BMI, or body mass index, is a number that provides most people with a good, indirect estimate of how much body fat they have. Calculating it is simple and requires knowing only your weight and height. The calculations can be done using metric or English/Imperial measurements. The Centers for Disease Control and Prevention, the National Heart, Lung, and Blood Institute and other organizations also provide online means of calculating BMI values.
Before You Can Calculate
You need to know your height and weight in order to calculate your BMI. Stand upright, but relaxed, without shoes against a wall or door frame and mark it at a point that corresponds to the top of your head. Measure the distance from the floor to the mark in either meters or inches. Record the figure. Weigh yourself, preferably in the morning after getting up, in minimal clothing using an accurate scale. Record your weight in kilograms or pounds.
Calculation in Pound and Inches
Use this formula to determine your BMI using English measurements:
(weight in pounds) ÷ (height in inches x height in inches) x 703.
For example, say you weigh 135 lbs. and you are 5 feet, 7 inches tall. Convert your height to inches: 5 x 12 = 60 inches and 60 inches + 7 inches = 67 inches.
Square your height in inches: Height in inches x height in inches, or 67 x 67 = 4,489.
Divide 4,489 into 135 and multiply by 703: (135 lbs. ÷ 4,489) x 703 = 21.1, your BMI.
Calculation in Kilograms and Meters
If your measurements are metric, use this formula: (weight in kilograms) ÷ (height in meters x height in meters) to determine your BMI. If your height measurement is in centimeters, you will need to convert it into meters. Since there are 100 centimeters in a meter, just divide your height in centimeters by 100 to get your height in meters.
Say you were 1.7 m tall and weighed 61.3 kg. Your BMI would be (61.3 kg) ÷ (1.7 m x 1.7 m) or (61.3 kg) ÷ (2.89 m squared) = 21.2 , your BMI.
Interpreting Your BMI
Unless you have a very athletic and/or muscular physique, your BMI reading will provide an indication of where you fall in terms of body fat composition compared to the general population. Significant muscularity can result in a misleading, elevated BMI reading. For most people, however, a BMI reading of 18.5 or lower indicates underweight; 18.5 to 24.9 means normal weight; 25.0 to 29.9 shows overweight and 30.0 or higher marks obesity. Discuss with your physician the implications of your BMI reading. The doctor can determine if other tests or interventions might be needed if the number is outside the normal range.
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