The BMI Equation

The BMI equation (BMI = Body Mass Index) was originally framed by Adolphe Quetelet, a Belgium mathematician and scientist, between 1830 and 1850. Adolphe was the first person to think of relating weight to height in a statistical, expressible manner. It's kind of an odd thing to think about - if you think about it!

In the 1950's and 1960's (yes, that far back!) we were getting noticeably fatter, so Adolphe's nineteenth century BMI equation was dusted off and used to express relative thinness or fatness across a large population.

Whether working with average weight people, over weight people, or the anorexic the BMI calculation provides a snapshot of relative weight.

Quetelet saw BMI as a straight forward way of classifying physically inactive individuals with average body composition. For these 'average' people the current interpretations of BMI are here in this table:

Classifications - The BMI Equation

Description	Lower BMI	Upper BMI
Anorexic	Below 17.5
Underweight	17.5	18.5
Optimal	18.5	25
Overweight	25	30
Obese	30	40
Morbid Obesity	Over 40

How to Calculate BMI

Here's the formula for calculating BMI. The easiest calculation applies to you if you live in the Metric zone where you think of weight in Kg and height in meters.

For most North Americans we're still thinking of body weight in terms of pounds and height in inches, so we apply the same formula and multiply by 703. As for you lucky ducks in the UK with the curious but wonderful combination of stones and meters the multiplying factor is 6.35!

In all cases the core formula is the same: weight is divided by height squared.

Calculating BMI - an example

Suppose you weigh 160 pounds and are 5 feet 10 inches tall. Here's how you calculate your BMI;

1. Convert you height into inches: (5 feet x 12 inches) + 10 inches = 70 inches

2. Now (the hard part!) square your height in inches: 70 x 70 = 4900

3. Divide weight by the squared height: 160 / 4900 = .0326

4. Multiply by the correcting factor of 703 since you're not using metric: .0326 x 703 = 22.9 BMI

You've got to wonder about old Adolphe back there in 1835... It must have been a slow news day when he doodled that formula out! Here's the actual math for the BMI formula:

Metric units	$Wikipedia Formula Metric$
USA	$Wikipedia Formula USA$
Imperial	$Wikipedia BMI Formula Imperial$

(Source: Wikipedia.org)

BMI Equation Summary

Notice that Adolphe and others saw BMI as a statistical measurement for sedentary people of average European build. For instance, Southeast Asians have a slightly different BMI chart:

Category	BMI range
Anorexic	less than 14.9
Underweight	from 15 to 18.4
Normal	from 18.5 to 22.9
Overweight	from 23 to 27.5
Obese	from 27.6 to 40
Morbidly Obese	greater than 40

Also, the BMI calculation doesn't apply to kids in the same way it applies to adults.

BMI is a useful tool to check body mass index for a population-wide "snapshot" measure of fatness or thinness, but the BMI equation may not apply directly to you, especially if you are already athletic and more muscular than average, or if you're not of European background.

That's the history and use for the BMI Equation! Don't forget...