Body Mass Index (BMI)

Body mass index (BMI) is a measurement based on your weight and height that helps indicate whether your weight falls within a range associated with certain health risks. Calculating your BMI is straightforward, but interpreting the result requires more than looking at a number.

BMI can help identify a possible risk of weight-related health problems, such as type 2 diabetes and heart disease, but it does not directly measure body fat or show where fat is stored. Your age, muscle mass, waist circumference, medical history, and other health indicators can all affect what your result means for you. 

This guide covers how BMI is calculated, what the different BMI ranges mean, and how to interpret your result in the context of your overall health.

What Is Body Mass Index (BMI)?

Body mass index, commonly called BMI, is a calculation that compares your weight with your height. Healthcare professionals use it as a screening tool to identify weight ranges that may be associated with health risks. For adults, BMI categories can indicate whether someone is underweight, within the standard healthy-weight range, overweight, or living with obesity. These categories can help determine whether further assessment may be useful, but BMI alone cannot diagnose a disease or establish a person's overall health.

BMI is widely used because it is inexpensive, easy to calculate, and does not require specialized equipment. However, it does not distinguish between muscle, bone, and fat. For example, a person who exercises regularly and has substantial muscle mass may have a higher BMI without having excess body fat. Another person with the same BMI may have less muscle and more body fat, particularly around the abdomen, which can be associated with different health risks. BMI is therefore best understood as an initial indicator rather than a complete picture of body composition or health.

How BMI Is Calculated

You can calculate BMI using either the metric system or the imperial system. Both methods compare weight with height, but the imperial formula includes a conversion factor to account for the different units of measurement.

• Metric formula: Divide your weight in kilograms by your height in meters multiplied by itself (height squared).

• Imperial formula: Divide your weight in pounds by your height in inches squared, then multiply the result by 703.

For example, if you weigh 70 kg (154 lb) and are 1.75 m tall (about 5 ft 9 in), the calculation is:

BMI = 70 ÷ (1.75 × 1.75) = 22.9

For an adult, a BMI of 22.9 falls within the standard healthy-weight category of 18.5 to 24.9. This means your weight is within the range commonly associated with lower weight-related health risks at your height. It does not, however, confirm that you have a healthy body-fat level or rule out conditions such as high blood pressure or elevated blood sugar. Your BMI result is one piece of information that may be considered alongside your waist measurement, medical history, lifestyle, and other health checks.

Where BMI Came From and What It Measures

BMI is sometimes called the Quetelet index, after Belgian statistician Adolphe Quetelet, who developed the measure in the 19th century. It became widely used because it provides a consistent way to compare weight and height across large groups of people.

The important thing to remember is that BMI measures weight in relation to height, not body fat directly. It cannot tell whether your weight comes mainly from muscle or fat, or whether fat is building up around your internal organs. Your result is a useful starting point, not a complete picture of your health.

BMI Chart and Categories for Adults

For adults aged 20 and older, BMI falls into standard categories. The chart below shows the commonly used ranges for underweight, healthy weight, overweight, and the three classes of obesity.

What Should You Take From Your Result?

If your BMI is outside the healthy-weight range, it does not automatically mean you have a health problem. It means your weight may be worth looking at alongside other factors, such as your waist measurement, blood pressure, blood sugar, and medical history. Equally, a BMI in the healthy range does not rule out every health concern.

These categories are designed for adults and should not be used to interpret a child's BMI. They may also be less informative for very muscular people, some older adults, and people whose health risks differ from those represented by the standard ranges.

Is BMI Different for Women and Men?

The BMI formula and standard adult categories are the same for women and men. However, women generally have a higher percentage of body fat than men at the same BMI, and fat may be distributed differently throughout the body.

This means a BMI chart for women or a BMI chart for men cannot tell you exactly how much body fat you have. If you want a clearer picture of your health, your waist measurement and other findings may be just as important as your BMI.

Does Ethnicity Affect BMI?

Yes. The relationship between BMI and health is not the same in every population. Many people of Asian backgrounds can develop type 2 diabetes and cardiovascular disease at lower BMI values than people of European backgrounds, partly because body fat and its distribution can differ at the same BMI.

