Understanding BMI: Meaning, Calculation, Health Impact & More

The latest figures from a Lancet study mentioned by the WHO show that 2.5 billion adults and 390 million children and teens in the world have an unhealthy BMI (overweight or obese).

That should point to a reason many people today are overly conscious about maintaining a healthy weight. After all, weight and good health are closely related.

For these people, and we should say for everyone, understanding the meaning and everything there is to Know About Body Mass Index (BMI) is vital.

So, what is BMI? How is BMI calculated? What is a normal BMI? What does your BMI score mean? How useful is BMI in health matters?

This post has easy-to-grasp answers to all these questions.

Body Mass Index is a popular measure used to assess body fatness by referring to your body weight to height ratio.

The basic principle is to determine if one is within the healthy BMI range with a balance between body weight and height or if they have an unhealthy amount of body fat.

It is crucial to understand right away that BMI is not a diagnostic tool for body fatness or your health status. Instead, it is a screening tool considered a reliable indicator of body fatness.

The fact that BMI is not a diagnostic tool underlines an essential disclaimer: Calculating your BMI should only be one aspect of a complete health assessment when evaluating one’s health status.

As such, BMI should be considered alongside other factors like medical history, physical assessment, and lab testing.

As I explain in detail later, BMI categorizes individuals according to the amount of weight-related fat in their body. So, one can be in a BMI category of underweight, normal weight, overweight, or obese.

These BMI categories point to the potential health risks for weight-related diseases such as diabetes and heart disease.

Therefore, understanding the Body Mass Index and how to calculate BMI can help determine if it’s the right assessment tool for you.

The most straightforward answer is that BMI is calculated by dividing your weight in kilograms by your height in square meters.

Put that way, that answer can seem rather generalized. That’s because how you calculate the BMI can vary based on factors like:

The units of measure (kilogram vs pound; feet & inches vs meters).

Age (adults from 20 years and above vs. children and teens between 2 and 19 years).

The first step in calculating your BMI is measuring your weight in kg and height in meters. But the BMI formula can also be adapted for weight in pounds and height in inches.

You can take your weight and height measurements at home using a scale and tape measure.

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If you doubt you’ll pull off accurate measures, you can also visit a healthcare provider’s office.

Once you have the figures for weight and height, BMI is calculated the same way for male and female adults.

As mentioned, the formula for calculating BMI is weight (kg) divided by height (m2). This is the metric system.

In the imperial system, the formula for BMI is weight (lbs) divided by height (in2) multiplied by a conversion factor of 703.

Example: What is the BMI of an adult with a height of 165 cm and weight of 70 kg?

1. In the metric system
Weight = 70kg
Height = 165cm (1.65m)
BMI = 70 ÷ (1.652) = 25.72

2. In the imperial system
Weight = 154lbs
Height = 154lbs
BMI = 154 ÷ (652) x 703 = 25.62

Note: Results from metric and imperial systems may differ slightly due to the differences that occur when rounding for weight and height conversions.

Also, you can find digital tools that automatically calculate your BMI like the Adult BMI Calculator by the CDC.

Professionals and healthcare centers can also share the adult BMI calculator widget by the CDC by adding the code for the adult BMI calculator to their websites or social media sites. That way, their clients and other visitors can easily and automatically calculate their BMI.

Similar to adults, BMI for children and teens (age 2-19) is calculated like that of adults: weight divided by height. As such, the first step in calculating the BMI for children is accurately taking their weight and height.

However, to be correctly interpreted, sex and age factors are considered when calculating BMI for children and teens, explaining why it is also called BMI-for-age. These factors are used to convert the BMI results to BMI percentiles and BMI categories.

The BMI percentile is then plotted on a standard growth chart that places the results within a BMI category. The standard growth charts were made by measuring the BMI of US children in a survey carried out from 1963-1965 and 1988-1994.

Often, designated calculators are used to generate BMI for children and teens like the Child and Teen BMI Calculator by the CDC.

We mentioned that BMI is basically calculated the same for adults and children. However, the age and sex considerations for children’s and teens’ BMI calculations allude to differences in interpreting their BMI calculations.

So, how is BMI interpreted for adults and children, and what are the implications on health?

BMI scores are interpreted differently for adults from children and teens.

For both male and female adults 20 years and above, your BMI score places you in a standard category based on the amount of body fat-related weight.

Here’s a quick summary of adult BMI categories:

So, from our previous example, a person with a weight of 70kg (154lb) and a height of 1.65m (65 inches) is in the 25.0-29.9 range and would be categorized as overweight.

Children and teens are still in the age of rapid growth. For that reason, their BMI values are interpreted relative to age and sex in percentiles.

The standard percentiles for interpreting children and teens BMI are as follows:

A BMI percentile is a rank that shows where a child or teen lies in relation to others of the same age and sex.

For example, if a child is ranked in the 85th percentile, his/her BMI value is higher than 85% of children of his/her age and sex, and s/he is rated overweight.

Let’s take an example.

If a 9-and-half-year-old boy has a BMI-for-age of 22, their plot would look like this on a BMI chart.

His BMI falls on the 95th percentile, meaning his weight is higher than 95% of children of his age and sex, and he’s, therefore, obese.

