BMI, or body mass index, is the fastest, easiest, cheapest and yet reasonably accurate means of estimating your body fatness. BMI is usually calculated by cross-referencing your height and weight on a chart that automatically yields your BMI. But if you don't have access to BMI charts, you can calculate your own BMI by inputting your height and weight into a mathematical formula.
Because BMI doesn't measure your body fat directly, but instead extrapolates from your height and weight, there is some room for error. If you're very muscular, for example, your BMI reading may designate you as overweight, even though the weight comes from muscle instead of body fat.
Step 1
Ask a friend to measure your height, or ask the nurse to measure your height during your next doctor visit. Convert the measurement to inches: Multiply your height in feet by 12, then add any leftover inches. So if you're 5 feet, 3 inches tall, your height in inches is 5 x 12 = 60 + 3 inches, or 63 inches.
Step 2
Weigh yourself on the most accurate scale at your disposal. If possible, visit your doctor's office and use their scale. Note this measurement down as well.
Step 3
Square your height measurement. So if you're 63 inches tall, your height squared is 63 * 63 = 3,969 inches.
Step 4
Divide your weight, in pounds, by the result from Step 3. So if the example subject weighs 125 lbs, it is 125 / 3,969 = 0.03149.
Step 5
Multiply the result from Step 4 by 703. The result is your BMI. To continue the example, take 0.03149 * 703 = 22.13747. Because the subject's BMI falls between 18.5 and 24.9, the subject is considered to have a normal weight status.
Tips and Warnings
- For adults, a BMI below 18.5 is considered to be underweight. If your BMI falls between 18.5 and 24.9, you're of normal weight. If your BMI is between 25 and 29.9, you're overweight, and a BMI of 30 or higher means you are obese, as published in "About BMI for Adults" by the Centers for Disease Control and Prevention. You calculate a child's BMI with the same formula you use for an adult's BMI, but the child's BMI result is then interpreted with charts that relate that BMI to others of the same age and gender.
Things You'll Need
- Measuring tape
- Accurate scale
- Calculator
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