The body mass index, or BMI, is a ratio between your weight and your height. Its most common use is to determine if your weight is in the recommended range for your height. The World Health Organization, or WHO, publishes standard BMI values that allow you to classify your weight.
Measurements
The calculation of your BMI requires you to measure your height and your weight. In the United States, you typically measure your weight with a set of scales that uses pounds. A tape measure or other measuring device typically provides your height in inches.
Conversion of Units
Standard references such as the ones published by the WHO provide BMI values in metric units. This will require you to convert your height and weight into metric units. Convert your weight from pounds to kilograms by dividing by 2.2. Convert your height from inches to meters by multiplying by 0.0254.
Calculation
Your BMI is your weight divided by the square of your height. Obtain your BMI in metric units by dividing your weight in kilograms by your height in meters. This will provide your BMI in units of kg/m^2.
Evaluation
Compare your BMI to standard BMI values to classify your weight. According to the WHO, a BMI below 18.5 kg/m^2 indicates you are underweight. The recommended BMI range is between 18.5 and 25 kg/m^2. Pre-obesity BMI values range between 25 and 30 kg/m^2, and a BMI value greater than 30 kg/m^2 indicates obesity.
Limitations
The WHO makes a number of simplifying assumptions in its BMI classifications. It assumes the individual is relatively sedentary and has an average body fat percentage. An individual with a lower body fat percentage will tend to have a higher BMI. This means that the WHO BMI values will tend to classify athletes as overweight. Health care workers should therefore use the BMI only as a preliminary screening tool and not as a diagnosis.
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