Why does BMI have units of kg/m^2 when we are three dimensional? Wouldn’t kg/m^3 or g/cm^3 be more accurate?

Question

Why does BMI have units of kg/m^2 when we are three dimensional? Wouldn’t kg/m^3 or g/cm^3 be more accurate?

In: 100

Answer 1

It is basically just a very unsufficiant method to guess the area of your waistline in proportion to body height (which isn’t the worst method to estimate if someone is obese)

​

It is based on two assumptions that are both untrue but close enough to the truth that the result is not completely meaningless.

1st assumption: The human body has the shape of a solid cylinder

2nd assumption: The density of the human body is consistent (bones, muscles, fat all have the same density). Let’s assume the density is 1 for the rest of the argument but it works with any density in principle.

Density = BodyWeight / BodyVolume

When we replace density with 1 and BodyVolume with the formula of a cylinder, we get:

1 = BodyWeight / (BodyHeight * CircularAreaOfWaist)

Using Algebra we get

CircularAreaOfWaist = BodyWeight / BodyHeight

CircularAreaOfWaist / BodyHeight = BodyWeight / BodyHeight^2

CircularAreaOfWaist / BodyHeight = BMI

​

So if you would just measure your waist, calculate the area and devide it by your body height, you would get a much better version of BMI. There is never a good reason to use the BMI for individuals. According to it’s inventor it was only meant to be used to get estimations about large groups where you only have little data (weight and height are commonly available in big medical datasets). Like comparing all military conscripts this year with military conscripts one decade ago.