that it doesn't distinguish between lean mass and bodyfat?

3. Originally Posted by EliKoehn
Isn't that a prevailing reason why this metric is falling out of favor, the fact that it doesn't distinguish between lean mass and bodyfat?
That's not what I mean.

I mean why can't it just be: weight/height

That's not what I mean.

I mean why can't it just be: weight/height

Beats me. In fact, I thought it literally was just a simple ratio.

5. Height is a 2 dimensional variable, hence squaring it. Weight is one dimensional, so there is no exponent. That's my S.W.A.G.

Scientific
Wild
Ass
Guess

I studied number crunching in college. I studied advance number crunching in grad school. Now I do sales and marketing to make money.

6. Are the various numerical results using the exponent more "user friendly" / easier to categorize in any way?

I find the stomach jiggle test to be a more accurate measure.

7. Originally Posted by air2fakie
I find the stomach jiggle test to be a more accurate measure.
That sounds like a variation of the Dickie Do test.

That's if your stomach sticks out further than your Dickie Do, you are fat.

8. Originally Posted by air2fakie
Are the various numerical results using the exponent more "user friendly" / easier to categorize in any way?

I find the stomach jiggle test to be a more accurate measure.
I mean it's just a ratio so I can't imagine why it would matter at all.

like if BMI was Height In Inches / Weight in LB... or convert it to KG... etc.

That sounds like a variation of the Dickie Do test.

That's if your stomach sticks out further than your Dickie Do, you are fat.
Much like the shortcomings of BMI, that doesn't take into account any Dickie Do deficiencies.

I mean it's just a ratio so I can't imagine why it would matter at all.
It has something to do with normalizing numbers if you square them. When calculation a standard deviation or variance (I don't remember which), all the data numbers are squared.

11. Originally Posted by air2fakie
Are the various numerical results using the exponent more "user friendly" / easier to categorize in any way?
I mean it's just a ratio so I can't imagine why it would matter at all.

like if BMI was Height In Inches / Weight in LB... or convert it to KG... etc.
If BMI is used with imperial units it has to be multiplied by 708 to read out like the BMI numbers we normally see. The numbers would likely be way too small to make sense of without doing so.

Interesting to me is that the units kg/m^2 are also units for pressure (like psi in your car tire). Not sure I understand why that is?

kg is a measure of force and m^2 is an area...

12. Originally Posted by air2fakie
Much like the shortcomings of BMI, that doesn't take into account any Dickie Do deficiencies.
The Dickie Do test only works for guys, not for women. So I've invented a test for women. It's called the Boobie Do. You all can figure it out.

13. Originally Posted by DougyF7
If BMI is used with imperial units it has to be multiplied by 708 to read out like the BMI numbers we normally see. The numbers would likely be way too small to make sense of without doing so.

Interesting to me is that the units kg/m^2 are also units for pressure (like psi in your car tire). Not sure I understand why that is?

kg is a measure of force and m^2 is an area...
Seems like an odd choice to me… I mean we use small numbers for many health metrics like hba1c and measuring blood counts and such….

I’m just not seeing how squaring a number ‘normalizes’ it

14. The actual reason why is that by squaring it you get similar numbers across adult age and variable heights (ie, a BMI of 25 is similar for someone 5'10 and 6'5"). That's not perfect but when examining the allometric relationship between weight and height using the power of 2 works best.

In kids using 3 works a lot better, but we use 2 as that is what is used for adults, and as a result if you look at the BMI curves for children the expected BMI changes significantly with age. https://www.google.com/search?q=male...D_MsijDTK-U5TM

15. Originally Posted by Heisman2
The actual reason why is that by squaring it you get similar numbers across adult age and variable heights (ie, a BMI of 25 is similar for someone 5'10 and 6'5"). That's not perfect but when examining the allometric relationship between weight and height using the power of 2 works best.

In kids using 3 works a lot better, but we use 2 as that is what is used for adults, and as a result if you look at the BMI curves for children the expected BMI changes significantly with age. https://www.google.com/search?q=male...D_MsijDTK-U5TM
Interdasting

Interdasting
There are also some statistical things where you take the mean of all the data sets and then subtract the mean from each data set. You'll have a bunch of positive and negative numbers. Doing a sigma of each number from the deviation from the mean will yield a 0.0000. If you square each deviation the number will be positive and then do a sigma and square root it will give you a number that can be used in some quantitative analysis.

