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.