# The Naked Scientists Forum

### Author Topic: How is Body Mass Index - BMI - calculated?  (Read 3911 times)

#### DonBrown

• Jr. Member
• Posts: 18
##### How is Body Mass Index - BMI - calculated?
« on: 08/10/2008 21:37:12 »
Listening to a program on R4 today, references to Body Mass Index said that BMI of 40 was clinically obese and for some people, 35 was reason for serious concern.
Since my body mass index is about 50 kg/m and most people think I'm a bit skinny, I'm puzzled!
I tried inverting it to get a BMI of 0.02 m/kg, but that didn't work.  So perhaps they're using old fashioned imperial units: but 2.3 stone/ft doesn't work either.
In my experience, BMI is invariably referred to as an index with no units (aka a ratio?), so I wonder how this is achieved?*

Can any life scientist enlighten me?

Don.

*
The best thing that I can think of, is to compare ones mass to the mass of a lump of lard whose dimensions are related to ones height - say my mass (90kg) divided by the mass of 7.64m^3 of lard (vol=my height cubed) = 90kg / 7026kg = 0.0128  (dimensionless!)
Since this is inevitably much less than 1, I would suggest working in milliBMI's, so that would be about 13 mBMI. (If people accidentally failed to mention the unit / dimension, then it would not matter as there would be no confusion between BMI, mBMI, uBMI etc.,because the factor of 1000 would be much greater than the range of observed values.)
« Last Edit: 11/10/2008 10:17:27 by chris »

#### RD

• Neilep Level Member
• Posts: 8134
• Thanked: 53 times
##### Re: How is Body Mass Index - BMI - calculated?
« Reply #1 on: 09/10/2008 00:12:52 »
Quote
you can calculate your own BMI. Divide your weight in kilograms by the square of your height in metres,
or divide your weight in pounds by the square of your height in inches, then multiply by 703.

http://news.bbc.co.uk/1/hi/health/5297790.stm

The BMI can be misleading if the person is very muscular, e.g. a bodybuilder can have an obese BMI.
« Last Edit: 09/10/2008 00:14:27 by RD »

#### DonBrown

• Jr. Member
• Posts: 18
##### Re: How is Body Mass Index - BMI - calculated?
« Reply #2 on: 09/10/2008 12:22:20 »
The formulae explain why I was not getting the right numbers.
It doesn't of course explain how everyone gets it to be a dimensionless number: these formulae would give units of kg/m^2 (I haven't checked yet, but presumably the 703 is a combination of the conversion factors for ft^2 to m^2 and lb to kg.)

I'm very disappointed to find from the BBC graph that I'm overweight: I've always thought of myself as rather skinny.  But at least people won't laugh any more when I say I'm trying to lose weight.

#### Bored chemist

• Neilep Level Member
• Posts: 8676
• Thanked: 42 times
##### Re: How is Body Mass Index - BMI - calculated?
« Reply #3 on: 09/10/2008 19:36:37 »
If you ask people their age they usually say something like "I'm 25", they don't bother with units. It's bad practice but common.

#### The Naked Scientists Forum

##### Re: How is Body Mass Index - BMI - calculated?
« Reply #3 on: 09/10/2008 19:36:37 »