17. Originally Posted by Heisman2
The actual reason why is that by squaring it you get similar numbers across adult age and variable heights (ie, a BMI of 25 is similar for someone 5'10 and 6'5"). That's not perfect but when examining the allometric relationship between weight and height using the power of 2 works best.

In kids using 3 works a lot better, but we use 2 as that is what is used for adults, and as a result if you look at the BMI curves for children the expected BMI changes significantly with age. https://www.google.com/search?q=male...D_MsijDTK-U5TM
why is BMI different for people of Asian decent? Ive noticed that on online calculators

18. Originally Posted by snailsrus
why is BMI different for people of Asian decent? Ive noticed that on online calculators
I'm guessing lower bone density and size relative to height.

19. Originally Posted by TolerantLactose
I'm guessing lower bone density and size relative to height.
that was my guess but I wasn’t sure

It seems like they have to weigh less to be considered a healthy weight

20. Its really hard to sell something or sound super smart if you make things too simple. I mean you can’t really call yourself a genius if you invent a 1st grade division problem. But if you tel people they need to divide it by the cosine of the square root of your height in inches squared and then multiply it by pi, then all of a sudden you sound like a genius who demands respect and must not be questioned.

Its kind of like how everyone assumed Einstein was a genius because he came up with the formula E=mc squared. But then later people learned he was wrong about a lot of stuff. But he was long dead, thus , he died a genius.

21. Originally Posted by snailsrus
why is BMI different for people of Asian decent? Ive noticed that on online calculators
Originally Posted by TolerantLactose
I'm guessing lower bone density and size relative to height.
It's actually because they have health risks start to increase at a lower BMI. I don't recall reading anything regarding their bone density being different, though admittedly I haven't looked. So while overweight/obesity by BMI is typically considered 25/30, for people of Asian descent it is 23/27.

22. Originally Posted by EliKoehn
Isn't that a prevailing reason why this metric is falling out of favor, the fact that it doesn't distinguish between lean mass and bodyfat?
I think the metric is actually quite good for most of the population. In fact I think it's more likely to lead to wrong conclusions in the other direction - i.e. concluding that people are normal weight when they should be classed as overweight. Most people who are "overweight" according to BMI probably actually are. I tend to think of BMI + waist measurement together as an excellent indicator.

People who are overweight and really lean are quite rare. Even many people who lift (naturally) for years will struggle to be really lean and overweight measured by BMI.

I obviously don't mean that this applies to you, you natty mass monster!

23. Originally Posted by EiFit91
Most people who are "overweight" according to BMI probably actually are. I tend to think of BMI + waist measurement together as an excellent indicator.

I!
Or you could just use your eyes. /shrug. I think a better test would be the airplane seat test. You sit in a middle seat and have the person sit next to you. If they roll into your seat they are obese.

24. To provide a serious answer, my guess is it has to do with charting/plotting the data. After taking statistics at a University and seeing presentations by Tufte, there are times when data is massaged to enhance the data visualization in whatever chart or plot you are attempting to show it in. Ie, maybe when it originated, i believe for military enlistment?, then it was worked to fit a certain chart.

25. Originally Posted by desslok
Or you could just use your eyes. /shrug. I think a better test would be the airplane seat test. You sit in a middle seat and have the person sit next to you. If they roll into your seat they are obese.
lol of course, but I normally don't stare at men without clothes that often so I'd rather have them tell me those measures

People can look really thin in clothes even at like 25% BF

26. Originally Posted by EiFit91
lol of course, but I normally don't stare at men without clothes that often so I'd rather have them tell me those measures
Just jiggle their stomachs, it's really a foolproof rest. :P

I find it funny how many people rely on numbers like BMI or "what's my bf%?" to tell themselves if they're overweight. Seriously, find a mirror.

27. Originally Posted by EiFit91
I think the metric is actually quite good for most of the population. In fact I think it's more likely to lead to wrong conclusions in the other direction - i.e. concluding that people are normal weight when they should be classed as overweight. Most people who are "overweight" according to BMI probably actually are. I tend to think of BMI + waist measurement together as an excellent indicator.

People who are overweight and really lean are quite rare. Even many people who lift (naturally) for years will struggle to be really lean and overweight measured by BMI.

I obviously don't mean that this applies to you, you natty mass monster!
Lol thanks

My two cents..

While I don't disagree with the subjective impression of leanness being the only really important final say for informal purposes as recreational lifters, that doesn't mean that there aren't meaningful, categorical thresholds (especially for people who carry this hobby out with seriousness and a large chunk of their own personal time).

