In a medical setting what is 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 does not 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 should not 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.

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

In general, the following BMI ranges (in kg/m2) classify different weight types:

• Underweight: Less than 18.5.
• Optimum range: 18.5 to 24.9.
• Overweight: 25 to 29.9.
• Class I obesity: 30 to 34.9.
• Class II obesity: 35 to 39.9.
• Class III obesity: More than 40.

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 higher risk for developing the following conditions:

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:

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:

1. 180 x 703 = 126,540.
2. 126,540 / 65 = 1,946.769.
3. 1,946.769 / 65 = 29.95.

Their BMI would be 29.9.

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.

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 adults assigned male at birth (AMAB) and adults assigned female at birth (AFAB) even though adults AFAB typically have more body fat than adults AMAB.
• 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.
• Pregnant people.
• People 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.

A note from Cleveland Clinic

Body mass index (BMI) is a quick tool that healthcare providers can use to assess your risk for certain health conditions. However, BMI is not always an accurate measurement of body fatness and is not 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.

• body builders
• some high-performance athletes
• pregnant women.

BMI calculations will underestimate the amount of body fat for:

• the elderly
• people with a physical disability, who are unable to walk and may have muscle wasting.

BMI is also not an accurate indicator for people with:

• eating disorders like anorexia nervosa
• extreme obesity.

Why isn’t BMI always the best measure of health?

Generally speaking, the more body fat you’re carrying, the higher your health risk. However, BMI cannot differentiate between body fat and muscle mass.

This means there are some exceptions to the BMI guidelines:

• Muscles – body builders and people who have a lot of muscle bulk will have a high BMI, but are not overweight.
• Physical disabilities – people who have a physical disability and are unable to walk may have muscle wasting. Their BMI may be slightly lower, but this does not necessarily mean they are underweight. In these instances, it is important to consult a dietitian who will provide helpful advice.
• Height – BMI is not totally independent of height and it tends to overestimate obesity among shorter people and underestimate it among taller people. Therefore, BMI should not be used as a guide for adults who are very short (less than 150 cm) or very tall (more than 190 cm).
• People of different ethnic groups – Asians and Indians, for example, have more body fat at any given BMI compared to people of European descent. Therefore, the cut-offs for overweight and obesity may need to be lower for these populations. This is because an increased risk of diabetes and cardiovascular disease begins at a BMI as low as 23kg/m2 in Asian populations. Some populations have equivalent risks at a higher BMI, such as people of Torres Strait Islander and Maori origin.

What is a healthy BMI range for children?

BMI calculations used for adults are not a suitable measure of weight for children or adolescents. BMI calculations for children and adolescents are interpreted differently from an adult’s and take into account the age and sex of the child or adolescent.

The current BMI charts for children have been developed by the US Centres for Disease Control and Prevention. They are useful for the assessment of overweight and obesity in children aged over two years.

To calculate a child's BMI, you can use the body mass index calculator for children and teenagers.

However, BMI charts should be used only as a guide to indicate when to make small lifestyle changes, and when to seek further guidance from a doctor or a dietitian.

Being overweight or underweight can affect your health

The link between being overweight or obese and the chance you will become ill is not definite. Research is ongoing.

Statistically, there is a greater chance of developing various diseases if you are overweight. For example, the risk of death rises by 20 to 30 per cent as BMI rises from 25 to 27kg/m2. As BMI rises above 27 kg/m2, the risk of death rises more steeply (by 60 per cent).

Risks of being overweight (high BMI) and physically inactive

If you are overweight (with a BMI over 25kg/m2) and physically inactive, you may develop:

Risks of being underweight (low BMI)

If you are underweight (BMI less than 18.5kg/m2), you may be malnourished and develop:

Waist circumference is a better indicator of increased disease risk

A person’s waist circumference is a better predictor of health risk than BMI. When identifying health risk in adults, it is recommended that you combine your BMI classification with your waist circumference as a measurement of disease risk. Having fat around the abdomen or a ‘pot belly’, regardless of your body size, means you are more likely to develop certain obesity-related health conditions. Fat predominantly deposited around the hips and buttocks doesn’t appear to have the same health risk. Men, in particular, often deposit weight in the waist region and therefore have an increased risk of obesity-related disease. Studies have shown that the distribution of body fat is linked to an increased prevalence of diabetes, hypertension, high cholesterol and cardiovascular disease.

Generally, the associations between health risks and body fat distribution are:

• least risk – slim (evenly distributed body fat)
• moderate risk – overweight with no pot belly
• moderate to high risk – slim with pot belly
• high risk – overweight with excess belly fat.

Waist circumference and health risks

Waist circumference can be used to indicate health risk for chronic diseases.

For men:

• 94 cm or more – increased risk
• 102 cm or more – substantially increased risk.

For women:

• 80 cm or more – increased risk
• 88 cm or more – substantially increased risk.

Although the tendency to deposit fat around the middle is influenced by a person’s genes, you can still take this genetic tendency into account and do something about it.

Being physically active, avoiding smoking, and eating unsaturated fat instead of saturated fat have been shown to decrease the risk of developing abdominal obesity.

Where to get help

• Eat for health: Australian dietary guidelines (summary) , National Health and Medical Research Council, Australian Government. 
• About child and teen BMI , US Centers for Disease Control and Prevention. 
• Mooney SJ, Baecker A, Rundle AG 2013, 'Comparison of anthropometric and body composition measures as predictors of components of the metabolic syndrome in a clinical setting ', Obesity Research and Clinical Practice, vol 7, no. 1, pp. 55–66.
• Body mass index – BMI , 2019, World Health Organization (WHO). 

This page has been produced in consultation with and approved by:

This page has been produced in consultation with and approved by:

This page has been produced in consultation with and approved by:

Content on this website is provided for information purposes only. Information about a therapy, service, product or treatment does not in any way endorse or support such therapy, service, product or treatment and is not intended to replace advice from your doctor or other registered health professional. The information and materials contained on this website are not intended to constitute a comprehensive guide concerning all aspects of the therapy, product or treatment described on the website. All users are urged to always seek advice from a registered health care professional for diagnosis and answers to their medical questions and to ascertain whether the particular therapy, service, product or treatment described on the website is suitable in their circumstances. The State of Victoria and the Department of Health shall not bear any liability for reliance by any user on the materials contained on this website.