It’s important to note that, due to the increased prevalence of childhood obesity in the US, the CDC has developed new percentiles for very high BMI values among children and teens.

These changes refer to data collected from 1988 to 2016. The data cover BMI values above the 97th percentile (98th, 99th, 99.9th, and 99.99th percentiles). This extension allows plotting for BMI values of up to 60 kg/m2 for children 2 to 20 years.

From a health perspective, the extension allows professionals to plot and interpret the BMI of children and teens with severe obesity so they can:

See here what the extended BMI-for-age growth charts look like.
As with adults, you can add the child and teen BMI calculator widget code to your website or social media page.

You can also learn about the children's BMI tool for schools here.
So, is the BMI the only tool you can use to assess body fatness? That’s what you’ll find out in the following section.

The BMI is an easy and cost-free method for assessing body fatness, as it only requires measures for weight and height.

That said, it’s worth noting that there are other ways of assessing body fatness. They include:

Some of these methods (especially the last two) may require the expertise of a professional or designated equipment. That means they incur costs, making them less preferred over the BMI method.

Also, these BMI alternatives are hard to standardize and, therefore, not recommended for comparison across populations.
Both reasons answer the common question why is BMI used to measure overweight and obesity?

So, how reliable is BMI as an indicator of body fatness?

BMI is considered a strong index of body fatness. By showing a person’s weight category, BMI can help individuals make the right healthcare decisions.

That’s because there are many health risks associated with unhealthy body weight to height ratio, especially for overweight and obese individuals.

Both overweight and obesity are indicators of a high BMI value and are associated with an increased risk for diseases and health conditions such as:

Knowing that your BMI is in the overweight or obese range and you are at risk of these diseases can inform your decision for a healthier lifestyle.

Despite its advantages as an indicator of body fatness and a pointer to overall wellness and health, experts underline some BMI measurement limitations.

Here are the four most important ones:

Because it considers only weight and height, BMI does not isolate muscle mass from body fat. An example implication of that fact is an athlete with high muscle mass may record a high BMI value yet be healthy.

For example, a non-athlete adult may have a BMI of over 28, which would be interpreted as overweight. But, the same BMI for a muscular athlete does not necessarily count as overweight, as it is usually due to muscle build.

It is a science-backed fact that women have a higher tendency to become overweight than men. Also, older adults are a category that naturally loses muscle mass. Studies have also shown that different ethnic groups have differentiated body fat distribution.

Yet, the BMI measure in adults does not consider all these factors or differentiate between BMI for men vs. women. Instead, it measures the weight-to-height ratio in adults in a universal standard, largely based on white populations.

While it can point to unhealthy body weight, BMI does not show where body fat is accumulated. That makes it a poor indicator of individual susceptibility to health risks due to unhealthy body fat values. Generally, fat around vital organs, or visceral fat, poses higher health risks than subcutaneous fat.

One reason BMI is not a comprehensive health index is that it ignores some general health and fitness truths.

For example, a person with a BMI within the healthy weight range can have high body fat and be predisposed to metabolic risks. In the same way, one with a BMI value rated as overweight can live a healthy lifestyle and maintain metabolic health.

Regardless of these drawbacks, maintaining a healthy BMI score is generally a healthy practice. As such, maintaining or improving your BMI to a healthy range through a healthy lifestyle is recommended.

Improving your BMI entails working towards maintaining an ideal weight-to-height ratio. Here are three key ways to maintain a healthy BMI value.

A healthy, balanced diet includes all the food varieties that provide the entire range of nutrients your body requires.

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The 2020-2025 dietary guidelines for Americans, Make Every Bite Count suggests these key recommendations:

It’s healthy for everyone to do some type of exercise every day, regardless of age. This can include play for children or exercises like walking, cycling, or strength training for muscle building for adults.

The WHO recommends a certain amount of exercise for children and adults from 5 years and above:

A healthy lifestyle is not just about eating a balanced diet and exercising regularly. It also entails managing your stress levels and getting enough sleep. All that is key in supporting a healthy metabolism and maintaining an emotional balance, crucial for weight management.

BMI is a body fatness assessment tool that shows whether your body weight to height ratio is balanced.

While it is not a comprehensive diagnostic tool for overall health, it is a reliable measure of body fatness and can point to unhealthy weight levels: overweight and obesity.

An unhealthy BMI is a risk factor for diseases such as diabetes mellitus and ischemic heart disease.

You can calculate your BMI at home by measuring your weight and height, but available digital tools make it easier.

Make maintaining a healthy BMI part of your healthcare choices by working with a healthcare provider, especially for diet, exercise, and lifestyle choices.

1. WHO: Obesity and overweight.

2. The Lancet: Worldwide trends in underweight and obesity from 1990 to 2022: a pooled analysis of 3663 population-representative studies with 222 million children, adolescents, and adults.

3. CDC: About Body Mass Index (BMI).

4. CDC: Child and Teen BMI Calculator.

5. CDC: Plotting and Interpreting BMI-for-Age.

6. CDC: Background: CDC Extended BMI-for-Age Growth Charts.

7. CDC: Child and Teen BMI Categories.

8.WHO: Physical activity.