It is of limited use to compare one person's weight with another – and, for the same reason, impossible to cite a 'healthy weight' – because people of different ages and different heights are expected to have different weights. The body mass index (BMI), therefore, is used by most doctors and physicians to ascertain a 'healthy weight' for each individual. The index is based on both the height and weight of the individual, and is used to calculate the body fat. The index gives useful results for men or women between the ages of 18 and 65.

There are numerous websites where one can enter one's weight and height and have a BMI calculated instantly, but the formula used is as follows:

Weight / Height²

Both weight and height can be given in imperial or metric measurements – provided that the same form is used for both numbers.

The BMI calculation will usually be a number between 20 and 25; this indicates that the individual appears to have a 'healthy weight' for their height. A BMI calculation of more than 25 suggests that the individual is overweight, and above 30 suggests obesity. Those with a BMI calculation of less than 20 may be underweight – a condition as unhealthy as being overweight.