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BMI Calculator

Reviewed by BS Raquel Mendoza Hernández

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Table of Contents

What is BMI?

Also known as Quetelet Index. It is named after the Belgian astronomer Adolphe Quetelet and allows, through mathematical calculations, to know the weight-to-height ratio of a person.

The results obtained show the weight distribution per square meter and whether this distribution is healthy or not. (1)

BMI, broadly speaking, is a simple formula that determines the ideal mass that any person should have. It serves as a guide to know if we are below or above our ideal body mass in relation to our height.

The weight ranges of this index cannot be applied to those under 18 years old. Children or adolescents, because their bodies are constantly evolving, present constant changes in their body mass.

BMI in children and adolescents is calculated by age ranges and has values by sex.

Height plays an important role in the interpretation of BMI, since a taller individual can ideally have a greater weight than someone shorter in height.

Even so, Carmen Escalada, a nutritionist at the European Medical Institute of Obesity, comments that “it is a system that only takes into account the weight we should be according to our height”, referring to it as an incomplete system, given that it does not take into account other aspects such as the percentage of adipose or muscle tissue the person has.

Susana Monereo, a dietitian belonging to the Spanish Society for the Study of Obesity, adds that “BMI does not distinguish between muscle mass, fat mass, water, and bone mass, parameters that make up the weight”.

How to classify BMI?

The World Health Organization has created, in conjunction with BMI, a way to classify the nutritional status of a person, based on the results obtained from the calculation. (2)

The results shown below are demonstrated in kg/m2:

  • Underweight: Less than 18.50
  • Severe thinness: Less than 16.00
  • Moderate thinness: Between 16.00 and 16.99
  • Mild thinness: Between 17.00 and 18.49
  • Normal: Between 18.5 and 24.99
  • Overweight: Greater than or equal to 25.00
  • Pre-obese: Between 25.00 and 29.99
  • Obesity: Greater than or equal to 30.00
  • Mild obesity: Between 30.00 and 34.99
  • Moderate obesity: Between 35.00 and 39.99
  • Morbid obesity: Greater than or equal to 40.00

How to calculate BMI?

As we mentioned earlier, this index is based on just two basic variables: the mass and height of the individual, with the following formula being used to obtain the BMI of any person.

This is the formula with which you can calculate your BMI: BMI = mass * height / 2

In this equation, mass must be expressed in kilograms (kg) and height in square meters (m2) to obtain the BMI unit of measure in the International System of Units kg/m2.

As we said before, many specialists in the field claim that the BMI calculation tends to be quite inaccurate due to the omission of factors such as the different tissues in the body, being that a bodybuilder with greater muscle mass, according to BMI, has overweight, even when they do not present the same overweight symptoms as a normal person with a high BMI.

In the case of children, BMI must be calculated with the same formula presented, and then it must be compared with the age range based on the WHO charts. For children from 0 to 5 years old, the charts can be found at the following WHO link: BMI for Age Boys(3).

For ages between 5 to 18 years, the following link from the Food and Nutrition Technical Assistance can be used: BMI Tables for Children and Adolescents(4).

Advantages and disadvantages of this metric

Advantages (5):

  • Ease of use: To calculate your Body Mass Index you only need a calculator, your weight, and your height; with just these three elements you can quickly obtain a reference to know if you are within your ideal weight, or if, on the contrary, you have some type of overweight, severe or moderate thinness.
  • Diversity: The Body Mass Index can be applied to people of any age, men or women, and even to children or elderly people. The detail with children is that an extra process must be done after calculating the BMI to know their “body status”.
  • Range: The BMI is quite flexible in terms of its scores, in fact, if we take the normal classification as an example, it is between 18.5 kg/m2 and 24.99 kg/m2.
  • Generalization: Although it is considered to have errors, it serves to generally calculate the health of the people in the countries to which the WHO subjects to the BMI calculation.

Disadvantages (5):

  • Inaccuracy: Due to the factors mentioned above, in which many doctors in the area agree, it is considered that the Body Mass Index is quite inaccurate, and some even consider it a useless index.
  • Omission of values: BMI omits body fat, water, and even bone mass of the individual for its calculation, which, along with the previous point, makes it quite prone to errors.
  • Does not prevent diseases: Even by calculating BMI, the healthiest option is to visit a nutritionist doctor who can help us know the true state of our body, since some people could obtain a normal data while having a high amount of body fat, which could lead to heart diseases.
  • Not suitable for bodybuilders: People who exercise a lot and have a large amount of muscle tissue tend to fall into the overweight category, according to BMI.

Bibliographic resources

  1. Caponi Sandra. Quetelet, the average man and medical knowledge. Hist. cienc. saude-Manguinhos [Internet]. 2013 Sep; 20(3): 830-847. Available at:
  2. 10 facts about obesity [Internet]. WHO. 2017. Available at:
  3. BMI for age [Internet]. WHO. Available at:
  1. BMI tables for children and adolescents. [Internet]. FANTA III. 2013. Available at:
  2. Alex Valenzuela Montero. Diagnosis and Clinic How to Evaluate the Risk Associated with Excess Weight. Body Composition, its Changes and Associations. Chilean Society of Obesity, Chile. Available at:
BS Raquel Mendoza Hernández
Bachelor's degree in Nutrition and Dietetics from UCV. Postgraduate diploma in Public Management in Nutrition from UBV.
Raquel Mendoza Hernández holds a degree in Nutrition and Dietetics from the Central ... Read more »

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