The World Health Organization has identified BMI values of 23 and 27.5 as additional points for assessing health risk in Asian populations. Some clinical guidelines also use lower BMI thresholds when considering treatment. These are additional reference points, not universal replacement categories for everyone of Asian ancestry.

Health risks can vary across other ethnic groups, too. If you are unsure how to interpret your BMI, a healthcare professional can consider your background alongside your waist size, medical history, and other risk factors.

How Does BMI Change With Age?

BMI can become harder to interpret as you get older because the body changes with age. Muscle mass often decreases, while the proportion of body fat may increase. As a result, someone can lose muscle without seeing much change on the scale.

A low BMI in an older adult may be a sign of poor nutrition, muscle loss, or an underlying illness. A higher BMI also needs to be considered alongside mobility, strength, existing health conditions, and general well-being. The number alone cannot show whether someone is getting the nutrition and physical support they need.

BMI for Children and Teens

Children and teens are still growing, so their BMI is interpreted differently from an adult's. Instead of using fixed adult categories, healthcare professionals compare a child's BMI with reference measurements for children of the same age and sex. This comparison is called a BMI percentile.

For children and teens aged 2–19, the US Centers for Disease Control and Prevention uses these categories:

• Underweight: Below the 5th percentile.

• Healthy weight: From the 5th percentile to below the 85th percentile.

• Overweight: From the 85th percentile to below the 95th percentile.

• Obesity: At or above the 95th percentile.

For example, a child at the 75th percentile has a BMI higher than that of about 75% of children of the same age and sex in the reference group. This does not mean the child is 75% overweight.

These categories help track growth, but they do not provide a diagnosis on their own. A pediatrician can interpret the result alongside the child's growth pattern, development, family history, and overall health. Parents should not use adult BMI categories to assess a child or make changes to a child's eating habits based on the number alone.

What Is a Healthy BMI?

For most adults, a BMI between 18.5 and 24.9 is considered the standard healthy-weight range. Staying within this range is associated with a lower risk of some weight-related health problems, but it does not guarantee good health. Blood pressure, blood sugar, cholesterol, waist size, family history, and other factors also affect your risk.

You can have a BMI within the healthy range and still carry excess fat around your abdomen or have high blood pressure. On the other hand, someone with a higher BMI may have more muscle than average, which can make the number less useful as a measure of body fat.

Rather than focusing only on whether your BMI is normal, consider whether your weight has changed, how you feel, and whether you have any health concerns. If you are unsure what your result means, a doctor can help you decide whether any further checks would be useful.

Health Risks Linked to a High BMI

A higher BMI is associated with an increased risk of certain health conditions, particularly when it reflects excess body fat. It does not mean you will develop these conditions, and your personal risk depends on several factors.

Health problems associated with excess body fat can include:

• Type 2 diabetes: Excess body fat can make it harder for the body to use insulin properly, which can cause blood sugar levels to rise.

• High blood pressure and heart disease: Excess body fat can affect blood pressure and other factors that influence heart health.

• Sleep apnea: Fat around the upper airway can contribute to repeated interruptions in breathing during sleep.

• Fatty liver disease: Fat can build up in the liver and, in some people, cause inflammation and liver damage.

• Joint problems: Extra strain on weight-bearing joints can contribute to pain and conditions such as osteoarthritis.

• Some cancers: Excess body fat is associated with a higher risk of several types of cancer.

A doctor may recommend checking blood pressure, blood glucose, cholesterol, or liver health depending on your BMI and other risk factors. These checks can identify problems that BMI alone cannot show.

Health Risks Linked to a Low BMI

A BMI below 18.5 means your weight is below the standard range for your height. Some people naturally have a lower body weight, but for others, it can be linked to inadequate nutrition, an underlying illness, or unintentional weight loss.

If low weight is related to poor nutrition or loss of muscle and body fat, possible problems include weakened bones, anemia, reduced immune function, fertility difficulties, and slower recovery from illness. Unexplained weight loss, persistent fatigue, weakness, or digestive symptoms are reasons to seek medical advice.

A doctor can help investigate possible causes, including nutrition problems, thyroid conditions, medication effects, or digestive disorders. What matters is understanding why your weight is low and whether it is affecting your health.