A2F, this actually seems to be the very point you were making in another recent thread, relating meticulousness positively with enthusiasm rather than as an unhealthy preoccupation, per se - such as with stamp collecting. If being in a certain formal stat category is someone's arbitrary goal and it gives them recreational or personal satisfaction to achieve it, I see nothing wrong with that being an important criterion for their own measure of success. Imagine being a high school kid itching to tune their beater from the 70s into a ten second car, getting fairly close to that goal but not being quite there, and being told that as long as it's clearly fast, what does it matter that it sprints a quarter mile in ten seconds?

Also, EiFit does have a good point. I can't think of an example of someone (myself or anyone I know or have been privy to) who would be considered "overweight" by BMI and doesn't have at least a handful of pounds of fat they could lose, even if only that. But then, it often ends up being more than it looks like anyway; bearing in mind, that probably depends on what definition we're using for subjectively fat, as that is on a wide range, loosely speaking.

28. Originally Posted by air2fakie
Just jiggle their stomachs, it's really a foolproof rest. :P

I find it funny how many people rely on numbers like BMI or "what's my bf%?" to tell themselves if they're overweight. Seriously, find a mirror.
LOL

True, but also look at the number of people coming to the forum who are completely delusional about their body composition (underweight people who want to lose fat and overweight people who think they are way leaner than they are). Mirror only works if your body dysmorphia is under control!

I actually stopped using the tape measure except occasionally (e.g. at the start and end of a cut instead of daily) because it leads to unhealthy obsession and just go by the mirror instead, but if I'd done that back in my fattie days I'd probably have ended up as Jabba the Hutt.

29. Originally Posted by EliKoehn
A2F, this actually seems to be the very point you were making in another recent thread, relating meticulousness positively with enthusiasm rather than as an unhealthy preoccupation, per se - such as with stamp collecting. If being in a certain formal stat category is someone's arbitrary goal and it gives them recreational or personal satisfaction to achieve it, I see nothing wrong with that being an important criterion for their own measure of success. Imagine being a high school kid itching to tune their beater from the 70s into a ten second car, getting fairly close to that goal but not being quite there, and being told that as long as it's clearly fast, what does it matter that it sprints a quarter mile in ten seconds?

BMI IMO isn't a particularly useful metric for people who lift weights, there should be a disclaimer. Bf% is somewhat useless for most people because unless you slice apart & measure a recently deceased cadaver, the estimates are all over the place. You'd think the medical/health/fitness industry could come up with a better metric/tool for the average person to use.

My point on the other thread was that IF someone is achieving good fitness & nutrition progress by engaging in somewhat obsessive behavior, without disrupting other aspects of their life, it's not a bad thing. It might even be the reason why they are able to stay on track mentally. But to your point, sure, if shooting for certain BMI or bf% numbers helps someone make good progress or stay on track, great - I just think the mirror is still a better measuring tool than both.

In either case, there are tons of people who engage in obsessive behavior and shoot for numerical goals - while doing everything wrong from a fitness/nutrition perspective. In the other thread I thought it was wrong to label behavior as a problem when the person had fitness/nutrition in order and was achieving good results.

30. Originally Posted by air2fakie

BMI IMO isn't a particularly useful metric for people who lift weights, there should be a disclaimer. Bf% is somewhat useless for most people because unless you slice apart & measure a recently deceased cadaver, the estimates are all over the place. You'd think the medical/health/fitness industry could come up with a better metric/tool for the average person to use.

My point on the other thread was that IF someone is achieving good fitness & nutrition progress by engaging in somewhat obsessive behavior, without disrupting other aspects of their life, it's not a bad thing. It might even be the reason why they are able to stay on track mentally. But to your point, sure, if shooting for certain BMI or bf% numbers helps someone make good progress or stay on track, great - I just think the mirror is still a better measuring tool than both.

In either case, there are tons of people who engage in obsessive behavior and shoot for numerical goals - while doing everything wrong from a fitness/nutrition perspective. In the other thread I thought it was wrong to label behavior as a problem when the person had fitness/nutrition in order and was achieving good results.
Ha, nice F&F reference.

And fair enough, given that the "measure" itself is rather vague and hard to arrive at precisely. As far just going off of what you see in the mirror, however, I for one totally thought I was less fat than I was a couple of years ago, possibly even last year. It might still be best to get an honest opinion from someone else since there's no objective way of seeing through one's own delusions. In lieu of that and in that regard, waist/height probably still is the best way to go.

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