Health A-Z

Medical Content Created by the Faculty of the Harvard Medical School


What Is It?

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 more accurate 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:

  • High blood pressure

  • Diabetes

  • Heart disease

  • Some forms of cancer

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:

  • Genetic influences: Your genetic makeup plays a significant role in your chances of becoming obese. However, you still maintain most of the control when it comes to your weight.

    Some rare genetic diseases make it almost impossible to avoid obesity.

  • Physiological influences: Some researchers believe that every person has a predetermined weight that the body resists moving away from. Also, people of the same age, sex and body size often have different metabolic rates. This means their bodies burn food differently. Someone with a low metabolic rate may require fewer calories to maintain approximately the same weight as someone whose metabolic rate is high.

  • Food intake and eating disorders: If you eat a lot, especially foods that are high in fat and calories, you can become obese. Obesity also can result from eating disorders, such as a tendency to binge.

  • Lifestyle: If you lead a sedentary lifestyle, you are at a higher risk of becoming obese.

    • Your weight history: If you were overweight as a child or adolescent, you are more likely to be obese as an adult.

    • Pregnancy: Pregnancy can contribute to obesity. Many women weigh more after each pregnancy.

    • Drugs: Some drugs can cause obesity. These include steroid hormones and many drugs used to treat psychiatric conditions.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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