Is BMI Accurate? Understanding Its Limitations

BMI is useful because it is quick, inexpensive, and easy to compare across populations. It helps healthcare professionals identify weight ranges associated with health risks and decide when a closer assessment may be helpful. However, BMI cannot measure body fat directly or show how much of your weight comes from muscle.

This is why BMI can sometimes give a misleading impression. A muscular person may have a high BMI without having excess body fat, while someone within the healthy range may carry more fat around their waist than the number suggests. BMI is most useful when it is considered alongside other measurements and your overall health.

Can Muscle Affect Your BMI?

Yes. Muscle is denser than fat, so a person with a lot of muscle may weigh more for their height and have a higher BMI. This can happen in athletes and people who regularly do strength training.

The opposite can also occur. Someone may have a BMI in the healthy range but have relatively little muscle and a higher proportion of body fat. If your BMI does not seem to match your build or activity level, a waist measurement or body composition assessment may help clarify the picture.

Does BMI Show Where You Store Fat?

No. BMI cannot tell whether fat is stored around your waist, hips, thighs, or elsewhere. This matters because visceral fat—fat stored deep inside the abdomen around internal organs—is associated with a higher risk of metabolic and cardiovascular problems.

Two people with the same BMI may have different waist measurements and different health risks. Measuring your waist or comparing your waist size with your height can help identify abdominal fat that BMI alone may miss.

When Is BMI Less Useful?

BMI needs careful interpretation for children, pregnant people, very muscular adults, and some older adults. It may also be less informative for people whose body composition differs from that of the populations used to develop standard categories.

During pregnancy, BMI before pregnancy can help guide recommendations for weight gain. However, weight naturally changes during pregnancy, so BMI is not used to assess weight in the same way during this time.

If your BMI seems inconsistent with your body composition or health, a healthcare professional can help you understand the result and decide whether other measurements would be useful.

What Can You Use Alongside BMI?

A few additional checks can help explain what your BMI means. You may not need every test or measurement; the most useful options depend on your health, age, and medical history.

• Waist circumference: Measures around your waist and helps assess abdominal fat.

• Waist-to-height ratio: Compares your waist measurement with your height, using the same units for both. In adults, a ratio of 0.5 or higher can indicate increased health risk, although it is not a diagnosis.

• Waist-to-hip ratio: Compares your waist size with your hip size to give another indication of how fat is distributed.

• Body composition testing: Methods such as dual-energy X-ray absorptiometry (DEXA) can estimate body fat and lean tissue while also measuring bone mineral density. These results can provide additional information about body composition and bone health that BMI cannot show, although testing is not necessary for everyone.

• Blood pressure and blood tests: Checking blood pressure, blood glucose, and cholesterol can reveal health concerns that BMI cannot detect.

For many adults with a BMI below 35, NICE recommends using waist-to-height ratio alongside BMI to help assess abdominal fat and related health risks. These measurements add useful information, but none gives a complete picture of health on its own.

What Are Clinical and Preclinical Obesity?

In January 2025, an international expert commission proposed distinguishing between preclinical obesity and clinical obesity. The proposal reflects an important point: having excess body fat and already experiencing health problems because of it are not the same thing. This is an evolving expert recommendation and has not been adopted universally.

Under the proposed approach, preclinical obesity means a person has excess body fat but no current evidence that it is affecting organ function or daily activities. Clinical obesity refers to excess body fat that is already affecting how the body works or significantly limiting everyday activities.

The commission recommends looking beyond BMI to confirm excess body fat and assess how it affects a person's health. This may involve waist measurements, body composition assessment, symptoms, and signs of organ problems. The goal is to make treatment decisions based on a person's health needs, rather than relying on a BMI number alone.

What Should You Do With Your BMI Result?

Your BMI can help you decide whether to seek further advice, but you do not need to draw conclusions from the number alone. Think about whether your weight has changed recently, whether you have any symptoms, and whether you have health conditions that may be linked to weight. A healthcare professional can help you work out whether further checks or treatment would be useful.

If Your BMI Is in the Healthy Range

A BMI between 18.5 and 24.9 falls within the standard healthy-weight range for adults. Regular physical activity, a varied diet, adequate sleep, and routine health checks can help support your well-being.

A healthy-range BMI does not rule out every health concern. If you have concerns about abdominal fat, blood pressure, or blood sugar, it is still worth discussing them with your doctor.

