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Body Mass Index (BMI): How Does It Work?

Medically reviewed by Jobelle Ann Dela Cruz Bigalbal, MD · General Practitioner

Written by Tracey Romero · Updated Jan 03, 2023

Body Mass Index (BMI): How Does It Work?

A lot of fitness enthusiasts regularly refer to “Body Mass Index” or “BMI” in their work, which typically refers to body fat percentage. Many people who are unfamiliar with it may find themselves confused. However, it can be easily understood. Here is what you need to learn about body mass index, its relation to determining normal weight range, and how it can help you better gauge your physical health.

What is the Body Mass Index? 

Adolphe Quetelet created the BMI back in the 1830s by using a math formula. BMI is a type of measurement and formula that lets you know if you are in the right weight range for your height and gender. It calculates that by taking your weight and height. Many health professionals have used BMI for more than 100 years to decide if their patients are underweight or overweight. With this approach, Quetelet was able to establish the normal weight range of different sexes for different ages. 

How does BMI work? 

BMI divides a person’s weight by the height squared. Other factors like measurements can complement the BMI calculations to get a somewhat more accurate estimation. 

BMI numbers and what they mean in relation to normal weight range

BMI of 30 and over

These individuals are deemed obese

BMI of 25 – 29.9

This connotes that the individual may be overweight. 

BMI of 18.5 – 24.9

This describes a person as being within the normal weight range.

BMI of 18.5 and under

A person is deemed underweight, below normal weight range

Those who fall between 23 – 24.9 can be deemed at-risk for Obesity. Those who fall between 25 – 29.9 can also be considered Obese Level 1.

normal weight range

Why is the Body Mass Index popular? 

BMI is very easy for people to use and remember. Since it only needs to factor in weight and height for the calculation (with the occasional measurements), the general public and some doctors can use it for practical reasons. However, it is not an accurate diagnostic tool. It can only be used for screening and measuring a person’s body fat percentage. 

Since it is easy to use, it can easily correlate the weight of people at a population level and their chances of getting health problems based on their weight. Therefore, people who want to compare the BMIs of people from different countries often use this formula.

However, there are cut-off values that have to be taken into account when using BMI. This is especially true when used to calculate the BMI of a large number of people. 

Adolescents and children vary in height and weight, and so this throws off the calculation of the average normal weight range. This is because their bodies are still developing. Therefore, the cut-off values for them are age- and gender-specific, especially for kids and teens who are below 19 years old.

Does it really work? 

While the body mass index is popular and widely-used, there are still a lot of arguments surrounding it. For one thing, it does not always take into account whether the person is muscular. A physically fit person with a good amount of muscle may be confused by the BMI calculator. This is because BMI does not distinguish between fat and muscle.

Let us compare.

A bodybuilder who is 221 lbs and 6 ft. tall will have a BMI of 28. A person who doesn’t exercise weighs 203 lbs and is 6 ft. tall, and he will have a BMI of 27. So a bodybuilder may be deemed as overweight or obese because of their height to weight ratio. While their heights are the same and their weight is similar, they will not be of the same health. Therefore, there are times that BMI is not accurate. 

While it is possible to be healthy and overweight, being overweight or obese still holds more risk to certain illnesses and conditions.

Other than the fat to muscle ratio being unclear when it comes to body mass index, BMI also cannot distinguish whether a person is healthy or not.

While it is possible to be healthy and overweight, being overweight still holds more risk to certain illnesses and conditions.

Additionally, BMI can over- or underestimate adiposity. For instance, one study showed that women who had a spinal cord injury and used the BMI calculation found that it had limited use for them. That is because BMI underestimated adiposity for women with their injury.

Another point to note is that the body mass index may not be as helpful when determining what the normal weight to height ratio is for different races and ethnic groups. For instance, some people of Asian ethnicity tend to have smaller frames than Western people. So, a slim Asian woman may be deemed as underweight, despite being healthy and normal.

As stated earlier, measurements can be helpful and can complement the body mass index. A few scientists say that using the waist to height ratio is better and easier than solely using the body mass index calculator. It can be helpful because people who have fat around their abdomens have higher risks of metabolic disorders and heart disease.

Another  study states that waist circumference can also indicate whether a person has a higher risk of getting heart disease and type 2 diabetes. That study was proven true by the authors even after they had accounted for people’s BMI. 

The new BMI calculator

When Quetelet first created the body mass index, he made a simple system because there were no electronic devices, calculators, and computers at the time. So, a mathematician, Professor Nick Trefethen, believed that the body mass index was misleading and confusing. 

Trefethen then devised another formula that he called the new BMI calculator. He created this because he believed that assigning a number to a person isn’t ideal because humans are complex beings. Therefore, he created the new body mass index calculator to help people get a better estimation of what their normal weight range should be for their heights.

How can I calculate my BMI? 

Use our health tool to find out if your Body Mass Index falls within the healthy range. 

Key Takeaways

While the body mass index may not be the perfect calculation to estimate whether a person is healthy or not, it has practical value. It can help determine if you fall within the normal weight range, or above or below it. It is also still very helpful when you want to compare people’s weights in large groups.


Hello Health Group does not provide medical advice, diagnosis or treatment.

Medically reviewed by

Jobelle Ann Dela Cruz Bigalbal, MD

General Practitioner

Written by Tracey Romero · Updated Jan 03, 2023

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