


Body Mass Index Formula
The body mass index formula gives the body mass index (BMI), or Quetelet index, is a measure which compares a person's weight and height. Though the body mass index formula does not actually measure the percent of body fat, it is used to estimate a healthy weight based on a person's height. Due to its ease of measurement and calculation, BMI is the most widely used diagnostic tool to identify weight problems within a population It was invented between 1830 and 1850 by Adolphe Quetelet a Belgian. Body mass index is defined as the individual's body weight divided by the square of his or her height. The formulae universally used in medicine produce a number with units in kg/m^{2}. BMI can also be determined using a BMI chart. or you can use the formula below:
While the BMI formula dates to
the 19th century, the term "body mass index" for the ratio and its
popularity date to a 1972 paper by Ancel Keys, which found the BMI to
be the best
way to estimate body fat percentage among the various ratios of weight
and
height. This
interest in measuring body
fat being due to obesity becoming a obvious issue in prosperous Western
societies. BMI was explicitly cited by Keys as being appropriate for population
studies, and inappropriate for individual diagnosis. Nevertheless,
due to its simplicity, BMI came to be widely used for
individual diagnosis, despite its inappropriateness. Now a little math to show why some people think that there should be a 3 (cubed) in the formula rather than a 2 (squared)
If these cubes were made of the
same material then the cube that doubled its height would have a weight
that
was 8 times (2³) as much as the smaller cube.
This is why the BMI formula is not accurate. If you took the
weight of a
person that was 3 feet tall and the weight of someone who had the same
shape
that was six feet tall, the weight of the taller person should be about
8 times
as much. Does that make
sense? Let us see.
If you go to the cdc site http://apps.nccd.cdc.gov/dnpabmi/calculator.aspx and plug in for a male 3
feet and 30 ¾ pounds
one gets a BMI of 16.7 which would be too small for an adult but is
about
average for a child (53^{rd} percentile). Take 30¾ times 8 = 246
pounds which is not
bad for a six foot man. If the BMI was
correct then: That 6 foot man
that was twice as
tall should weight four times (2x2) as much as the 3 foot child or 30 ¾
x 4 =
123 pounds. Too Skinny! This is why
another scale was
invented called the Ponderal index PI.
We already saw the Body Mass Index
formula problems
start to show up for very tall people and very short people. It results
in
taller people having a reported BMI that is uncharacteristically high
compared
to their actual body fat levels. Very short people ie children will
have low
BMIs in comparison to their fat level. Now the problem with the Pondeal
index: Tall
people are not just "scaled up" short people, but tend to have
narrower frames in proportion to their height. It has been suggested
that
instead of squaring the body height (as the BMI does) or cubing the
body height
(as the Pondeal index does), it would be more appropriate to use an
exponent of
somewhere between 2.3 to 2.7, but alas too many people are using the
BMI. Just like SI
units are better than Imperial
units but too many people are using them. Another problem
(in case no one
has noticed) is that men and women are not shaped alike…indeed no one
is shaped
alike…..Use these charts as a starting place only. Keep weighing but
remember: There are
other ways to measure weather or
not you are in shape: % Fat Body Measurements Blood pressure How many push ups How fast
am I? How much can I
lift? Do you float
easily? Blood sugar How do I feel? PS Here is how to interpret BMI:
If you run the body mass index formula and find that you are obese, the categories above make for excelent goals. For example if you have a BMI of 45, your first goal is under 40, second under 35, third under 30 and finally under 25. Once you get to a normal weight you can refine your goal by coming back to this site to study % fat, ideal body measurements, and other goals. 