If Your BMI Is Above the Healthy Range

A higher BMI does not automatically mean you are unhealthy or need a particular treatment. Your doctor may look at your waist measurement, blood pressure, blood glucose, cholesterol, sleep, mobility, and medical history to understand your overall health.

If treatment would be helpful, options may include nutrition support, changes to eating and activity habits, prescription medication, or bariatric surgery in selected cases. The right approach depends on your health and personal circumstances. Crash diets and quick fixes are not a substitute for a treatment plan that is safe and suitable for you.

If Your BMI Is Below the Healthy Range

If your BMI is below 18.5, consider whether you have lost weight without trying or have experienced fatigue, weakness, appetite changes, or digestive symptoms. A doctor can help investigate possible causes, including inadequate nutrition, thyroid problems, medication effects, or digestive conditions.

The aim is to understand why your weight is low and whether it is affecting your health. The appropriate next steps depend on the cause, not just the BMI number.

A Note on Body Image and Eating Habits

BMI is a health screening tool, not a judgment about your appearance or personal worth. If concerns about weight are causing distress, affecting how you eat, or leading to restrictive or compulsive behaviors, consider speaking with a doctor or mental health professional. BMI alone cannot diagnose an eating disorder, and support is available whatever your result.

BMI and Weight Loss Treatment

BMI is one factor healthcare professionals use when considering medical weight management or bariatric surgery. It can help identify people who may benefit from further assessment, but it does not determine treatment on its own. Your medical history, weight-related health conditions, previous treatment, and personal circumstances all matter.

Medication and Medical Weight Management

Some prescription weight management medicines may be considered for adults with a BMI of 30 or higher, or a BMI of 27 or higher when weight-related health conditions are present. The criteria depend on the medicine, its approved use, and local prescribing rules.

Medication is not suitable for everyone and is generally considered alongside nutrition support, physical activity, and follow-up. Availability and prescribing criteria in Turkey should be confirmed with a treating specialist. A BMI result alone cannot tell you whether medication is appropriate.

What BMI Is Needed for Weight Loss Surgery?

International guidelines recommend metabolic and bariatric surgery for appropriately selected adults with a BMI of 35 or higher, even if they do not have other obesity-related conditions. Surgery may also be considered for adults with a BMI of 30–34.9 who have metabolic disease or have not achieved lasting improvement with nonsurgical treatment.

For people of Asian backgrounds, lower BMI thresholds may be appropriate because some weight-related health conditions can develop at lower BMI values. International guidelines recognize this difference, but eligibility depends on clinical assessment and the criteria used by the treating team.

Procedures such as gastric sleeve surgery and gastric bypass surgery require a thorough assessment before a decision is made. The specialist team considers your health conditions, previous treatment, potential benefits and risks, nutritional needs, and ability to attend long-term follow-up. Meeting a BMI threshold does not automatically mean surgery is the best option.

Weight Management and Metabolic Health Care at Medical Park

At Medical Park hospitals in Turkey, weight management begins with a comprehensive assessment of each patient's health, rather than BMI alone. Depending on individual needs, a comprehensive check-up may include BMI and waist measurements, blood pressure monitoring, blood tests for blood sugar and cholesterol, and further investigations to identify conditions that may affect weight or increase the risk of complications. These findings help the medical team assess metabolic health and determine appropriate next steps.

Medical Park takes a multidisciplinary approach to weight management, bringing together specialists in Endocrinology, Gastroenterology, General Surgery, and Nutrition and Dietetics, along with other specialties when needed. This approach allows patients to receive care tailored to their medical history, weight-related health concerns, and individual needs.

For patients who may benefit from bariatric surgery, specialists experienced in these procedures assess eligibility and explain options such as sleeve gastrectomy and gastric bypass. The assessment takes into account the patient's medical history, existing health conditions, potential surgical risks and benefits, and the nutritional support and follow-up required after surgery.

Ongoing follow-up is also an important part of weight management, whether a patient receives medical treatment, nutritional support, or bariatric surgery. Medical Park’s International Patient Services Center helps international patients coordinate comprehensive check-ups, specialist consultations, diagnostic appointments, treatment planning, and follow-up arrangements in Turkey.