Calculating BMI - Body Mass Index

The body mass index, BMI better known by the abbreviation is an index adopted by WHO (World Health Organization), which is used for the diagnosis of overweight and obesity. BMI can easily be calculated from two simple data: height and weight.

Body mass index
Body mass index
 

The body mass index is an important indicator of health, supported by several studies which show that, in general, the higher the BMI of an individual, the higher the risk of early death, mainly due to cardiovascular diseases.

BMI is a valid index to identify the excess or lack of weight in any person from 2 years old. Their results are quite reliable, but its main flaw is that it may overestimate the amount of fat in people who have a lot of weight due to a large muscle mass, as in the case of high-performance athletes and body builders.

It is also important to note that BMI results are not valid for pregnant women, as these plus baby weight, also have large fluid retention.

In this article we will explain how to calculate BMI and what are the optimal values in each of the following groups: children, adolescents, adults and seniors.

How to calculate BMI?

Easy to calculate the body mass index. Just follow the following formula:

BMI calculation = weight (kg) ÷ height² (meters)


For example, we calculate the BMI of a person of 80 kg, which measures 1.60 m

BMI = 80 ÷ 1,6²
BMI = 80 ÷ 2.56
BMI = 31.25 kg/m²


What is the optimal BMI for me?

In adults, BMI results should be interpreted as follows:
  • Low weight very serious = BMI below 16 kg/m²
  • Low serious weight = BMI between 16 and 16.99 kg/m²
  • Underweight = BMI between 17 and 18.49 kg/m²
  • Normal weight = BMI between 18.50 and 24.99 kg/m²
  • Overweight = BMI between 25 and 29.99 kg/m²
  • Obesity class I = BMI between 30 and 34.99 kg/m²
  • Obesity class II = BMI between 35 and 39.99 kg/m²
  • Obesity class III (morbid obesity) = BMI greater than 40 kg/m²

The goal obviously is to have a BMI in the normal range, between 18.50 and 24.99 kg/m². Values above or below this range are associated with a higher risk of disease. The further the ideal BMI range you are either more or less, the greater your risk of developing health problems.

BMI values given above are for the adult population between 20 and 65 years. The calculation of BMI for seniors, children and adolescents will be explained below.

BMI table

BMI table
BMI table
 
In addition to the BMI calculator and the calculation done by hand with the formula BMI, another way to calculate the body mass index is through the BMI tables.

The table provided below is one of many examples available. Look for the line that matches your height and weight and see what their classification according to BMI.

BMI calculation in elderly

The elderly are less muscle mass in young adults and, therefore, the calculation of BMI does not usually have in the aging population the same meaning as in the rest of the adult population. While in adults a BMI over 25 kg/m² is clearly associated with an increased incidence of disease and an increased risk of premature death in the elderly it does not appear to be true.

More and more studies show that optimal weight values suggested by BMI are not suitable for the elderly. Therefore, it proposed a new BMI chart, specially designed for people over 65 years. How to calculate BMI is exactly the same, however, the results should be interpreted as follows:

a) BMI for women over 65
  • Low weight = low BMI of 21.9 kg/m²
  • Normal weight = BMI between 22 and 27 kg/m²
  • Overweight = BMI between 27.1 and 32 kg/m²
  • Obesity class I = BMI between 32.1 and 37 kg/m²
  • Obesity class II = BMI between 37.1 and 41.9 kg/m²
  • Obesity class III (morbid obesity) = BMI over 42 kg/m²

b) BMI for men over 65
  • Low weight = low BMI of 21.9 kg/m²
  • Normal weight = BMI between 22 and 27 kg/m²
  • Overweight = BMI between 27.1 and 30 kg/m²
  • Obesity class I = BMI between 30.1 and 35 kg/m²
  • Obesity class II = BMI between 35.1 and 39.9 kg/m²
  • Obesity class III (morbid obesity) = BMI greater than 40 kg/m²

BMI calculation in children

The adult BMI values are also not the most appropriate for children and adolescents. As in the elderly, BMI calculation is the same, but interpretation of the results is different in children.

The process should be done as follows: you must first calculate your BMI by traditional formula (or the BMI calculator), then, with the old table and sex provided below, see which percentile the value found BMI fits. The results should be interpreted as follows:
  • Underweight = BMI below the 10th percentile
  • Normal weight = BMI between the 15th percentile and 85
  • Overweight = BMI between the 85th and 95th
  • Obesity = BMI above the 95th percentile

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