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.

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 could 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 the problem and I'll show you what that was.

So, my weight is 160 pounds and my height is six foot, one 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, "Alright, well one pound equals 0.454 kilograms. So far, so good.

And now one inch, which is pretty small, is going to be a small number relative to a meter, which is huge, right?

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, "Okay, 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 can say, "Okay, 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, "Okay, so my BMI is 160 divided by 73 squared times 703."

Let me erase that just to make it clear.

Okay, so this math works out to 21.

So my BMI is 21.

Wonderful. The next question is: well, what does that mean?

You know, 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 on here starting with zero 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 catergories

and they said, "Okay, well if your BMI as an adult is somewhere between zero and 18.5

somewhere in this range. 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 can say, "Well how about W equals BMI times H squared over 703, right?"

Because that's the conversion. So if I rewrite the equation like this

then now I can solve for my weight. I can say, "Okay, 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 anymore 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.

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.