How To Calculate Your BMI and Assess Your Health

BMI formula  •  Is BMI a body fat measure?  •  Obesity in adults  •  BMI for children and teenagers  •  Risks of being obese  •  Risks of being underweight  •  The gist

BMI is a measure of your weight in relation to your height. It is calculated by dividing your weight in kilograms by the square of your height (in meters). There are distinct BMI categories that are used to assess whether an individual is considered underweight, normal, overweight, or obese.

The World Health Organization (WHO) classifies BMI categories as follows (1):

• Underweight: <18.5
• Normal range: 5–25
• Overweight: 25–30
• Obese: >30

The BMI is a statistical measure of the weight-to-height ratio. It does not directly measure body fat, but rather provides an estimate of body fat based on height and weight measurements.

How to calculate BMI: Using the BMI Formula

There are several ways to calculate BMI. The most common method is to use the following formula: BMI = weight in kilograms/height in meters × height in meters.

For example, if your weight is 65 kg (143 lbs) and your height is 1.7 meters (5’ 7”), then your BMI would be: 65 / (1.7×1.7) = 23

Based on your BMI, you are assigned one of the following weight statuses:

• Underweight (Below 18.5)
• Healthy (18.5 to 24.9)
• Overweight (25.0 to 29.9)
• Obese (30.0 and above)

Based on your weight status, you might be required to make dietary or lifestyle changes to reach a healthy weight.

Is BMI a measure of your body fat?

BMI is not a direct measure of body fat. However, it is a fairly good indicator of body fat for most people, but it does have some limitations. For example:

• A person with more muscle will have a higher BMI than someone with less muscle mass even if they are equally overweight.
• At the same BMI, women tend to carry more body fat than men.
• At the same BMI, men tend to have a higher bone density compared to women.
• A person who is very muscular and has a low percentage of body fat may have a high BMI because the amount of muscle tissue contributes to their weight.
• A person with a lot of body fat but also a very high percentage of muscle may have a low BMI. For example, some professional athletes and bodybuilders have very low BMIs but are very muscular.
• A person who is very tall may have a high BMI even if they are not overweight. This is because height contributes to weight.

BMI is a tool that helps you gauge a rough estimate of your health – it is a ratio of your weight and height

BMI can be a screening tool, but it does not diagnose the body fat or health of an individual. It is not, however, an accurate way to measure the physical fitness of athletes, people with low body fat, or a high lean body mass.

Body fat can be more accurately measured in a number of ways, with the two most common being skinfold measurements and hydrostatic weighing.

Skinfold measurement involves measuring the thickness of subcutaneous body fat at multiple locations with a measuring caliper and then applying these numbers to a formula.

Hydrostatic weighing is a more accurate alternative to skinfold measurements because it takes into account both the amount of fluid in your body and air spaces.

BMI and obesity in adults

Being underweight or obese is associated with multiple health risks. Body mass index (BMI) has been used as an assessment of a large population’s health for a long time. A study assessed the trends in adult BMI in 200 countries from 1975 to 2014. This included nearly 19.2 million participants, of which 9.9 million were men and 9.3 million were women, aged 18 years or older (2).

For adults 20 years and older, the BMI is interpreted using standard categories, ranging from underweight to morbidly obese. According to the study, the global age-standardized mean BMI (in kg per meter squared) increased from 21.7 in 1975 to 24.2 in 2014 in men. Similarly, it increased from 22.1 in 1975 to 24.2 in 2014 in women.

Over 40 years, the prevalence of underweight individuals decreased from 13.8 percent to 8.8 percent in men, and from 14.6 percent to 9.7 percent in women. Obesity, however, increased from 3.2 percent in men to 10.8 percent. Similarly, it increased from 6.4 percent to 14.9 percent in women.

The study suggested at the current rate, the global obesity prevalence will reach 18 percent in men and over 21 percent in women. Severe obesity will surpass 6 percent in men and 9 percent in women.

