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
Literally because the formula is mass (kg) / (height (m))^2.
The end calculation has the units from the input.
It sounds like you’re maybe asking why we care about kg/m^2 and not kg/m^3. That’s a decent question. BMI is a very rough guide so it doesn’t need to be perfect. It’s useful in that a result of less than 18 or over 25 probably means you should double check. There are perfectly legit reasons for those results, but there are also lots of unhealthy reasons.
BMI is a very rough tool to give an estimate on a complex topic. It doesn’t need to be perfect, it just needs to say if you should probably dig a bit deeper.
Literally because the formula is mass (kg) / (height (m))^2.
The end calculation has the units from the input.
It sounds like you’re maybe asking why we care about kg/m^2 and not kg/m^3. That’s a decent question. BMI is a very rough guide so it doesn’t need to be perfect. It’s useful in that a result of less than 18 or over 25 probably means you should double check. There are perfectly legit reasons for those results, but there are also lots of unhealthy reasons.
BMI is a very rough tool to give an estimate on a complex topic. It doesn’t need to be perfect, it just needs to say if you should probably dig a bit deeper.
It has units of kg/m^2 because it is weight over height squared, basically.
On a population level it works: BMI has a correlation with various things, and because it works off data you *have*, it is better than no measure.
On an individual level it is not a good measure of obesity: it just isn’t a very good measure of body fat percentage for people who are not of average height. Better individual measures exist but they use data most people haven’t provided, like the circumference of the neck or a caliper measurement.
In other words, your surmise that something without the right units is not a good measure is a decent engineering intuition.
Literally because the formula is mass (kg) / (height (m))^2.
The end calculation has the units from the input.
It sounds like you’re maybe asking why we care about kg/m^2 and not kg/m^3. That’s a decent question. BMI is a very rough guide so it doesn’t need to be perfect. It’s useful in that a result of less than 18 or over 25 probably means you should double check. There are perfectly legit reasons for those results, but there are also lots of unhealthy reasons.
BMI is a very rough tool to give an estimate on a complex topic. It doesn’t need to be perfect, it just needs to say if you should probably dig a bit deeper.
All this answer miss the point: the BMI *is* a measure of density, just the superficial density.
In other words it measure the average density of the slices that stacked together makes us. The higher the density of the slice the larger we are.
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.
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.
All this answer miss the point: the BMI *is* a measure of density, just the superficial density.
In other words it measure the average density of the slices that stacked together makes us. The higher the density of the slice the larger we are.
Yes, you are right. The human body actually scales with an exponant of about 2.3. So if someone is 10% larger, they should weigh 1.1^2.3 ≈ 1.25 times as heavy. The fact that you still need a chart to see what BMI is healthy at a certain length is a direct consequence of using the wrong exponent in the formula.
All this answer miss the point: the BMI *is* a measure of density, just the superficial density.
In other words it measure the average density of the slices that stacked together makes us. The higher the density of the slice the larger we are.
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