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Body Mass Index (BMI): Calculator And Definition

The body mass index or Quetelet index is a value used for the assessment of overweight and obesity
The BMI calculator is a useful tool that indicates our ideal weight.

 

The body mass index or BMI is used to help you figure out if you are at a healthy weight for your height. BMI is a number based on your weight and height and in general, the higher the number, the more body fat a person has.

Although the BMI is used to diagnose medical conditions such as anorexia or obesity, the limitations that this tool faces have chipped away at its popularity and credibility. 

We will have a detailed look at what the BMI is, and how to calculate it either with a mathematical formula or an online calculator. 

What is the body mass index (BMI)?

The body mass index, usually abbreviated to BMI, is a tool that analyzes the relationship between a person's height and weight. It is also known as the Quetelet index due to its creator, Belgian astronomer, mathematician, and statistician Lambert Adolphe Jacques Quetelet. 

The purpose of the BMI is to indicate whether a person is underweight or if they have a healthy weight, excess weight, or obesity.

The weight categories are divided into subcategories that indicate the degree of severity. For example, excess weight can be classified as "overweight", "obese", or "morbidly obese". 

The biggest use of the BMI is predicting medical conditions - the higher the BMI value, the higher the possibility of developing cardiovascular diseases, type 2 diabetes, sleep apnea, and osteoarthritis among others. 

On the other hand, a low BMI is indicative of hypotension, alopecia, dry skin, infertility, and premature death due to inanition, which occurs in extreme cases of anorexia. 

These values differ based on the age, gender, and ethnicity of the person which makes the body mass index less accurate for individual use. When looking for an exact measurement we should do a cross-reference with groups that share our characteristics. 

You can calculate your BMI using an online calculator or a special formula.

 

How to calculate body mass index 

Body mass index can be calculated using the metric system by dividing your weight (kg) to your height (m2), thus the formula is BMI = kg/m2.

The internet is home to many BMI calculators that process your data automatically once you fill in your height and weight. There are also body mass index charts that show you exactly where you are on the weight spectrum.

You can also find out your BMI by using the simple formula and a calculator (or mentally, if you're feeling up for it): we multiply our height in meters by itself first and then divide your weight in kilograms by the result gotten from the previous multiplication. 

For example, a person standing at 1,70 meters and weighing 65 kilograms will have to do the following operations: 

  • 1,7 x 1,7 = 2,9
  • 65 ÷ 2,9 = 22,4
  • BMI = 22,4 

This result means that the person in our example, according to the Quetelet index categories shown below, is classified as having normal weight. 

Weight ranges according to BMI

Once we know our body mass index, the next step is to situate ourselves in one of these five categories. 

  • Underweight: BMI is less than 18.5

      

  • Normal weight: BMI is 18.5 to 24.9

      

  • Overweight: BMI is 25 to 29.9

     

  • Obese: BMI is 30 or more

      

  • Morbidly obese: BMI is 40 or more

As we can see, the ideal weight is not a singular value but a range which is important to keep in mind when faced with misleading information indicating that a given person should weigh a certain number of kilos.  

If we look at our previous example, according to the BMI result, the weight range of someone who measures 1,70m is between 53,6 kg and 72,5 kg, approximately. 

Globally speaking, 5% of the population is underweight, 90% fall under the normal weight category and the remaining 5% is classified as obese. 

The BMI offers a general guideline regarding our ideal weight.

 

Limitations of the BMI 

The body mass index faces a series of limitations that have determined many nutritionists and human biologists to discard it as an accurate measuring tool. Specialists have yet to prove its effectiveness in predicting the risk of developing medical conditions, especially in connection to excess weight. 

One of the issues of BMI is the fact that it overestimates the nutritional values of taller people and places them in overweight/obese groups and conversely, it underestimates the status of shorter persons, which makes it easier to believe they are underweight. 

Another limitation of the body mass index is the inability to distinguish between fat and muscle mass. Consequently, the BMI results tend to miscalculate the weight range for muscular people. 

The BMI, therefore, is not an exact measuring tool that helps us evaluate our nutritional status. It can, however, offer us a general view of our weight, especially if we use alternative measurements as well, such as body fat measuring, waist circumference measurement, or waist-to-hip ratio. 

 

Family physician Dr. Richard Honaker with the Family Medicine Associates of Texas talks about why it is important to know your BMI, or body mass index.

References: 

Keys, A., Fidanza, F., Karvonen, M. J., Kimura, N., Taylor, H. L. (1972). Indices of relative weight and obesity. Journal of Chronic Diseases, 25 (6): 329–43.

Knoyan, G. ( 2008). Adolphe Quetelet (1796-1874)--the average man and indices of obesity. Nephrology, dialysis & transplantation, 23(1): 47–51.

Romero-Corral, A., Somers, K., Sierra-Johnson, J., Thomas, R. J., Collazo-Clavell, M. L., Korinek. J. et al. (2008). Accuracy of body mass index in diagnosing obesity in the adult general population. International Journal of Obesity, 32(6): 959–966.

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