You can use a measurement called a body mass index, or BMI, along with your waist size , to decide whether your weight is dangerous to your health. The BMI is a combination of your height and weight. If you have a BMI of 30 or higher, unhealthy eating patterns, and too little physical activity, your extra weight is putting your health in danger.

Use the Interactive Tool: Is Your BMI Increasing Your Health Risks? to find out your body mass index.

Use the Interactive Tool: What Is Your Child’s BMI? to check BMI in children ages 2 to 19.

People who carry too much fat around the middle, rather than around the hips, are more likely to have health problems. In women, a waist size of 35 in. (88 cm) or more raises the chance for disease. In men, a waist size of40 in. (101 cm) or more raises the chance for disease.

If you are Asian, your health may be at risk if you have a BMI of 27.5 or higher and you have unhealthy eating patterns and too little physical activity. Also, health problems are seen with a smaller waist size. In Asian women, a waist size of 32 in. (80 cm) or more raises the chance for disease. In Asian men, a waist size of 36 in. (90 cm) or more raises the chance for disease.

Check this table to find your risk for disease using your body mass index and waist size.

What causes obesity?

When you take in more calories than you burn off, you gain weight. How you eat, how active you are, and other things affect how your body uses calories and whether you gain weight.

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If your family members are obese, you may have inherited a tendency to gain weight. And your family also helps form your eating and lifestyle habits, which can lead to obesity.

Also, our busy lives make it harder to plan and cook healthy meals. For many of us, it’s easier to reach for prepared foods, go out to eat, or go to the drive-through. But these foods are often high in saturated fat and calories. Portions are often too large. Work schedules, long commutes, and other commitments also cut into the time we have for physical activity.

You’ve tried diets, but you always gain the weight back. What can you do?

Focus on health, not diets. Diets are hard to stay on and usually don’t work in the long run. It is very hard to stay with a diet that includes lots of big changes in your eating habits.

Instead of a diet, focus on lifestyle changes that will improve your health and achieve the right balance of energy and calories. To lose weight, you need to burn more calories than you take in. You can do it by eating healthy foods in reasonable amounts and becoming more active. And you need to do it every day.

Little steps mean a lot. Losing just 10% of your body weight can make a difference in your health.

Make a plan for change. Work with your doctor to create a plan that will work for you. Ask family members and friends for help in keeping with your plan. Ask your doctor to recommend a dietitian to help you with meal planning.

When you stray from your plan, don’t get upset. Figure out what got you off track and how you can fix it.

How can you stay on your plan for change?

It’s hard to change habits. You have to be ready. Make sure this is the right time for you. Are you ready to make a plan and stay on it? Do you have the support of your family and friends? Do you know what your first steps will be? Becoming healthier and staying that way is a lifelong effort.

Most people have more success when they make small changes, one step at a time. For example, you might eat an extra piece of fruit, walk 10 minutes more, or add more vegetables to your meals.

Studies show that people who keep track of what they eat are better at losing weight. Keep a notebook where you can write down everything you eat and drink each day. You may be surprised to see how much you are eating. Use a calorie counter to add up your calories. (You can find calorie counters online and at bookstores.)

As you keep track of calories, look at whether you skip meals, when you eat, how often you eat out, and how many fruits and vegetables you eat. Keep track of when you eat beyond feeling full and if you eat for reasons other than being hungry. This will help you see patterns that you may want to change.

You may want to write down the amount of physical activity you’ve had each day and compare the calories you burned to those you took in. Use the Interactive Tool: How Many Calories Did You Burn? to see how many calories you burn through daily activities.

Can you take medicines or have surgery to lose weight?

Surgery and medicines don’t work by themselves. Most people also need to make changes in what they eat and how active they are.

Before your doctor will prescribe medicines or surgery, he or she will probably want you to work on healthier eating and activity for at least 6 months. Even if your doctor gives you medicines or recommends surgery, you will need to keep your new healthy habits for the rest of your life.