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### Course: Health and medicine > Unit 10

Lesson 6: BMI and diet# Calculate your own body mass index

Find out how simple it is to calculate your BMI and the common BMI categories. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

## Want to join the conversation?

- This is such a frustrating video for me as a health professional, as while accross a population BMI has a strong correlation with obesity and related health problems, but the BMI numbers themselves are for epidemiological purposes, not for individuals. A 210lb body builder with 5% bodyfat may show up just as obese via BMI as someone who is 180lbs but 35% bodyfat. Obviously you can look at their body and their BMI and know if this is the case...but it is a fundamentally flawed and inaccurate measurement none the less. BMI does not look at WHAT the weight is made of and comes from, ignoring if you are more barrel chested/stocky a build, or are very muscled. These things make a MASSIVE difference. I am 245 pounds but have over 200 pounds of lean body mass; via BMI I am considerably obese, but have a reasonably healthy degree of body fat. Using circumference measurements is a method to arrive at a fairly accurate bodyfat percentage calculation, and anyone can do it. This actually looks at lean and fat body weight in a more reasonable fashion. BMI is misleading and incorrectly founded and used.(28 votes)
- Just because it's outdated doesn't mean we do not use them. What if technology broke, what if your mirror broke? I'm looking at this useful and valuable video right now because I have a competition later in March to solve this (BMI). Nothing is allowed to be brought on stage (that means no real person or mirrors). I think this video is quite helpful, compared to other websites that tells you to buy a mirror(1 vote)

- BMI is relatively easy to calculate, but is it always accurate as to the state of a person's health? The Michelin Man and a NFL pro football player may have the same BMI, but their diet and fitness levels may differ significantly. Also, in the U.S. in 1998, the curves were adjusted DOWN so that some people who were in a normal weight range were now considered overweight. Why did this change occur?(8 votes)
- If I remember correctly, BMi was supposed to be used on groups of people in the first place, and not on individuals, since many of its shortcomings are less significant when used for statistical research.(11 votes)

- I'm at BMI 32, so I'm obese. I always imagined obese meant you can't move well, but I can run 13km or swim 2.5km in just over an hour and generally feel fit. So I never imagined I would be officially obese.(5 votes)
- You need to take into consideration the fact that BMI is a simplified way of determining a person's shape and adiposity (percentage of of body mass that is fat). This measure was initially intended to be used as a statistical device and assumes a "normal" amount of fat and muscle density within an individual. However, this assumption is clearly flawed when one considers individual cases--for example bodybuilders which by the BMI standard are also obese. Obesity, at its core, describes a situation wherein a person carries so much body fat that it begins to have an adverse effect on their health. The BMI measures how much mass you carry per unit area of your body (kg/m^2) with the area parameter being derived from a person's height squared. This measurement does not discriminate between body fat and lean mass. Therefore, in individual cases it can lead to misleading diagnoses of obesity. Not to say that you are perfectly fit (I don't know what your frame is or your %body fat), but remember to take your BMI with a grain of salt if your build is not that of the statistically average human being.(11 votes)

- is there a place where you can get various bmi ranges? like 10 or 12 year olds.(5 votes)
- This is a chart with percentiles: http://www.cdc.gov/growthcharts/data/set1clinical/cj41c023.pdf

Age group 2 to 20 years. Boys.

SOURCE: The National Center for Health Statistics in collaboration with the National Center for Chronic Disease Prevention and Health Promotion (2000).

See http://www.cdc.gov/growthcharts for more charts.(6 votes)

- Ask a question...In the video at2:37what do you mean by height squared?(1 vote)
- It is the height of the object times itself one time.(6 votes)

- Why do Americans still use imperial measurements? They make no sense! It would make it easier for everyone to just update (sure it would be challenging for a generation or two but the end result would be worth it) :)(3 votes)
- Technically, it makes absolutely no sense to measure weight in kilograms when kilograms is a measure of mass and weight is mass x acceleration due to gravity(3 votes)

- Is the way to calculate childrens BMI different than adults?How?Why?(3 votes)
- No. The only thing that is probably different in that children would be in adults under weight to have good weight.(3 votes)

- I am student nurse. We learn to use the BMI scale to estimate whether or not a person is overall healthy, need a change in diet, is at risk for certain pathologies etc.

What continues to worry me is the already pointed out inaccuracy of the BMI.

My BMI is just over thirty, making me obese. I weigh 115 kg and measure 1,94 metres.

I have recently lost about 15 kilos worth of fat and put on about 25 kilos worth of muscle. I have never felt better. Because of my BMI I am considered unhealthy!

