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Medically reviewed by Last updated on Mar 24, 2023.

What is Obesity?

Harvard Health Publishing

Obesity is an excess of body fat.

It is difficult to directly measure body fat. Body mass index (BMI) is a popular method of defining a healthy weight. BMI should be used as a guide, along with waist size, to help estimate the amount of body fat.

BMI estimates a healthy weight based on your height. Because it considers height as well as weight, it is a somewhat better guide than body weight alone.

To calculate your BMI:

  1. Multiply your weight in pounds by 703.
  2. Divide that answer by your height in inches.
  3. Divide that answer by your height in inches again.

Then use the chart below to see what category your BMI falls into.



Below 18.5


18.5 – 24.9


25.0 – 29.9


30.0 – 39.9


Over 40

Morbidly obese

Obesity can shorten your life. It can also put you at risk of developing a number of conditions. These include:

Many other health risks are higher for people who are obese. These risks may increase as the degree of obesity increases.

Where you carry the extra weight is also important. People who carry extra weight around their waist may be more likely to experience health problems caused by obesity than those who carry it in their legs and thighs.

People become obese for a number of reasons. Often, several of these factors are involved.

Some of the most common reasons for obesity are:


The primary warning sign of obesity is an above-average body weight. If you are obese, you may also experience:

Obesity increases your risk for:


Obesity is diagnosed by calculating your BMI. BMI is based on your height and weight. A BMI of 30 or more defines obesity. In general, this means your body weight is 35% to 40% more than your ideal body weight.

Your body fat also can be calculated by using skin calipers. Calipers are an instrument that measures the thickness of your skin.

Body shape is also important. People who carry most of their weight around the waist (apple shaped) have a greater risk of heart disease and diabetes than do people with big hips and thighs (pear shaped).


Waist circumference is a good measure of abdominal obesity. Ideally your waist size should be no more than one-half of your height. Women with a waist more than 35 inches or men with a waist more than 40 inches are at increased risk of cardiovascular disease, fatty liver and type 2 diabetes.

Expected duration

Obesity is often a lifelong problem. Once excess weight is gained, it is not easy to lose. Once lost, you will have to work at maintaining your healthier weight.

The length of time it takes to reach your weight goal depends on:

Diseases and conditions caused by obesity often improve as you lose weight.


To prevent obesity and maintain a healthy body weight, eat a well-balanced diet and exercise regularly.

Preventing obesity is important. Once fat cells form, they remain in your body forever. Although you can reduce the size of fat cells, you cannot get rid of them.


Weight reduction is achieved by:

Structured approaches and therapies to reduce weight include:

Types of surgical procedures include:

When to call a professional

Call your doctor if you need help losing weight. Also call if you have any of the symptoms or complications of obesity.


Others, however, find it difficult to maintain the weight loss for long. Most people return to their pretreatment weight within five years.

Additional info

Academy of Nutrition and Dietetics

National Heart, Lung, and Blood Institute (NHLBI)

Centers for Disease Control and Prevention (CDC)

Center for Food Safety & Applied Nutrition

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.