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


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.

21 thoughts on “Calculate your own body mass index | Miscellaneous | Heatlh & Medicine | Khan Academy

  1. 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.

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

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