BMI Calculations

Body mass index, or BMI, is often used to assess your body weight and to monitor changes over time. It can help answer questions about the appropriate weight for your height and whether your weight is jeopardizing your health or is associated with good health and longevity. If your weight falls within the healthy range for your height, your fat distribution pattern indicates a low risk of illness and premature death, and your medical history reflects no obesity-related risk factors, then you may not gain any health advantage by changing your weight. If you do not meet those criteria, you may want to consult with a health care professional. Calculate your BMI as a first step to help answer those questions.

Identification

BMI describes relative weight for a given height. It can be calculated using a formula based on metric measures of weight in kilograms and height in meters, or it can be approximated by a formula using decimal measures of weight in pounds and height in inches.

Metric Calculation

Determine your weight in kilograms and your height in meters. Some bathroom scales provide your weight in both pounds and kilograms. If yours does not, divide your weight in pounds by 2.2 to get the metric equivalent. Tape measures often provide both inches and centimeters. If yours does not, multiply your height in inches by 2.54 to get your height in centimeters. Divide the number of centimeters by 100 to get meters. Keeping two decimal positions, multiply the number of meters by itself to get meters squared. BMI is equal to your weight in kilograms divided by your height in meters squared.

Decimal Calculation

A close approximation of the metric calculation can be derived from the decimal version of the BMI formula. Your BMI is approximately equal to your weight in pounds, divided by your height in inches squared, multiplied by 705.

Interpretation

Interpretation of BMI measurements for adults is based on weight classification. Healthy weight is reflected by a BMI of 18.5 to 24.9. Underweight is a BMI below 18.5; overweight is 25.0 to 29.9, and obese is 30 and above.

BMI for children is interpreted differently, because their normal growth pattern causes body composition to change continually throughout the growing years. A healthy BMI for a child depends on sex and age, in years and months. Calculate your child's BMI the same way as for adults, then plot it on the BMI clinical growth charts that are found on the website of the Centers for Disease Control and Prevention.

Considerations

Although BMI measures are accurate, easy and inexpensive to calculate, they have limitations. BMI does not reveal how much of your weight is fat and where the fat is located. For instance, fat accumulated around the abdomen presents a higher risk for heart disease than does fat carried lower in the body. A bodybuilder may have an overweight BMI but not be overfat. The elderly and others who have lost muscle mass may have a BMI that can be misleading. You need enough lean tissue to support health and the right amount of fat to meet your body's needs. If you have difficulty with the calculations, use an online BMI calculator such as the one provided by MayoClinic.com.