BMI or Body Mass Index: how to calculate it

BMI or Body Mass Index: how to calculate it

The body mass index is an important indicator for evaluating one's weight. Here's how it's calculated

When we talk about ideal weight or diet, it is common to refer to a parameter indicative of our state of form. This indicator is the BMI, or Body Mass Index (from English, Body Mass Index). It is a biometric index (ie obtained from unique physical or behavioral characteristics and identifying a person) used as a basis by specialists to have an approximate indication of the ideal weight of an individual and thus build an effective strategy to improve the state of personal health.


  • What it is and how it is calculated
  • What is the right weight for my height?
  • BMI and diabetes
  • Calorie needs and BMI

What it is and how it is calculated

The body mass index is an indicator that expresses the ratio between weight (kg) and the square of height (m2). Obviously this is an index that must be contextualized within a much broader assessment, which must necessarily include other types of important data such as age, bone constitution and structure, the percentages of fat mass and lean mass. , anamnesis of the individual (presence of particular pathologies, heredity) and gender. The BMI is therefore very important but must be included within an overall picture that must be traced by a specialist: using the BMI with a "do it yourself" interpretation is not a good idea!

Having made the necessary premises, therefore, we specify the reference values ​​identified by the World Health Organization (WHO), which define for the adult a condition of:

  • normal weight with a BMI between 18.5 and 24.9;
  • overweight with a BMI of 25 and obesity with a value equal to or greater than 30;
  • underweight, with a BMI that does not reach the value of 18.5.

The threshold values ​​of BMI in children or young people up to 19 years of age are instead reported in ad hoc reference tables.

What is the right weight for my height?

First of all, as we have already mentioned, it is essential that the calculation of the BMI is also combined with the evaluation of all the other variables involved in identifying the "ideal weight". Therefore, even if the body mass index can be calculated in a simple and fast way by oneself, to obtain a complete evaluation of one's state of form it is advisable to consult a specialist (dietician, nutritionist), especially where there is a BMI value far from the normal weight range.

For the calculation, just measure the height and weight (this value should be measured several times in a row in the morning, fasting and without clothes, always at the same time) and then calculate the ratio between the weight and high height squared. At that point, the exact value of the BMI will be obtained, which must then be compared with the reference tables. The formula in short is as follows:

BMI = (weight in kg): (height in meters x height in meters)

Let's take a practical example. If you are 1.70 meters tall and weigh 70 kg, you will divide 70 by the square of 1.70. Therefore 70: 2.79. The result (24,22) is the BMI, in this case it is a normal weight value but close to the overweight threshold.

On the Ministry of Health website, it is possible to use an automatic calculation tool.

BMI and diabetes

The BMI is an index that has also proved very useful in some research fields, especially for the study of metabolic diseases. For example, with regard to type 2 diabetes, it has been found that the hereditary factor certainly counts but that the value of the individual's body mass index is also fundamental. The latter, in fact, would contribute to decreasing or increasing the probability of developing a form of diabetes in those who are already genetically predisposed.

The research on the topic was conducted by the University of Cambridge in collaboration with the State University of Milan and its results were presented at the ESC 2020.1 Congress. The study took into consideration both the hereditary risk and the BMI of a group of individuals, highlighting that the sample with the highest BMI had an 11 times greater risk of developing diabetes than those with a higher body mass index. bass.

A high value of body mass index was therefore found to be a more relevant risk factor than the genetic component alone, as regards the predisposition to develop diabetes. Maintaining a healthy lifestyle and a normal weight are therefore essential bases to avoid running into this pathology. All the more so if there is familiarity and with the passing of the years.

Calorie needs and BMI

As we have said, the body mass index is a value that must be inserted in a broader context of variables. If the BMI of an individual falls within a range other than that of normal weight, it will be necessary to consult a specialist to evaluate all the other variables (age, sex, previous diseases, plicometry with measurement of abdominal or visceral fat, percentage of lean mass and of fat mass) and thus establish an adequate amount of calories to be consumed during the day.

In conclusion, the BMI can prove useful as a general indication of one's own weight and as a starting point for deciding if necessary to rely on a specialist in order to get closer to one's ideal weight and prevent pathologies and disorders related to excess or lack of weight.

Read also

  • Covid-19, the impact of the pandemic on obesity surgery
  • Heart at risk? The strength of the arms says it (also)
  • How to calculate fat mass and lean mass
  • Viola Zulian: on tiptoe in the world of the obese
  • Breast cancer, physical activity as prevention

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