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Why You Need to Know Your BMI

Your weight alone won't give you your total health picture.

man on scale in doctor's office

BMI stands for body mass index. For most people, BMI can be used as an indicator of body fatness. The National Institutes of Health (NIH) uses it as a way to help define obesity and being overweight.

Knowing your BMI is important. For example, if your BMI is too high, you may be at an increased risk for many chronic health problems. These include high blood pressure, type 2 diabetes, coronary heart disease, stroke, gallbladder disease and osteoarthritis. It may also raise your risk for certain cancers including endometrial, breast and colon cancers.

Making sense of the numbers

BMI is calculated using your height and weight.  It does not measure body fat directly. BMI = [weight in pounds ÷ height in inches ÷ height in inches] x 703. For example, the BMI of a person who is 6 feet tall and who weighs 210 pounds would be 210 pounds ÷ 72 inches ÷ by 72 inches and multiplied by 703 = 28.5.

According to the NIH, these are BMI categories:

  • Underweight: below 18.5
  • Normal: 18.5 to 24.9
  • Overweight: 25 to 29.9
  • Obese: 30 and above

Note that it is not accurate for everyone.  It is possible to have a higher BMI, but very little body fat. Bodybuilders and highly-trained athletes may fall into that category. But most people who are overweight have too much body fat. BMI also may not be accurate for older people and those who have lost muscle. For these groups, BMI may underestimate their amount of body fat.

Children, too

Overweight children are more likely to be overweight adults. To help fight the rising obesity rate among children, the Centers for Disease Control and Prevention have also published BMI charts for children.

Children’s BMI charts help doctors identify potential weight problems at an early age. Even though BMI for children and teenagers is calculated the same way as for an adult, the way it is interpreted is different.

For children and teens, interpretation of BMI takes into account both age and sex. That is because body fat for children changes with age. Boys and girls also differ in the amount of body fat they have. BMI in children and teens is reported as percentiles compared with other children of the same sex and age.

Waist circumference

Besides knowing your BMI, waist circumference (WC) can be a helpful screening measure for certain health risks associated with being overweight or obese. Carrying weight around your middle, rather than on your hips and thighs, may put you at higher risk of heart disease and type 2 diabetes.

The risk goes up for women with a waist of greater than 35 inches around. It goes up for men with a waist greater than 40 inches. For an accurate WC reading, stand, place a tape measure around your middle just above your hip bones and measure after you breathe out.

Toward a healthier BMI

If you’re overweight or obese and need to lose weight, reduce your calories and increase your physical activity.

Here are some tips for losing weight and keeping it off.

  • Talk to your doctor before you start any weight-loss program. Be wary of fad diets and rapid weight-loss programs. They may give you dramatic short-term results, but may not be effective or may be dangerous in the long term. If you don’t get enough calories, your body can go into starvation mode, your metabolism can slow down and weight loss will be even harder.
  • Be active. Guildelines call for at least 150 minutes of moderate-intensity aerobic activity every week and muscle-strengthening activities on two or more days a week that work all major muscles. Keep in mind that you haven’t been active for a while, ask your doctor the types of activities that may be right for you. If you have a health condition or a disability, ask if you should take any special precautions when you exercise.
  • Know what you're eating. Read food labels and pay attention to portion sizes, fat, sugar and sodium content.
  • Track your eating and exercising habits by recording them in a diary.
  • Stick with it. Don’t give up just because you reached a plateau or binged on potato salad at yesterday’s barbecue.
  • Set realistic weight-loss goals, such as no more than 1/2 to 2 pounds a week. Even a small amount of weight loss can make a huge difference to your health.

By Diane Griffith, Contributing Writer

Weight-control Information Network. Weight-loss and nutrition myths. Accessed: September 9, 2016.
Centers for Disease Control and Prevention. Assessing your weight. Accessing: September 9, 2016.
National Heart, Lung, and Blood Institute. Aim for a healthy weight: Assessing your weight and health risk. Accessed: September 9, 2016.
Centers for Disease Control and Prevention. About BMI for children and teens. Accessed: September 9, 2016.

Last Updated: September 12, 2016