# Measuring BMI **URL:** \ **Category:** Factual Questions\ **Created:** [June 18, 2009, 2:29am UTC](https://boards.straightdope.com/t/measuring-bmi/500137 "2009-06-18T02:29:29Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Loach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/loach/32/350_2.png) [@Loach](https://boards.straightdope.com/u/Loach)\ **Post date:** [June 18, 2009, 2:29am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/1 "2009-06-18T02:29:29Z")
So I joined a new gym. As part of the sign up I got a free session with a trainer. It was part orientation, part sales pitch for paying for a personal trainer. During this they claimed to measure my BMI. They made me hold a device in front of me with both hands and after a bit it said what my BMI was. Where my hands held it were metal plates just like the sensors which measure heart rate on cardio machines. What was this machine? How can that thing measure BMI more accurately than just a simple height and weight[calculator](http://www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/english_bmi_calculator/results_obese.html?pounds=210&inches=69)? Like most I have a certain ammount of extra weight but I am also carrying a good bit of muscle. The machine did not seem to take that into account. So is this machine just snake oil or is there something to it? ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [June 18, 2009, 2:34am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/2 "2009-06-18T02:34:50Z")
Sounds to me like a device to measure body fat %. It may have a BMI function. Body fat % is far more accurate anyway. ---
**Author:** ![Otara](https://avatars.discourse-cdn.com/v4/letter/o/a88e4f/32.png) [@Otara](https://boards.straightdope.com/u/Otara)\ **Post date:** [June 18, 2009, 2:48am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/3 "2009-06-18T02:48:31Z")
Its a device that sends a small current through your body and estimates your body fat on that basis, because fat conducts differently to muscle or bone etc. Its more accurate than BMI in theory, but I find it very hard sometimes to find what level of precision they really have, let alone for a particular product. Otara ---
**Author:** ![Loach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/loach/32/350_2.png) [@Loach](https://boards.straightdope.com/u/Loach)\ **Post date:** [June 18, 2009, 2:50am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/4 "2009-06-18T02:50:37Z")
> [@runner pat](#): > > Sounds to me like a device to measure body fat %. It may have a BMI function. > > Body fat % is far more accurate anyway. But they told me what my BMI is, not body fat %. It is quite possible that they were using it wrong. ---
**Author:** ![silk1976](https://avatars.discourse-cdn.com/v4/letter/s/8edcca/32.png) [@silk1976](https://boards.straightdope.com/u/silk1976)\ **Post date:** [June 18, 2009, 3:03am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/5 "2009-06-18T03:03:12Z")
It sounds like there is [some discussion](http://www.webmd.com/diet/features/body-fat-measurement) regarding the usefulness of each - but I suspect that like most things, the truth lies somewhere in the middle, and BMI is useful for certain things, and body fat % is useful for others. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [June 18, 2009, 3:24am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/6 "2009-06-18T03:24:35Z")
You input weight and height - that’s all BMI is. It doesn’t take any additional measurements to calculate BMI. I had the same thing last week. > **[Amazon.com: Omron HBF-306C Handheld Body Fat Loss Monitor : Health &...](https://www.amazon.com/Omron-HBF-306C-Handheld-Body-Monitor/dp/B000FYZMYK)** > > The Omron handheld Fat Loss Monitor (HBF 306) helps you answer an important question in regards to your weight goals: am I losing fat or muscle? Clinically proven bioelectrical impedance technology gives you an accurate percentage in a matter of... This is the one they used on me. ---
**Author:** ![Hirka\_T\_Bawa](https://avatars.discourse-cdn.com/v4/letter/h/db5fbb/32.png) [@Hirka\_T\_Bawa](https://boards.straightdope.com/u/Hirka_T_Bawa)\ **Post date:** [June 18, 2009, 3:50am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/7 "2009-06-18T03:50:52Z")
A good measure to see if you are overweight is your Ideal Body Weight, the formula for that is: Males: 50kg +2.3kg for each inch above 5 foot or Female 45.5kg +2.3kg for each inch above 5 foot. If you are 20% over your Ideal Body Weight you are considered obese by the medical community. Oh, and that gives your weight in kilograms, to convert to pounds multiply your answer by 2.2 ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [June 18, 2009, 4:06am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/8 "2009-06-18T04:06:11Z")
