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The Body Mass Index is supposed categorize you into a place in a scale to determine how healthy you are. What components would one need to provide to do a BMI test, and why?

If one were to do a BMI test, how might they proceed in doing so - what are the steps involved and how must the calculation be executed?

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Hi Mochan, welcome! I think some of these could be good questions, but could you please separate them into individual questions? –  Kate Dec 11 '12 at 8:58
    
-1 As some of the question might have already been answered on the site, I highly dislike question lists. Most of the questions are either off topic here, or could be counted as this question does not show research effort. Additionally some of the questions are highly theoretical or even unanswerable, please check our FAQ. –  Baarn Dec 11 '12 at 9:18
    
I see, @Informaficker. I will do as both comments have suggested and focus on only a couple crucial points. I agree with what you are thinking, also. Thanks ^^ –  Grace Dec 11 '12 at 9:21
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I removed the -1. But I am not sure if the question is too basic and simple to answer. Simply reading the Wikipedia Article on BMI should give more than enough information. –  Baarn Dec 11 '12 at 9:27
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Thanks for asking the question, part of the beta is determining the scope of the site. I posted a question on Meta about this. –  Baarn Dec 11 '12 at 11:27

3 Answers 3

up vote 4 down vote accepted

I am taking most of the data from Wikipedia here, but most consider this common knowledge anyway.

The BMI or Body Mass Index is calculated by dividing your weight in kilograms by your squared height in meters.

BMI = kg / m²

So for example if you weight 65kg and are 1.73m tall you would get:

65 / 1.73² = 21.7

This index is then matched to a table to give you a rough estimate if you are over or underweight. I copied this table partially from Wikipedia, too.

Very severely underweight               less than 15.0
Severely underweight                    from 15.0 to 16.0
Underweight                             from 16.0 to 18.5
Normal (healthy weight)                 from 18.5 to 25
Overweight                              from 25 to 30
Obese Class I (Moderately obese)        from 30 to 35
Obese Class II (Severely obese)         from 35 to 40
Obese Class III (Very severely obese)   over 40

BMI tables might vary, for example there are other values used for children or even for people from different nations.

These values are, as stated above, only a rough measure, some people with completely good health might fall into the overweight categories easily when they are for example very tall athletes.

BMI is particularly inaccurate for people who are fit or athletic, as the higher muscle mass tends to put them in the overweight category by BMI, even though their body fat percentages frequently fall in the 10–15% category, which is below that of a more sedentary person of average build who has a healthy BMI number.

The Wikipedia article gives a lot of other reasons why BMI is inaccurate for a single person to determine their health. BMI is mostly useful for statistical comparison of groups of people:

The BMI is generally used as a means of correlation between groups related by general mass and can serve as a vague means of estimating adiposity. […] Generally, the index is suitable for recognizing trends within sedentary or overweight individuals because there is a smaller margin for errors.

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Great answer. Thanks again :) –  Grace Dec 11 '12 at 9:47
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FYI, the index posted looks like the index for women. There are different indices based on gender and age. Also remember that since it is a ratio of height to weight, Arnold Schwarzenegger was considered morbidly obese back in his lifting years too. –  Grohlier Dec 11 '12 at 11:20
    
@Grohlier I took the table directly from the English Wikipedia (cutting some data about BMI prime) there was no notion about gender. However the German Wikipedia states that the ranges for men are a bit higher (eg normal 20-25kg/m²) and for women a bit lower (19-24kg/m²). But these are adjustments have been done by the DGE (Deutsche Gesellschaft für Ernährung – German Nutrition Association). –  Baarn Dec 12 '12 at 7:59
    
Weird. Generally women are given the higher BMI-normalcy ranges because they normally carry more fat (breasts and buttocks). Thanks for the clarification. –  Grohlier Dec 14 '12 at 2:34

There are some other ways to calculate body fat other than the weight to height ratio or measuring waist and hip circumference.

1) Bio-electrical Impedance. This method uses a machine to send a small electrical impulse through your body. You normally have to input your Age, Gender, Height, Weight, and Physical Activity Level. The more information you put in the more accurate these types of machines are. Depending on how much fat is present the signal will slow down denoting more fat is present (because fat is a poor conductor of electricity). The machine then calculates your body fat. Being dehydrated can skew the results of this test.Link Here

2) Skin fold test. This test involves using a caliper to measure the thickness of a specific area of the body. There are anywhere from 3-9 sites from which a trained professional can take the measurements from to calculate the results. The more sites you use the more accurate the measurement will be. If the calipers are calibrated appropriately and the person taking the measurements is trained appropriately, the test is 98% accurate.Link Here

3) Underwater weighing. This test can be done 2 different ways. One is in a tub meant for this type of measurement with a trained professional and computer for calculating water displacement and underwater weight measurements. The second can be done in a pool and, using a special scale, measures the subject's weight under water. This method relies on physics and the known buoyancy of fat, muscle, and bone. A common "hiccup" can be the learning curve. The person being weighed has to be very still and used forced expiration (exhaling as much air as possible, forcibly) and remain under water until the professional can get all of the readings they need. Link Here

There are more Listed Here tests; but, the first three are the more commonly used (common because they don't require the purchase of machines that cost thousands of dollars).

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Screening Tool:

BMI is primarily a screening tool. It can be done quickly and easily with a simple a tape measure and a scale. The measurements are run thru a formula that gives you a number to determine where your measurements fall between an underweight to overweight chart. @Informaficker had given you a formula and chart. Here is another formula: BMI = (Weight in Pounds/( Height in inches )x (Height in inches) )x 703 And a link to the CDC's BMI calculator and their BMI calculator for Teens.

Validity:

Although it is a good screening tool, it cannot actually determine how healthy you are. Muscular people for example, may show up as being overweight due to their heavier muscle mass. Older, overweight people may show up as normal due to muscle wasting.

Other Measurements

In addition to your BMI, a waist and hip measurement will add more information to your health assessment. And there are other ways to calculate your body fat percentage more directly.

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