How to Calculate BMI

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Body Mass Index, abbreviated BMI, is a simple way to screen for possible weight problems. While BMI does not match actual body fat percentage, it is a fairly reliable way of estimating this figure. Even though a resulting BMI may indicate riskier levels of excess weight, a final determination may need to be made through further individual medical information. You can calculate BMI at home by following these steps.

Things You'll Need

  • Scale
  • Yardstick or tape measure
  • Measure your current weight on an accurate scale.

  • Stand against a wall or door and place a flat piece of cardboard on top of your head. Have someone make a light pencil mark on the wall just atop the cardboard. Then measure its height in either feet and inches or meters and centimeters.

  • Use either of these formulas for U.S. units, depending on how the measurements were made: BMI = weight in lbs. times 703 divided by height in inches squared; or BMI = weight in lbs. times 4.88 divided by height in feet squared. Here is an example for a person weighing 172 lbs. and who is 6 feet 1 inch tall: BMI = (172 X 703) ÷ (73 X 73) = 120916 ÷ 5329 = 22.7 rounded to a single decimal.

  • Calculate BMI using metric units with this formula: BMI = weight in kilograms divided by height in meters squared. Using the same equivalent height and weight, which converts to 1.85 meters and 78.2 kilograms, yields BMI = 78.2 ÷ (1.85 X 1.85) = 78.2 ÷ 3.4 = 22.8 rounded to a single decimal. Note that, due to rounding errors, the metric result may differ slightly from the U.S. measure.

  • Decide where the resulting BMI fits on the scale from underweight to obese:Underweight: 17.9Normal: 18 to 25Overweight: 25.1 to 29.9Moderately obese: 30 to 40Severely obese: More than 40

Tips & Warnings

  • These formulas assume that male and female BMI figures are comparable, although opinions differ on this issue. However, the Centers for Disease Control (CDC) use BMI as a starting point because of its inexpensiveness.
  • There are more detailed methods to determine true body fat percentage, such as skin fold measurement, weighing a person underwater, bioelectrical impedance and other expensive tests.
  • Many automatic BMI calculators can be found on the Internet.
  • Consult medical personnel when the BMI is either over or under normal readings.

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