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Calculate your own body mass index | Miscellaneous | Heatlh & Medicine | Khan Academy

February 14, 2020


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.

22 Comments

  • Reply iibornfighter October 20, 2012 at 11:32 am

    First 😀

  • Reply Paul Miller October 30, 2012 at 4:43 pm

    Is there such a thing as 'age-related' BMI? As a person gets older they tend to get 'fatter' (could be related to what they are doing more of of course) so being the 'right weight for height' would get more demanding without some adjustment to guidelines I'd have thought.

  • Reply q7w7e7r7t7y7u7i7o7p February 8, 2013 at 4:07 am

    damn my bmi is 29.5

  • Reply Andriy papirov July 5, 2013 at 4:19 am

    Hi! I'm Hannah.I did -10 lbs in two months.Open hddiet.gs#Odll

  • Reply David 110 September 28, 2015 at 10:21 pm

    mines 5.3 I don't know what I did

  • Reply davidbennett60 January 10, 2016 at 9:36 am

    Whoa! Did you just calculate the factoral of a half, multiply it by 2 and square it?

  • Reply YVONNE BRUCE January 26, 2016 at 10:57 pm

    dont know how u got the 7.3 ugh

  • Reply Sanda St Fort February 16, 2017 at 4:28 am

    How does 1 pound equal 0.454kg? I thought 1lb=2.2kg

  • Reply Castro Clare February 16, 2017 at 5:18 am

    Isn't 1 lb = 2.2 kg ? Where did you get the 0.454?

  • Reply StudentNurseJayJay February 16, 2017 at 1:06 pm

    Bardoush? lol

  • Reply damini zaveri July 7, 2017 at 7:25 pm

    simpal calculations height in inches = weight eg. 60 inches heights your weight should be 60 kg that's simple

  • Reply damini zaveri July 7, 2017 at 7:27 pm

    jitnaa inches heights hai utnaa kg weight honaa chaiya

  • Reply Flamingo Flamingo August 27, 2017 at 5:11 pm

    My BMI is 15. DANG IT

  • Reply Joey Guerrero October 5, 2017 at 6:24 am

    .454/.0254 is not 703

  • Reply Matt Obra January 5, 2018 at 4:33 pm

    Does women and men have the same BMI scale?

  • Reply Mani Potlada October 10, 2018 at 9:34 am

    My hight cm 148, waight 60.5 now tell me my bmi

  • Reply Angiel Stahley November 11, 2018 at 11:19 pm

    A lot fluff

  • Reply Insam Max February 26, 2019 at 8:54 am

    My weight 90 kg.my height 5.7 wat is my BMI.

  • Reply Tari Baron March 23, 2019 at 4:51 pm

    arrrg. We are not allow to use calculators! That's why I am watching the video!

  • Reply Angela Ortigoza March 25, 2019 at 6:45 pm

    I googled it you can plug in your info and the number comes out and it tells you what you are

  • Reply Cami BluTube July 17, 2019 at 6:29 am

    Learned in dietician classes that you can convert this in two ways, not just one!

  • Reply Weight Foods January 14, 2020 at 7:05 am

    Calculate BMI Calculator https://www.weightfoods.com/diets/bmi-calculator :Age Male Female KG/CM/LBS/POUND/INCH

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