How to calculate your body mass index and a good BMI?

What is body mass index?

BMI or body mass index is a universal tool for assessing and classifying the factor of obesity in the human body. This indicator is widely used in the field of public health to identify and prevent the risks of various diseases associated with obesity. For example, an elevated BMI may be associated with the risk of developing type 2 diabetes, hypertension, and cardiovascular disease.

This metric was first reported by professional statistician Dr. Lambert Adolphe Jacque Quetelet under the name “Quetelet Index” in the 1800s. BMI got its modern name in 1972 thanks to Ancel Keys who published an article on this topic in the Journal of Chronic Diseases

How to calculate your BMI using a formula?

BMI is defined as your weight in kilograms divided by the square of your height in meters. Squaring a person’s height normalizes the distribution of body weight and reduces the effect of leg length on the final result, since the bulk of fatty deposits is located in the human body.

Metric BMI calculation formula (kg and cm)

BMI = Body Mass / ( Body Height * Body Height)

Where Body Mass measured in (kg) and Body Height measured in (m)

Imperial BMI calculation formula (inches and pounds)

To calculate BMI in imperial units, you need to make a small conversion by adding the conversion ratio to the formula, and now this formula will look like this:

BMI = ( Body Mass * 703 ) / ( Body Height * Body Height)

Where Body Mass measured in (lb) and Body Height measured in (inches)

Example of how to calculate BMI

Okay, now that you know what formula is used to calculate BMI, let’s see how this is done with a real example.

Suppose Mr. Smith has the following data:

  • Weight – 90 kg
  • Height –186 cm

1) First of all, we need to translate the height from centimeters to meters

Height (m) = Height (cm) / 100 = 186/100 = 1.86 (m)

2) Now we need to substitute our data into the formula

BMI = 90 (kg) / (1.86 * 1.86) (m)  = 26.01

Now let’s assume Mrs. Johnson has the following metrics:

  • Weight – 120 lbs
  • Height – 5′ 2”

1) First of all, we need to translate the height into inches

Height (inches) = Height (ft) * 12 + Height (inches) = 5 * 12 + 2 = 62 (in)

2) Now you need to substitute the received data into the formula

BMI = 703 * 120 (lbs) / (62 * 62) (in) = 22

How to interpret BMI calculations?

Now that you have calculated your body mass index, you can determine how much your body weight differs from normal using the special range table below. This classification was created by the World Health Organization and published in a special report in 2000 1. The main purpose of this classification is to facilitate the statistical classification of people who are underweight, overweight or obese.

CategoryRange
Very severely underweight<15
Severely underweight15-16
Underweight16-18.5
Normal or healthy weight18.5-25
Overweight25-30
Obese30-35
Obese class 235-40
Obese class 3>40

If your BMI is below 18.5 (underweight), it means that you may be malnourished or have a higher risk of respiratory disease, digestive disorders, or weakened immune function.

In contrast, if, for example, your BMI is over 25 (overweight) and you are physically inactive, this means that you may have a higher risk of high blood pressure, diabetes and mental disorders.

For example, let’s look at what categories the indices of Mr. Smith and Mrs. Johnson fall into. As we already know that Mr. Smith has a BMI of 26 and according to the table he falls into the Overweight category, he should pay attention to his health in order to avoid the appearance of serious diseases. Now let’s look at Mrs Johnson’s result, her BMI 22 falls into the healthy weight category. Keep it up 🙂

What is a good body mass index for men and women?

The formula for calculating body mass index does not take gender into account. Therefore, for men and women, a healthy index value will be in the range of 18.5-25. Moreover, according to research published by The Lancet in 2009, the optimal BMI range is 22.5-25 2.

Body mass index for teenagers and kids

For children and adolescents, two main factors play a key role in determining the BMI indicator: age and gender. This is because the body is at the stage of growth, and body fat changes with age for children and teens. Girls tend to accumulate relatively large amounts of fat during and after puberty. At the same time, the volume of bone and muscle mass increases in boys. New research confirms that females aged from 12 to 16 years have BMI is higher than males in the same age category.

Considering the above, calculations of BMI for children is slightly different and involves the following steps:

  1. First of all, you need to measure the height and weight of your child.
  2. Secondly, you need to calculate your BMI using the standard formula above.
  3. Next, you need to take the resulting BMI and use the BMI-for-age chart to determine the percentile value. A percentile is a statistical indicator that allows you to find the relative position of a child’s BMI among children of the same gender and age.
  4. Finally, using the table below, you can determine which category your result falls into.
Weight Status CategoryPercentile Range
Underweight> 5 %
Healthy weight5-85 %
Overweight85-95 %
Obese> 95 %

Some limitations of the BMI method.

Although body mass index is a pretty popular indicator for determining obesity levels, it still has some limitations. 

Firstly, BMI does not take into account measurements of adipose tissue, which can lead to inaccuracies in the calculation result 3. For example, let’s consider professional athletes or bodybuilders. Often, they will have a high BMI, which will allow them to be classified as overweight or obese. However, the real reason for the overestimation is their increased muscle mass. 

Secondly, the formula does not take into account the location and position of body fat in the human body and their effect on various diseases. For example, fat accumulation in the lower parts of the body can lead to heart problems and diabetes, while fat accumulation in the lower parts of the body can lead to asthma and high blood pressure. 

Finally, BMI does not take into account many different factors that influence the distribution of body fat and its calculations. For example, it can be people with physical disabilities. They may have lower body weight due to limb amputation, and this may have an impact on the calculation results. Moreover, the formula does not take into account such characteristics of a person as his sex, ethnicity, genetic predisposition and various environmental factors.

  1. Obesity: preventing and managing the global epidemic. Report of a WHO consultation. World Health Organ Tech Rep Ser. 2000;894:i-xii, 1-253.
  2. Prospective Studies Collaboration, Whitlock G, Lewington S, Sherliker P, Clarke R, Emberson J, Halsey J, Qizilbash N, Collins R, Peto R. Body-mass index and cause-specific mortality in 900 000 adults: collaborative analyses of 57 prospective studies. Lancet. 2009 Mar 28;373(9669):1083-96. doi: 10.1016/S0140-6736(09)60318-4. Epub 2009 Mar 18. PMID: 19299006; PMCID: PMC2662372.
  3. Nuttall FQ. Body Mass Index: Obesity, BMI, and Health: A Critical Review. Nutr Today. 2015 May;50(3):117-128. doi: 10.1097/NT.0000000000000092. Epub 2015 Apr 7. PMID: 27340299; PMCID: PMC4890841.

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