The body mass index (BMI) was developed in the mid nineteenth century (~1850s) by the Belgian Adolphe Quetelet in an effort to identify a simple screening parameter for reporting one's weight as a function of his/her height. In the twentieth century, as many societies experienced historically unprecedented affluence, and as excessive weight gain and obesity came to be associated with many diseases, the use of BMI gained in popularity as a health risk factor for the general population. In particular, the BMI is used as an indicator of body fat content and, based on established threshold values (see below), it provides a quantifiable index of one's health status and risk factors with respect to body weight. Although, the BMI provides a simple and useful screening tool, we will see below that it also suffers from several limitations. Therefore, any attempt to strictly interpret one's BMI should be coordinated with a primary care provider.

Body mass index (BMI) is obtained by measuring a subject's weight in kilograms (kg) or pounds (lb) and the subject's height in meters (m) or inches (in). The BMI is then calculated by dividing the subject's weight by the square of his/her height. Therefore, weight is normalized for height. By convention, the calculated BMI has the SI units of kg/m². If the Imperial (English / U.S.) units of weight (pounds, lb) and height (inches, in) are used, the calculated value must be multiplied by a conversion factor of 703.0704 in order to convert the units to kg/m².

Table 1 shows the BMI values that are considered to be normal. "Normal" values are defined as those values that are associated with the highest life expectancy and range from 18.5 to 24.9 kg/m². Excess accumulation of body fat is generally associated with an increased risk of disease and, concurrently, a reduction in life expectancy. A body weight that is 20% greater than the ideal weight increases the risk for such conditions as diabetes (diabetes mellitus type II), hypertension, and cardiovascular disease. Table 1 also summarizes the weight classifications as well as the relative risk of disease associated with different BMI values.

Each calculator cell shown below corresponds to a term in the formula presented above. Enter appropriate values in all cells except the one you wish to calculate. Therefore, at least two cells must have values, and no more than one cell may be blank. The value of the blank cell will be calculated based on the other values entered. Reverse calculations are also possible. For example, in addition to using weight and height data to calculate the BMI, the height and a target BMI may be used to calculate the optimal weight for that height. Or, less practical but equally informative, the weight and a target BMI may be used to calculate the optimal height for that weight.

First and foremost, any attempt to strictly interpret one's BMI should be coordinated with a primary care provider. There are many cases in which BMI values may be misleading and may not accurately reflect one's body fat content. Some are listed below.