What's Your BMI?

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The Body Mass Index (BMI) is one of a series of measures to determine the level of excess fat in the body. Although other measures such as hydrodensitometry (underwater weighing), skinfold measurements (using calipers) and magnetic resonance imaging can provide more accurate determinations of body fat, BMI can be useful in most cases.

How to Calculate Your BMI

Your BMI is simply determined by the ratio of your weight and height. More specifically, BMI is calculated using the following formula:

BMI = weight / (height)2

This formula uses the metric system, with weight in kilograms and height in meters. To calculate your BMI based on height in inches and weight in pounds, multiply the result by 703.

BMI = 703 x weight (lbs.) / (height [in.])2

Interpreting Your BMI

The National Heart Lung and Blood Institute (NHLBI) considers BMI scores for average individuals to be ranked as follows:

Condition BMI
Underweight

Below 18.5

Normal 18.5-24.9

Overweight

25.0-29.9

Obese

30.0 and above

It is important to note, however, that BMI scores may fail to properly estimate the level of body fat in certain individuals. Athletes and others with muscular builds, for example, may fall into the overweight category despite having near perfect physiques, while the elderly and those who have lost muscle mass may fall into the underweight category despite having excess body fat.

The other important thing to note is that BMI assessments vary by sex and age. BMI scores for boys are slightly lower than for girls (aged 7-16), and scores for women are slightly lower than for men (aged 18 and up).

Despite these caveats, for most people, the BMI is a simple and useful tool for determining disease risk due to excess fat.

Risk Factors Associated with High BMI

There are a number of risk factors linked to high BMI scores that put individuals at significant risk for diabetes, heart disease, high blood pressure, gall bladder disease and some types of cancer. These include:

  • High LDL cholesterol (considered "bad" cholesterol)
  • Low HDL cholesterol (considered "good" cholesterol)
  • High blood sugar
  • High triglycerides
  • Sedentary lifestyle
  • Smoking

How much BMI do I need to loss to see improvements?

It may seem that a significant amount of exercise is needed to lose weight but even a small drop of 5 to 10 percent body weight can help lower the risk for obesity related diseases. As with all exercise programs, take care when getting started. Check with your doctor before you begin and discuss the fitness program you intend to start.

Depending how many additional risk factors you have, your doctor may advise a weight control program rather than a weight loss program. For dangerously high BMI scores, your doctor may advise more drastic measures, which may include surgery. However, for most individuals, a sensible fitness program and healthy diet that is incorporated into daily life and maintained in the long term can significantly lower BMI scores and thus lower the chances for excess fat related disease.