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Body Mass Index
(BMI)

Overview

Body mass index (BMI) is a tool that healthcare providers use to estimate the amount of body fat by using your height and weight measurements. It can help assess risk factors for certain health conditions. The BMI isn’t always an accurate representation of body fatness.

What is body mass index (BMI)?

Body mass index (BMI) is a medical screening tool that measures the ratio of your height to your weight to estimate the amount of body fat you have. Healthcare providers calculate BMI by using weight in kilograms (kg) divided by the square of height in meters (m2).

In most people, BMI correlates to body fat — the higher the number, the more body fat you may have — but it’s not accurate in some cases. BMI alone doesn’t diagnose health. Healthcare providers use BMI and other tools and tests to assess someone’s health status and risks.

High body fat may lead to heart disease, stroke and Type 2 diabetes. Low body fat may be related to malnutrition. Just the right amount of body fat helps vitamins and minerals get into your body. It also provides a source of energy for your body, helps maintain body temperature and protects your organs.

You shouldn’t use the standard BMI chart to evaluate a child’s or teenager’s weight. Talk to your child’s healthcare provider about the optimum weight range for their age and height.

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What is BMI used for?

Healthcare providers use BMI to help diagnose weight types and as a screening tool for certain health conditions.

Diagnosing weight types with BMI

Body mass index scores are broken down into the following categories:

Underweight: BMI less than 18.5

Normal weight: BMI of 18.5 to 24.9

Overweight: BMI of 25 to 29.9

Obesity: BMI of 30 or higher

Obesity is sometimes broken down into additional categories:

Class 1 (mild obesity): BMI 30 to 34.9

Class 2 (moderate obesity): BMI 35 to 39.9

Class 3 (severe obesity): BMI of 40 or higher

BMI isn’t the only tool providers use to classify weight types. Other tools include:

Measuring waist circumference

Measuring skin thickness using skinfold calipers in certain areas of your body, such as the back of your upper arms and under your shoulder blades

DEXA scan and air displacement plethysmography (ADP) — these are used less often

Screening for health risks with BMI

If you have a BMI less than 18.5 (underweight), you may be at a higher risk for developing the following conditions:

Malnutrition

Anemia

Weakened immune system, which could lead to more frequent infections and illnesses

Osteoporosis

Infertility

If you have underweight, your healthcare provider will likely order certain blood tests and other tests to check your overall health and to see if you’re malnourished.

In general, the higher your BMI, the higher your risk for the following conditions:

Heart disease

High blood pressure (hypertension)

Type 2 diabetes

Liver diseases

Gallstones

High cholesterol

Osteoarthritis

Sleep apnea

Certain cancers, including colon, breast, endometrial and gallbladder

Depression and other mental health conditions

It’s important to remember that you could have any of the above health conditions without having a high BMI. Similarly, you could have a high BMI without having any of these conditions. Genetics and other factors, such as smoking cigarettes, play a large role in the development of these conditions.

If your BMI reveals you may have obesity, your provider will likely order certain blood tests to check your general health, such as a comprehensive metabolic panel and lipid panel.

How do I calculate my BMI?

You can calculate BMI yourself with these steps:

Multiply your weight in pounds by 703.

Divide that answer by your height in inches (there are 12 inches in 1 foot).

Divide that answer by your height in inches again.

For example, a person who weighs 180 lbs. and is 5 feet and 5 inches tall (65 inches total) would calculate their BMI in the following way:

180 x 703 = 126,540

126,540 / 65 = 1,946.769

1,946.769 / 65 = 29.95

Their BMI would be 29.95.

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What is a healthy BMI?

The optimum range for a healthy BMI is considered to be 18.5 to 24.9.

It’s important to remember that body fatness isn’t the only determiner of overall health. Several other factors, such as genetics, activity level, smoking cigarettes or using tobacco, drinking alcohol and mental health conditions all affect your overall health and your likelihood of developing certain medical conditions.

BMI for Children and Teens

Weight Status Category	Percentile Range
Underweight	Less than 5th percentile
Normal or healthy weight	5th percentile to less than 85th percentile
Overweight	85th to less than 95th percentile
Obesity	95th percentile or greater
Severe obesity	120% of the 95th percentile

BMI of Adults Ages 20 and Older

BMI	Classification
18.5 to 24.9	Normal, or healthy, weight
25 to 29.9	Overweight
30+	Obesity (including severe obesity)
40+	Severe obesity

What are the limitations of BMI?

The standard BMI chart has limitations for various reasons. Because of this, it’s important to not put too much emphasis on your BMI.

Even though the BMI chart can be inaccurate for certain people, healthcare providers still use it because it’s the quickest tool for assessing a person’s estimated body fat amount.

Limitations of using BMI to help diagnose weight types

The standard BMI has limitations in regards to diagnosing weight types, including:

BMI doesn’t differentiate between lean body mass (the weight of everything in your body except fat) and fat mass. Because of this, a person can have a high BMI (by being muscular) but still have a very low fat mass and vice versa.

The same BMI chart is used for males and females even though adult females typically have more body fat than adult males.

The BMI chart hasn’t been adjusted for the increasing average adult height over the years.

You shouldn’t use the standard BMI chart to assess the amount of body fat of the following populations:

Athletes and bodybuilders

Children and teenagers

Anyone who’s pregnant

Anyone over the age of 65

People who have muscle atrophy (wasting) due to medical conditions

Limitations of using BMI as a screening tool for health conditions

The BMI as a screening tool for assessing the risk of certain health conditions, such as Type 2 diabetes and heart disease, has some limitations, including:

The BMI doesn’t measure the location or distribution of body fat. This is an issue because excess fat accumulation in certain areas of your body, such as in your belly (abdomen), is associated with a higher risk of health conditions than excess fat accumulation in other areas of your body, such as in your thighs.

The relationship between BMI and rate of death often doesn’t account for such factors as family history of diabetes, high blood pressure, cardiovascular disease and high cholesterol (dyslipidemia), familial longevity (average lifespan) or family history of cancer.

Prevalence of Overweight and Obesity

Adults

Age-adjusted percentage of US adults with overweight, obesity, and severe obesity by sex, 2017–2018 NHANES Data

	All (Men and Women)	Men	Women
Overweight	30.7	34.1	27.5
Obesity (including severe obesity)	42.4	43.0	41.9
Severe obesity	9.2	6.9	11.5

As shown in the above table

Nearly 1 in 3 adults (30.7%) are overweight.

More than 1 in 3 men (34.1%) and more than 1 in 4 women (27.5%) are overweight.

More than 2 in 5 adults (42.4%) have obesity (including severe obesity).

About 1 in 11 adults (9.2%) have severe obesity.

The percentage of men who are overweight (34.1%) is higher than the percentage of women who are overweight (27.5%).

The percentage of women who have severe obesity (11.5%) is higher than the percentage of men who have severe obesity (6.9%).

One Final Note...

Body mass index (BMI) is a quick tool that healthcare providers can use to assess your risk for certain health conditions. However, BMI isn’t always an accurate measurement of body fatness and isn’t the sole determiner of your general health.
If you have any questions or concerns about your weight or your risk for developing certain health conditions, such as heart disease, talk to your healthcare provider. They’re available to help.