# Talk:Body mass index

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## It ignores basic scaling laws

"it ignores basic scaling laws whereby mass increases to the 3rd power of linear dimensions" : Indeed! But in what strange medical world are we in to keep using something ridiculous like that ? — Preceding unsigned comment added by 144.85.157.254 (talk) 23:06, 15 June 2014 (UTC)

• Disagree. Pointing out the mathematical relationships between weight and height is an important part of explaining the shortcomings of BMI as a metric on which health decisions are based. It is appropriate to include it in the section on shortcomings, where it is currently. BakerStMD T|C 18:36, 14 December 2014 (UTC)

## Clarification on height

My problem with the Body Mass Index is that I have shrunk four inches since I was twenty. Do I use the height I am now (5'8"), or the height I was more than 20 years ago (5'11.75")? I have never been able to track this down, and even my doctor was uncertain. Almost all my family have shrunk with age, and it is common enough in the general population to be a significant factor in BMI calculation. Jgharston (talk) 18:22, 27 August 2010 (UTC)

Why would you even think of using the height that you were 20 years ago? (Except to make the BMI number lower, that is). You're measuring your weight as it is today, and you want to know your BMI as it is today, so why wouldn't you use your height as it is today? Chuntuk (talk) 13:26, 4 September 2013 (UTC)

@Jgharston, your BMI changes when your height changes. So your BMI is different now based on your current height than it was 20 years ago based on a different height. BakerStMD T|C 18:46, 14 December 2014 (UTC)

## Average BMI calculated from average weight and height

"Researchers ... calculated the average BMI ... using UN data on population, WHO estimates of global weight, and mean height ... ."

The cited source has a similar formulation. Clearly this is nonsense. The average BMI cannot be calculated from average weight and average height. The section should be removed. −Woodstone (talk) 08:08, 14 July 2012 (UTC)

Thank you very much for the response. A computer crash just deprived the universe of my long reply. This was probably a very good thing. Very much shorter version: I don't know enough to argue this properly; it would appear that you do; no-one else has come out to play! I suggest that, if you wish, you remove the offending bit (again: sorry) and see if it attracts further interest. WP:BRD and all that. Cheers, DBaK (talk) 07:18, 17 July 2012 (UTC)
Just saw your removal of the table. I don't see why it is not possible to calculate BMI that way. As the BBC is regarded as a reliable source, I'll restore the table. cmɢʟee 11:24, 31 July 2012 (UTC)
@User:cmglee, I believe the correct way to determine the average BMI for a group is to compute the BMI for the individual members of the group, then average the BMIs together. This will result in a different value than if you take the average height and average weight for the group and perform the BMI calculation on the average height and weight, because the relationship between height and weight are not the same for every person in the group. BakerStMD T|C 18:44, 14 December 2014 (UTC)
After doing this experiment, I'm now convinced that it's not possible to calculate average BMI from average mass and average height. So what do you do if a reputable source makes an error? Excluding it completely seems a drastic measure. How about adding a note that the calculation may be erroneous? cmɢʟeeτaʟκ 12:24, 16 December 2014 (UTC)
I like it, cmglee. Can we maybe let the WHO know? They're just people too. But for now I'd prefer excluding it. No reason to propagate erroneous information. Maybe we could replace it with a mention that the BMIs of the averages (wrong) is different from the average of the BMIs (Correct).BakerStMD T|C 21:14, 16 December 2014 (UTC)

I'm totally bollixed by the BMI definition. For the last 50 years I've been thinking it was the ratio of fat to muscle mass and that the only real way to measure it was through water immersion. I'd been taught that the height/weight chart was just a cheat sheet for the real (and complicated to administer) thing. deepsack (talk) 05:27, 5 January 2013 (UTC)

## Defining formula

Currently the defining formula is stated as:

 ${\displaystyle \mathrm {BMI} }$ ${\displaystyle ={\frac {{\text{mass}}({\text{kg}})}{\left({\text{height}}({\text{m}})\right)^{2}}}}$ ${\displaystyle ={\frac {{\text{mass}}({\text{lb}})}{\left({\text{height}}({\text{in}})\right)^{2}}}\times 703}$ †

The way units are indicated is rather sloppy. Formally correct would be:

 ${\displaystyle \mathrm {BMI} }$ ${\displaystyle ={\frac {{\text{mass}}/{\text{kg}}}{\left({\text{height}}/{\text{m}}\right)^{2}}}\ {\text{kg}}/{\text{m}}^{2}}$ ${\displaystyle ={\frac {{\text{mass}}/{\text{lb}}}{\left({\text{height}}/{\text{in}}\right)^{2}}}\times 703\ {\text{kg}}/{\text{m}}^{2}}$ †

This shows a dimensionless number times the unit it is expressed in. Would this form be too confusing for the average reader? Any better proposals? −Woodstone (talk) 08:22, 17 June 2013 (UTC)

Though strictly correct, BMI is not typically specified with units, as in http://www.nhs.uk/bmi and http://www.nhlbi.nih.gov/guidelines/obesity/BMI/bmicalc.htm so may lead to confusion unless specifically pointed out. cmɢʟee୯ ͡° ̮د ͡° ੭ 18:37, 18 June 2013 (UTC)

How is this not right?

