Women’s BMI through the decades
The healthy range of 18.5–24.9 stays the same at 25, 45 and 65, but what lies behind the number changes. In the 20s and 30s, muscle is usually at its peak; from the 40s, muscle declines and fat rises slowly; after menopause, fat moves to the waist; and after about 65 a BMI slightly above 25 is often considered acceptable, because unintentional weight loss and frailty become the bigger risks.
Girls under 20
For girls and young women up to the age of 19, adult categories do not apply. BMI is compared with girls of the same age and expressed as a percentile: the 5th to under 85th percentile is a healthy weight. Enter an age under 20 in the calculator to see the percentile, or use the Child BMI Calculator, which also uses months of age.
Where BMI falls short
BMI was designed to describe populations, not to diagnose individuals. It cannot tell muscle from fat, does not show whether fat sits around the organs or under the skin, and was built mainly on data from adults of European descent. It can underestimate risk for people of Asian background and for older adults who have lost muscle, and overestimate it for athletes. Doctors therefore combine BMI with waist size, blood pressure, blood sugar and cholesterol before drawing conclusions.