> [@Hirka\_T\_Bawa](#): > > A good measure to see if you are overweight is your Ideal Body Weight, the formula for that is: > > Males: 50kg +2.3kg for each inch above 5 foot > or > Female 45.5kg +2.3kg for each inch above 5 foot. > > If you are 20% over your Ideal Body Weight you are considered obese by the medical community. > > Oh, and that gives your weight in kilograms, to convert to pounds multiply your answer by 2.2 This still doesn’t account for differences in body composition. By this formula, I should weigh 66.1kg (145.2) I weigh 87.3kg(192) body fat 12% My lean weight is 76.8kg(169) If I lost every bit of fat, I would still be 10.5kg(23.1) overweight. ---
**Author:** ![Otara](https://avatars.discourse-cdn.com/v4/letter/o/a88e4f/32.png) [@Otara](https://boards.straightdope.com/u/Otara)\ **Post date:** [June 18, 2009, 4:20am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/9 "2009-06-18T04:20:00Z")
The BMI formula is for sedentary adults, with a medium frame. If you’re within those criteria its a useful rule of thumb. If you dont, its not. Otara ---
**Author:** ![Loach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/loach/32/350_2.png) [@Loach](https://boards.straightdope.com/u/Loach)\ **Post date:** [June 18, 2009, 4:20am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/10 "2009-06-18T04:20:20Z")
> [@Telemark](#): > > You input weight and height - that’s all BMI is. It doesn’t take any additional measurements to calculate BMI. I had the same thing last week. > > [Amazon.com: Omron HBF-306C Handheld Body Fat Loss Monitor : Health & Household](http://www.amazon.com/Omron-HBF-306-Body-Fat-Analyzer/dp/B00006WNPU) > > This is the one they used on me. That is not the exact model but it is very similar to what they used. Funny thing is they used only the BMI function and not the fat % but still had me hold it for a minute. I guess they could tell that the BMI would be the worse than the fat % due to muscle density. They were trying to sell personal training after all. ---
**Author:** ![dgrdfd](https://avatars.discourse-cdn.com/v4/letter/d/eb8c5e/32.png) [@dgrdfd](https://boards.straightdope.com/u/dgrdfd)\ **Post date:** [June 18, 2009, 4:27am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/11 "2009-06-18T04:27:20Z")
That’s why everything except for [hydrostatic](http://www.wisegeek.com/what-is-hydrostatic-weighing.htm) testing isn’t accurate. There are certainly arguments that maybe for large populations they are acceptable. BMI would say [Lebron James](http://dascoop.files.wordpress.com/2009/06/bron-bron2.jpg) is overweight (6’8 240 BMI 26.4 (he’s actually closer to 260, but conservatively)) Of course, people will counter that he’s a top level athlete so it doesn’t work for him. But it really doesn’t work for anyone who is somewhat of an athlete. Maybe it works for Couch Potato Bob though so it’s acceptable for large populations. I just can’t wrap my head around the idea that what is inaccurate for the individual is somehow accurate for the entire population. I would also recommend you be weary of personal trainers long term. They are great to introduce you to the equipment and help you feel comfortable in a new environment, but many will hold you back long term. And you can learn for yourself the information for free on the internet. I can recommend some good message boards if you would like. Good luck and work hard. ---
**Author:** ![Hirka\_T\_Bawa](https://avatars.discourse-cdn.com/v4/letter/h/db5fbb/32.png) [@Hirka\_T\_Bawa](https://boards.straightdope.com/u/Hirka_T_Bawa)\ **Post date:** [June 18, 2009, 4:48am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/12 "2009-06-18T04:48:45Z")
> [@runner pat](#): > > This still doesn’t account for differences in body composition. > > By this formula, I should weigh 66.1kg (145.2) > > I weigh 87.3kg(192) body fat 12% > My lean weight is 76.8kg(169) > > If I lost every bit of fat, I would still be 10.5kg(23.1) overweight. No, it doesn’t, it is one of those measurements that work for a population, but not for an individual. If you are someone that is in great shape, it will be inacurate for you. However, that doesn’t mean it isn’t used regularly in the medical establishment (mainly Pharmacy since that is what I know). One of the major calculations that use IBW is estimating your creatinine clearance which is used to determine your Kidney function (which is used to help dose certain drugs). It is normally used for the first dose, then blood work is used after that. Now, I know this doesn’t have too much to do with BMI, but as with all estimates, it is useful on a population level, not a individual level. So, if you are an athlete, or otherwise is really good shape, neither BMI or IBW, or any of these other measurements will be 100% accurate. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 18, 2009, 5:01am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/13 "2009-06-18T05:01:45Z")