5 ft, 8 in = 1.7272 m 220 lb = 100 kg

BMI in English measure 220 lb/(66 in ^2)=.0505

BMI in metric 100kg/(1.7272 m^2)=33.52083

BMI metric/BMI English = 704.5469

T Ashley --15:55, 6 August 2013 (UTC)65.88.43.2 (talk)

BMI in English measure 220 lb * 703/(68 in ^2)= 33.44 Omitting the 703 imperial adjustment changes everything 86.40.148.70 (talk) 16:58, 29 May 2014 (UTC)

If you measure your height in metres and your mass in kilograms, then divide the measured mass in kilograms by the square of the measured height in metres, they you will always get the right value. Then you don't have to worry if it is 702 or 703 or 704 or who cares? English measure is not designed to be exact, just to give Luddites a warm and fuzzy feel. — Preceding unsigned comment added by 174.69.64.122 (talk) 15:44, 4 January 2015 (UTC)

## Global statistics

What value or insight does the "Global statistics" table contribute to the article?

WP:INDISCRIMINATE Wikipedia is not an indiscriminate collection of information

--Nbauman (talk) 17:40, 12 April 2014 (UTC)

• Agree The table just clutters up the page and should be deleted. BakerStMD T|C 18:32, 14 December 2014 (UTC)
• The table data it is not supported by provided link. Also it clutters up the page. I removed it. Please provide WP:RS if you want to keep it. Innab (talk) 14:05, 12 May 2015 (UTC)

## New Body mass index

Body mass has nothing to do with height, rather it is the combination of fat, muscle and bone in a body. I'm a dweeb, so can't remember the name of the Greek who immersed himself in water and discovered the concept of body mass index. (along with some other hardly worthy of notice equations that are also used today.) The water displacement tool is somewhat cumbersome so people--being who they are--decided to go with the completely inaccurate but MUCH easier to do height/weight rule. These days the water displacement tool has been "ahem... displaced" by a tool that uses electricity to measure % of the three in the body. I've seen all three of them at work and I still consider a piece of paper that has not touched my body or in any way notices my ratio of Twinkies to exercise to be the stupidest idea ever considered. (by the way, that last sentence has been proven but apocryphal evidence is worse than no evidence.)deepsack (talk) 07:22, 18 August 2014 (UTC)

I think you're thinking of Archimedes with the water-displacement bit. Displacement is a measure of volume, which is related to weight by density, but volume is not included in BMI. Volume is correlated to height, but not perfectly, as more than just height goes into determining volume. BakerStMD T|C 18:30, 14 December 2014 (UTC)

## BMI Measuring Machines

Nearby grocery store has a machine next to the pharmacy that measures BMI amongst other things (BP, HR); it weighs you, asks age, sex, etc, ie enough to do a table lookup. But it also has hand grips with electrical contacts, implying that it measures resistance/impedance across a meaningful frequency range, to measure the body fat directly. Or maybe its just a pulse detector ? — Preceding unsigned comment added by 98.245.49.47 (talk) 20:38, 15 September 2014 (UTC)

Recent studies have shown that the waist-to-height ratio (WHtR) measurement is a better simple predictor of potential health conditions and outperforms other indices including BMI.

Your comparison with sportsmen is irrelevant. The problem I have with the lead is that it buys into the common understanding that BMI is the only simple measure of the issues which it has to deal with. As other comments on this talk page show, it's far from an ideal measure. It's not dimensionless, it doesn't directly work for children and it has a mediocre correlation with the problems it's assumed to be a predictor for: obesity, propensity for heart disease, diabetes etc. The only hint in the lead is the reference to "a range of perceived limitations and problems with the BMI". I'll maybe accept it's not the place to draw judgments since there isn't a widely agreed alternative, but it should at least draw attention to the fact that it is only one of a number of such simple measures and that specialists do not agree about them. Chris55 (talk) 10:49, 14 December 2014 (UTC)

## Problem with graphic

Obesity and BMI

The caption under the normal case is "25 < kg/m2 ", which could lead someone to think 25 is the lower bound. It should say "< 25 kg/m2 ". (Also, I get the vague feeling that something's missing from the graphic!) Loraof (talk) 14:55, 6 January 2016 (UTC)

Definitely the formula in the graphic is wrong. Not the unit "m", but the "height" should be squared.−Woodstone (talk) 16:18, 6 January 2016 (UTC)
I edited the image, changed to <25. Tom Ruen (talk) 23:29, 8 February 2016 (UTC)
Better, but you forgot to correct the displayed formula: height (m2) should be (height (m))2. −Woodstone (talk) 08:17, 9 February 2016 (UTC)

## Forms of weight and height

This article says Body Mass Index is defined as Body Mass divided by the square of body height. Perhaps it could clarify that this means weight in kilograms divided by the square of height in metres, so as to avoid confusion with imperial measures. Vorbee (talk) 08:32, 18 August 2017 (UTC)

Please read the second half of the sentence. −Woodstone (talk) 06:53, 19 August 2017 (UTC)