I worry about how misleading this scale is.(3 votes)- The scale is incredibly misleading because it doesn't account for muscle mass, which weighs more than fat! That 10 kilos of muscle you gained after all the loss is probably whats tipping you over. Sounds like you're feeling amazing, so keep enjoying that!! I have no idea why it is still used in medicine, it really baffles me!(2 votes)

- I am sure that BMI is a convention and its pratical index, considering that it is easy to calculate and also easy to check it out, but can we use BMI as a predictor of healthy style of life? I mean lot of people have a good BMI, an eutrophic one, but Im not sure if this mean that they are healthy.(2 votes)
- I am 45 year working in autoparts factory,having lower back pain,what types of exercises is best for me(1 vote)

## Video transcript

So let's talk about
body mass index, BMI. This is a term that's
often discussed, and so let's first figure
out how it's calculated. How do we figure this out? So if you have a little stick
person, what you could do is have them step on a scale. And you get their weight. So the first thing you
need is their weight. And then you go on,
and you ask them if they would mind
if you would take their measurement of height. And so you get
their H, or height. And a BMI is basically just
taking those two numbers and using them in a
little equation where you take the weight divided
by the height squared. Very simple, right? So now let me calculate my BMI. But before I do that, I have to
tell you one more thing, which is that the weight, this,
is in kilograms, kg. And the height is actually
in meters squared. So when I was actually figuring
out my own height and weight, I realized a problem. And I'll show you what that was. So my weight is 160 pounds and
my height is 6 foot 1 inch, which is the same as 73 inches. So I would love to use
this easy equation, but I have pounds
and inches, I don't have kilograms and meters. So what do I do? Well, what I need to do
first is convert over, right? So I can say, all right,
well, 1 pound equals-- and this is something
you can just look up on the internet--
is 0.454 kilograms. So far, so good. And now 1 inch, which
is pretty small, is going to be a small number
relative to a meter, which is huge. And so it is, it's
0.0254 meters. So now I've got my conversions. And actually, what I can do
is I can take these numbers and plug them in here to help
me easily convert from kilos and meters over to
pounds and inches-- make my equation much
simpler for me to use. So I can say, OK, well now
how about weight in pounds and height in inches squared. And I have to multiply, right? Because I have to multiply
by the conversion. And so I'll multiply by 0.454
divided by 0.0254 squared. And to make it even
easier, I can actually take this whole thing,
plug it into my calculator, and it gives me the number 703. So I could say, OK, so
really what I have here is, again weight in pounds
divided by height in inches squared multiplied by 703. So going back, now I can finally
throw in my own numbers, right? I can say, OK, so my BMI is
160 divided by 73 times-- let me write that out--
73 squared times 703. So let me erase that
just to make it clear. So this math works out to 21. So my BMI is 21. Wonderful. The next question is,
well, what does that mean? If I have a BMI of
21, is that normal? Or is that good? Or how should I feel about that? So if you put a scale out
here starting with 0 to, let's say 30. And I have 25 here. And let's say this
is about 18.5. What physicians have
done is basically divide up the BMI
into categories. And they've said, OK, well,
if your BMI as an adult is somewhere between 0 and
18.5, somewhere in this range. And I'm going to
do it in yellow, then you're underweight. I'm just going to write under. And if you're between 18.5 and
25, if you're in this range, you're in a very healthy range. And so I'll write a little
smiley face for that. And if you're above
25, let's say 25 to 30, then you're in the
overweight range. And finally, if
you're above 30-- let's say you're out
here-- then you're obese. And so that's where the word
obese really comes from. It comes from a category of BMI. And so going back to my
BMI, I'm right here at 21 and I'm doing pretty well. But the question might
come up in my head, what weight would I be
if I was overweight? And what weight would
I be if I was obese? How many pounds away am I from
being in those categories? So I can go back to my equation,
BMI equals W over H squared, and I can just rearrange it. I could say, OK, well, how about
W equals BMI times H squared over 703, because
that's the conversion. So if I rewrite the
equation like this, then now I can
solve for my weight. I can say, OK, well, let's say
that I wanted to figure out what to do if I
have a BMI of 25. And I know that my height
probably won't change. I'm not going to grow any more. And 703 is the
number we always have to use when we're using the
conversion from pounds to kilos and inches to meters. So what does that equal? Well, in this particular
case, using 25, I can see that for
me to be overweight, I would be around 190 pounds. So that's actually really,
really good to know. That's actually 30 pounds
above what I am today. So that gives me a
sense for how far away I am from being overweight. And I can even do
this for a BMI of 30. I can say, what
about a BMI of 30? My current height,
again, is 73 inches. That's not going to change. And I can do the math and
find out that, in this case, my weight would be 227 pounds. So if I was 227 pounds,
given my height, I would be considered obese.