How to Calculate BMI
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
Instructions

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.