BMI for adults and children: How to calculate Body Mass Index

How to calculate Body Mass Index for adults and children
By Emeh Joy

Body mass index (BMI) is the metric for measuring body size or body fat by measuring an individual's weight with respect to height. Find out in this article how to calculate BMI for adults and children.

Usually, BMI correlates with and individual's total body fat. This means the higher the BMI, the higher the body fat.

Obesity is becoming rampant, and BMI offers a standard way to determine if someone is obese, underweight, overweight or has a healthy weight.

While BMI measures body size, it is notable to add that it does not take into account factors like sex, age, muscle mass or ethnicity.

How to calculate BMI for adults

To calculate an individual's BMI, you have to measure the individual's body weight and height. There are two ways to calculate BMI:

Metric: Here, you calculate BMI in metric units using the formula BMI = kg/m (squared). This means you divide the body weight (in kilograms) by the square of the height (in metres).

If you measure height in centimetres, you can still convert it to metres by dividing it by 100.

Imperial: To calculate BMI using imperial units, you use the formula, BMI = lbs * 703/in (squared). This simply means that you multiply the individual's weight in pounds (lbs) by 703. Next, you divide by the height (which should be in inches squared).

Understanding the BMI result for adults

The BMI result will show if you are underweight, overweight, obese or have a normal weight

BMI result is categorized to show weight status. Here is a table that shows how obesity is classified.

This will help you interpret your result after calculating your BMI. It would show whether your weight is on the normal side or not.

Weight classification table



18.5 or below


18.5 to <25.0

normal weight

25.0 to <30.0


30.0 to <35.0

class 1 obesity

35.0 to <40.0

class 2 obesity

40.0 or over

class 3 obesity (morbid or extreme obesity)

Using a BMI calculator for adults

If you don't want to go through the stress of using mathematics to calculate BMI, you can use a calculator to calculate your BMI.

You can find a BMI calculator online. Enter into the calculator your height or weight either in metric or imperial measurements to find your BMI.

Using the BMI chart for adults

Asides from using the BMI calculator, another easy way to find out your body mass index is by using the BMI chart. The National Heart, Lung and Blood Insitute (NHLBI), has provided an easy-to-use chart for checking out body mass index.

All you have to do is to locate your height in inches on the side of the chart. Then, look across the chart to find your body weight in pounds.

To interpret the result, scan to the top to see whether the result corresponds to normal weight, obesity or overweight.

Calculating BMI for children and teens

When calculating BMI for adults, you don't take account of gender and age. However, things are slightly different when measuring BMI for children and teenagers.

BMI for children and teens takes into account gender and age

Because boys and girls grow at different rates and have varying amounts of body fats at different age ranges, measuring BMI for children and adolescents put sex and age into consideration.

To calculate the BMI for children and teenagers, doctors first measure the weight and height. Then, they locate the BMI number and the young one's age on a sex-specific BMI-for-age chart. In this way, they find out whether a child's weight is within a healthy range.

Using calculators and charts for children and teenagers' BMI

The Centers for Disease Control and Prevention (CDC) have provided a calculator for BMI and the corresponding BMI-for-age percentile on a growth chart for kids and adolescents.

After using the calculator, you use the charts to see if the young one's weight is okay for their age. There is a chart for girls aged 2 to 20 years and another chart for boys aged 2 to 20 years.

Understanding the BMI results for children and teens

To further understand the BMI results, the categories below explains the meaning.

Weight status categorisation for children

Weight status category

Percentile range


Below the 5th percentile

Healthy weight

5th percentile to less than the 85th percentile


85th to less than the 95th percentile


Equal to or greater than the 95th percentile

Application of the BMI

The body mass index might offer a more accurate way of measuring body fat for some people but might not accurately measure others.

For instance, very muscular people might be categorized as 'overweight' based on BMI when they are healthy and fit. Thus, someone with high muscle mass and low body fat might fall into the same BMI score as someone who is actually overweight.

This is because it fails to consider other body compositions like the amount of muscle, fat, bone and other tissues. However, despite these limitations, it can be used as a good metric for measuring obesity.

Healthcare providers have used BMI over the years to screen for obese, overweight or underweight individuals. It is helpful in assessing an individual's health risks associated with obesity.

Even if your BMI shows your weight is normal, it is still important that you adopt effective habits to avoid obesity or excessive weight gain.

However, if you have already gained some weight, there are still remedies for it. It would be best if you decided to start your weight loss journey today.

Other ways to measure a healthy body size

Other ways to measure body weight asides BMI include taking waist-high ratio and waist-to-height ratio.

There are other measurement systems that can be used to complement the BMI. Examples of other measures of body size include waist-to-height ratio,waist-hip ratio and body composition.

These measurement systems make it possible to measure the amount of fat in the body as well as how fat is distributed around the body.

When used together with BMI, these measures can help in making a more accurate assessment of the health risks associated with an individual's weight, particularly obesity and underweight.

Among the health risks of obesity are type 2 diabetes, heart disease, high blood pressure, stroke, lipid disorders, sleep apnoea, osteoarthritis and joint diseases.

Most times, we focus only on obesity but being underweight also has its health risks. Some health risks of being underweight include decreased immune function, osteoporosis, higher risk of complications from surgery, fertility issues, growth and developmental issues.

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