> [@](#): > > Males: 50kg +2.3kg for each inch above 5 foot Why on Earth would a formula use feet and inches for height, but kilograms for mass? ---
**Author:** ![Hirka\_T\_Bawa](https://avatars.discourse-cdn.com/v4/letter/h/db5fbb/32.png) [@Hirka\_T\_Bawa](https://boards.straightdope.com/u/Hirka_T_Bawa)\ **Post date:** [June 18, 2009, 5:20am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/14 "2009-06-18T05:20:22Z")
> [@Chronos](#): > > Why on Earth would a formula use feet and inches for height, but kilograms for mass? Trust me, I don’t understand it either! It is the only formula I know of that uses mixed units, Imperial and Metric! Unfortunately, the people who taught it to me were Pharmacists, so they didn’t know and or care about it. But it has bugged me every time I’ve seen it (Probably one of the reasons I can actually remember that formula off the top of my head!) ---
**Author:** ![Dog80](https://avatars.discourse-cdn.com/v4/letter/d/6bbea6/32.png) [@Dog80](https://boards.straightdope.com/u/Dog80)\ **Post date:** [June 18, 2009, 8:54am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/15 "2009-06-18T08:54:40Z")
> [@runner pat](#): > > I weigh 87.3kg(192) body fat 12% > My lean weight is 76.8kg(169) :eek: IIRC you are the bearded man riding that funny bicycle, right? Then you are heavier than you look on that picture. You look really skinny. ---
**Author:** ![Svejk\_1](https://avatars.discourse-cdn.com/v4/letter/s/ea666f/32.png) [@Svejk\_1](https://boards.straightdope.com/u/Svejk_1)\ **Post date:** [June 18, 2009, 9:50am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/16 "2009-06-18T09:50:30Z")
[QUOTE=Chronos] Why on Earth would a formula use feet and inches for height, but kilograms for mass? [/QUOTE] What does it matter? If you like your formulas metric-only, you can use ```auto Ideal Body Weight (kilos) = 50 + 0,91*(length-152.4) ``` ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [June 18, 2009, 10:09am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/17 "2009-06-18T10:09:31Z")
> [@Svejk\_1](#): > > What does it matter? If you like your formulas metric-only, you can use > > ```auto > > Ideal Body Weight (kilos) = 50 + 0,91*(length-152.4) > > ``` Yes, but the formula isn’t presented in that manner. Which is what he thinks is weird. In the US, I think it would make more sense to be all imperial. ```auto Ideal Body Waight (lbs) = 110 + 5.07*(height(in)-60) ``` ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [June 18, 2009, 10:27am UTC](https://boards.straightdope.com/t/measuring-bmi/500137/18 "2009-06-18T10:27:38Z")
> [@Chronos](#): > > Why on Earth would a formula use feet and inches for height, but kilograms for mass? I’m guessing it was once 110 lbs + 5 lbs for every inch above 5 foot. ---
**Author:** ![robardin](https://avatars.discourse-cdn.com/v4/letter/r/43a26b/32.png) [@robardin](https://boards.straightdope.com/u/robardin)\ **Post date:** [June 18, 2009, 12:47pm UTC](https://boards.straightdope.com/t/measuring-bmi/500137/19 "2009-06-18T12:47:20Z")
> [@Chronos](#): > > Why on Earth would a formula use feet and inches for height, but kilograms for mass? Because strictly speaking, “pounds” is not a measure of mass. This way you can use the formula on the Moon or on Mars with the same measuring tape (in inches) and scale (reading in both lbs. and kg). Not many scales give your weight in slugs! Or, it’s just one of those silly things that got written down by someone and copied over and over verbatim without question. ---
**Author:** ![Khadaji](https://avatars.discourse-cdn.com/v4/letter/k/9e8a1a/32.png) [@Khadaji](https://boards.straightdope.com/u/Khadaji)\ **Post date:** [June 18, 2009, 1:20pm UTC](https://boards.straightdope.com/t/measuring-bmi/500137/20 "2009-06-18T13:20:05Z")
I have one of those devices. Did it look like [this?](http://www.amazon.com/gp/product/B000FYZMYK/ref=ox_ya_oh_product) When I bought it I asked here how accurate it would be, and only got one response which said: not very accurate. I don’t know if that counts as a good cite or not though. [Next page](https://boards.straightdope.com/t/measuring-bmi/500137.md?page=2)