To determine the healthy weight for a woman over the age of 18, divide her weight in kilograms by the square of her height in meters to find her body mass index, states the U.S. Centers for Disease Control and Prevention.The ideal adult BMI is between 18.5 and 24.9.
A BMI below 18.5 most likely indicates that a person is underweight, while a person with a BMI of over 25 is probably overweight, states the CDC. Typically, a person with a BMI of greater than 30 is considered obese. However, BMI alone is only a screening tool, not a diagnostic indicator of how fat a person is. Physicians use a number of diagnostic tools to determine a woman's degree of body fat and overall health.
To calculate the BMI of adolescents and children, use the same formula as that for adults, but bear in mind that the norms vary by gender and age, explains the CDC. This is because boys and girls have different amounts of body fat as children, and normal levels of body fat fluctuate according to a child's age. Standardized growth charts for children are available on CDC.gov.
Some other measures of fatness or thinness include skin-fold measurements, underwater weighing and dual-energy x-ray absorption, or DXA. However, these methods are usually expensive and often unavailable, so scientists have adopted BMI as a standardized screening tool for obesity.