BMI is currently the most useful indicator of the health risks associated with being overweight and underweight.
The body mass index (BMI) is a value which makes it possible to estimate the build of a person. This calculation takes into account both height and weight. The BMI calculation was invented in order to objectively diagnose the health risks of being overweight. The objective is to assess corpulence without any personal judgment.
The objective of the BMI is to diagnose the health risks faced with certain diseases which are those of obesity (cardiovascular diseases for example). The BMI does not in any way give an ideal weight.
It is an index which is reliable but which is however to be taken with hindsight : the calculation of the BMI (which is made according to a single equation) does not take into account the muscle mass. Athletes are thus disadvantaged. Just as the result is not reliable for a pregnant or breastfeeding woman, etc.
There are no differences in how to calculate the BMI of a man and that of a woman; however, it is interesting to note certain particularities and statistics according to the sex of the individuals. This is the reason why this site has a page dedicated to BMI in men , and another page, dedicated to BMI in women .
This body mass index is also found under the name, in English, BMI (for Body Mass Index ).
WHO INVENTED BMI?
We owe this clue to Lambert Adolphe Jacques Quételet (1) (1796-1874), Belgian scientist, astronomer and mathematician. If originally this index was simply called “Quételet Index”. It was in 1972 that the term “Body Mass Index” (BMI) was coined (2) . It is now this term that is widely popular and widespread.
It was not until 1997 that the WHO (World Health Organization) decided to highlight this indicator in order to raise awareness among individuals and to compile statistics on a global scale (3) .
HOW IS IT CALCULATED?
The size and mass of a person are taken into account in the resolution of an equation which will lead to the calculation of the famous Body Mass Index. Basically, this equation was only designed for adults (18-65 years old). However, and because of the increasing number of overweight problems, it is now possible to calculate the BMI for children from 0 to 18 years old (4) .
For people over the age of 18, the BMI is calculated as follows:
I M C = W e ig h t ( k g ) / ( T a i l l e ( m ) 2 )
Where the result (the BMI) is provided in kg/m² . For people not using the metric system (meters and kilograms), but the imperial system (inches and pounds, inches and pounds) then a simple conversion of the final result is enough:
In both cases (adults or children), it is only an indication which is therefore not to be fatally linked to the general conclusions that we hear about BMI and the alarming opinions of people who do not know what exactly this indicator means. A person whose index is totally off the average can quite well remain in good health all his life. On the other hand, it is true that BMI is considered an important factor in the risk of certain diseases, such as cardiovascular diseases, for example.
Official classification of BMI according to WHO
The World Health Organization (WHO) provides an interpretation (classification) based on the value obtained from the BMI (5) . This interpretation can be done visually quite simply via a two-dimensional graph. Indeed the BMI only taking into account the weight and the height, it is possible to establish a graph (7) with in abscissa the height, and in ordinate the weight:
It is possible to learn more about this kind of IMC grid (or table) through a specially dedicated file. You will also be able, in this folder, to download these grids.
This graphic representation shows that the BMI is a linear indicator: the situation of proportionality is constant. This is not the case for calculating the BMI of children (4) . Another representation of the WHO classification consists in establishing a simple table with the correspondences between the ranges of BMI values and their interpretation. This is the table shown below:
| Classification according to WHO | BMI value (in kg/m 2 ) |
|---|---|
| Underweight | < 18.5 |
| Severe underweight | < 16.5 |
| Moderately underweight | 16.00 – 16.99 |
| Mild underweight | 17.00 – 18.49 |
| Normal build | 18.50 – 24.99 |
| Overweight | ≥ 25.00 |
| Pre-obesity | 25.00 – 29.99 |
| Obesity | ≥ 30.00 |
| Class I obesity | 30.00 – 34.99 |
| Class II obesity | 35.00 – 39.99 |
| Class III obesity | ≥ 40.00 |
This classification is official and provided by the WHO (World Health Organization). There are a lot of inaccuracies about this on the Internet. This is why we have devoted a detailed file to this question, you can access this file by clicking on the image below.
INTERESTS & LIMITS OF THE BMI
1. This is a particularly simple calculation, without any complexity.
2. This calculation makes it possible to take a completely objective look at one’s corpulence.
3. Since it is simple, the BMI has managed to establish itself internationally. It is therefore an index that is used all over the world.
But always keep in mind that the limits and flaws of the BMI are (very) numerous. This index, for example, does not take into account the “history of weight “, a fact that is fundamental in the field of obesity. Moreover, this index does not take into account the variance of age. Not applicable for those under 18, this index is also not valid for athletes. Indeed, BMI is not able to differentiate fat mass from muscle mass.
THE ALTERNATIVES
A math professor at Oxford suggested a “new BMI” in 2013 (which you can calculate here ). The alternatives are indeed numerous, but each calculation has its own disadvantages. CalculerSonBMI offers a multitude of other calculations (ideal weight according to the Lorentz formula, etc.) that you can discover on this page . The Adipose Mass Index ( IMA , or BAI for “Body Mass Index”) is for example an alternative which, by calculating the hip circumference, is supposed to produce more reliable results.
We have also dedicated a page to ideal measurements on our site.