Science Explains How to Calculate your BMI

Science Explains How to Calculate your BMI

calculate your BMIHealth

Have you ever asked yourself this health-related question? Knowing your BMI (Body Mass Index) can help you determine your total body fat. But what is the body mass index?

BMI is a mathematical formula that measures a person’s weight to his/her height. BMI does not provide a direct measure of a person’s total body weight. However, it indicates a person’s body fat.

According to a study report by the Center for Disease Control and Prevention (CDC), the Body Mass Index formula is used by health professionals to screen for several weight categories. Different weight categories are used to determine whether an individual’s BMI is prone to various common health problems or not.

How to Calculate the BMI

BMI calculation is based on two metrics; height and weight. To calculate Body Mass Index for adults, we use a mathematical formula of; Weight in kilograms/ height in meter squared. Alternatively, we can also use weight in pounds divided by height in inches and then multiplied by a conversion factor of 703.


Body Mass Index (BMI) is widely used as a tool for assessing overweight as well as obesity in the general population.

Health professionals use the BMI method for its inherent simplicity. Once you get your BMI score, you can, therefore, use that data to analyze your weight. And you can compare it to that of the general population.

The following table shows various BMI data ranges and their respective body weight status categories.

Below 18.5 Underweight
18.5 – 24.9 Normal
25.0 – 29.9 Overweight

Body Mass Index for Children and Teens

Calculating BMI for children under the age bracket of 2-19 years, a different BMI formula is used to put into account age and gender.

The initial procedure involves taking the child’s weight and height records. Once you have the data, you can then proceed to CDC BMI for age growth charts for both boys and girls.

The child’s BMI number is then closely scrutinized in comparison to the BMI of other children who are of the same gender and age bracket. Here, you will get a percentile cluster showing you where your child belongs.

For instance, if a child’s BMI is below the 5th percentile, it means that it’s lower than the BMIs of 95 % of other children of similar age and gender. In such a case, the child is considered underweight.

Check the following table, which indicates various BMI percentile range and their weight status.

Less than the 5th percentile Underweight
5th percentile to less than the 85th percentile Normal weight
85th to less than the 95th percentile Overweight
Equal to or greater than the 9th percentile Obese

In adults, BMI is still the measurement of choice used by many health professionals for determining weight-related health risks.

However, for more accurate results, BMI data should be used together with other disease risk measurements. Measuring waist circumference and cross-examining the data with a person’s BMI, can help health professional make a more informed decision.

For children, the BMI-for-age percentile range is a dependable pointer of body fat. However, it should be used for screening purposes only. When making a diagnosis, health professionals must also conduct other assessments. These include evaluating the physical activity, family history, and diet to get more accurate data.

What Does BMI Data Indicate?

According to the National Heart, Lung, and Blood Institute, a person’s BMI can be used to gauge the risk of getting diseases that are associated with increased body fat.

The higher the person’s BMI, the higher the risk of getting heart disease, high blood pressure, type 2 diabetes, respiratory problems, and certain types of cancer.

Besides, the Center for Disease Control and Prevention (CDC) emphasizes that BMI results should not be used exclusively as a diagnostic tool. Even if a person gets high BMI results, it might not automatically translate to being at a higher risk of getting weight-related diseases.

Proper assessment must be done by a qualified physician and use the BMI results in conjunction with other assessment results.

What are the limitations of BMI?

Due to the fact that BMI relies on a person’s height and weight only, it does not provide any reliable data that indicates body fat percentage or distribution. There are crucial elements that can affect the overall correlation between BMI and the amount of body fat. They include factors such as muscle mass, ethnicity, bone mass, age, and sex.

For instance, persons with more muscle mass, and they exercise more often are more likely to have a higher BMI compared to those individuals with less muscle mass who exercise fewer times. In such a situation, doctors might not classify a person with a higher BMI as less healthy.

BMI results are also not a good indicator of individuals who are at risk of unhealthy fat distribution in the body. Such an indicator can play a significant role in determining a person’s disease risk potential.

Other Considerations

Individuals who carry excessive weight in the abdominal area than other lower areas are at higher risk of getting heart disease, high blood pressure, and certain types of cancers compared to those who have a balanced fat distribution.

That’s why it’s paramount for health professionals to use both BMI data together with waist measurements to get a more reliable indicator of a person’s weight status as well as any potential risk of chronic diseases.

Also, elderly persons who are weak might fall in the normal weight category while in actual sense they may have little muscle and bone mass and a high body fat percentage.

Another drawback of calculating BMI is the fact that BMI calculation was initially based on Caucasian body types. It’s for this reason that BMI results may differ for individuals of different ethnicity.


A study result conducted on the ethnic difference in BMI indicated that indeed, there is a variance. When researchers compared White Europeans and Asians of the same BMI, Asians appeared had a four percent higher total body fat comparatively.

Maybe this could be a contributing factor to the high cases of abdominal obesity in South Asia.

History of BMI (Body Mass Index)

BMI concept is no new as you may have thought. Funny enough, the history of BMI dates back into the 1840s where a man called Adolphe Quetelet a Belgian statistician came up with a mathematical formula he called the Quetelet Index of Obesity.

The Quetelet Index of Obesity calculated obesity of a person using a simple mathematical formula abbreviated as w/h2. To get the obesity index, he would divide a person’s weight (in kilograms or pounds) by the square of his/her height (in inches or centimeters).

Quetelet famously once stated, “If a man increased equally in all dimensions, his weight at different ages would be as the cube of his height.”

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