The body mass index, or BMI, is your weight divided by the square of your height. It’s a common method of evaluating your weight, along with other factors. Health-care workers must also consider age in children when assessing the BMI because their height is still changing. They generally compare the child’s BMI with those of other children in the same age and gender group. Age isn’t a significant factor in BMI assessments for adults once they reach their maximum height.
Step 1
Measure your weight in pounds with a scale. Measure your height in inches with a tape measure. Divide your weight in pounds by the square of your height in inches. This will provide your BMI in English units of pounds per square inch (lb/in^2). Assume that you’re 65 inches tall and weigh 129 lbs. Your BMI is 129 / (65 x 65) = 0.0305 lb/in^2.
Step 2
Multiply your BMI in units of lb/in^2 by 703 to obtain your BMI in metric units of kilograms per square meter (kg/m^2). A BMI of 0.0305 lb/in^2 is equal to 0.0305 x 703 = 21.5 kg/m^2.
Step 3
Cross reference the 50th percentile column with your age on the appropriate BMI chart to obtain the average BMI for your age and gender. For example, the BMI charts for the Centers for Disease Control and Prevention show that the average BMI for a girl 16 years of age is about 20.4 kg/m^2.
Step 4
Determine your weight classification from your BMI percentile score. For example, a 16-year-old girl with a raw BMI of 21.5 has a BMI percentile score between 50 and 75. A BMI percentile score of 5 percent or less indicates you may be underweight, a percentile score between 85 and 95 percentile means you may be at risk for obesity and a percentile score above 95 percent indicates you may be obese.
Things You'll Need
- Scales
- Tape measure
- BMI Chart
- Calculator
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