Assessment of a healthy weight must take height into account. Medically, one of the best and simplest methods of doing this is to calculate the body mass index (BMI). The BMI is weight in kilograms divided by the height in metres squared. The healthy BMI range is 20-25. If we defi ne health as lowest mortality, then we fi nd that the healthy range increases with age. The lowest mortality is at BMI 19.5kg/m2 for women at age 20 and 27.3kg/m2 at age 60.