The U.S. seems to be the most affected by obesity. According to the Centers for Disease Control and Prevention (CDC), the prevalence of obesity in the U.S. was nearly 42 percent between 2017 to 2020, an over 10 percent increase from 1999-2000 (3).

The data also suggests that the annual medical cost of obesity in the U.S. was nearly $173 billion in 2019.

BMI in children and teens

BMI values in adults are generally not linked to age or sex, but the same cannot be said for children. Health professionals do not assign children weight ranges based simply on their BMI because:

• they undergo rapid growth
• male and female body types grow at different rates and have different caloric requirements
• females carry more body fat compared to males
• bone density increases at this age

BMI calculations for children and teens also vary based on age and sex.

Calculating BMI in Children and Teens

According to the CDC, a specific body mass index (BMI) calculator should be used for children and teens between ages 2 and 19, which you can find here.

You can use charts to check if a child is in the suitable weight range for their age according to their BMI. You can find the charts here:

• for girls aged 2 through 19
• for boys aged 2 through 19

The prevalence of specific weight status in children can be understood by the following table:

Obesity trends in children and teens

According to the CDC, obesity affected nearly 14.7 million children and teens between 2017 and 2020 alone. The highest prevalence of obesity was found to be among teenagers (aged 12 through 19) at 22.2 percent, followed by 20.7 percent in 6- to 11-year-olds.

The data also suggests that children of certain ethnic groups showed a higher prevalence of obesity, including:

• 2 percent among Hispanic children
• 8 percent among non-Hispanic Black children
• 6 percent among non-Hispanic White children
• 0 percent among non-Hispanic Asian children

Studies have also suggested that the risk of obesity is greater in urban areas, and boys are more likely to be overweight compared to girls (4).

Health risks of a high BMI and obesity

Obesity is a serious health risk, with nearly 65 percent of the world’s population living in obesity. According to the World Health Organization (WHO), at least 2.8 million people die each year due to obesity.

A high BMI and excessive body fat can lead to:

• increased blood pressure, cholesterol, and oxidative stress
• a greater risk of coronary heart disease, stroke, and other cardiovascular diseases
• increased chances of developing type 2 diabetes
• breathing problems and diminishing quality of life
• increased risk of developing some cancers
• a greater chance of all-cause mortality

Being overweight can also impact an individual’s mental and emotional health. According to the National Council on Aging (NCOA), obesity can cause:

• social isolation and loneliness
• weight bias and discrimination
• a poor body image
• a weakened immune system
• serotonin deficiency, mood disruptions
• irregular sleep patterns, insomnia, or sleep apnea
• chronic stress
• anxiety and/or depression

It, therefore, becomes important for both the physical and mental well-being that you follow certain practices that promote a healthy weight.

Health risks of being underweight

While obesity is attributed to be a serious health issue in developed countries, malnutrition can by no means be ignored.

In fact, nearly 150 million children under 5 were estimated to be stunted in growth in 2020 alone. Around 45 percent of deaths among children under 5 years of age are linked to undernutrition, a number which is even higher in underdeveloped countries.

The health risk of a low BMI and being underweight are:

• vitamin deficiencies
• weak or stunted bone and muscle growth
• anemia
• decreased immune function
• menstrual issues, irregular periods, and fertility issues
• increased risk of complications during certain diseases

The most common reason for being underweight is not eating enough food. The earliest symptoms of being malnourished include:

• feeling tired all the time
• getting sick very often
• irregular periods in females
• sickly appearance
• hair loss, weakening teeth, furrowed tongue

The easiest way to tackle malnutrition is to eat a balanced diet, higher in protein and if possible, and supplementing with essential vitamins and minerals.

The gist

BMI can be a convenient yet effective tool for predicting your health. However, it is important that you familiarize yourself with BMI weight ranges, and how it may not be an accurate assessment of health for some individuals.

For example, BMI does not factor in lean muscle mass, overall body composition, or activity levels. It is generally an ineffective manner of health assessment for athletes. For more accurate assessment of overall health and body fat, a skin-fold test using a caliper, a hydrostatic weighing test, or a DEXA scan can be more accurate.