What Should My BMI Be for My Age?

The BMI or body mass index is more accurate for determining obesity than just weight alone, but it is only a screening tool and not a diagnostic one; it does not account for people with muscular frames or other variations.

BMI stands for Body Mass Index, and it is a tool doctors use to determine if a person is overweight, and to what degree. It is a calculation based on height and weight that can provide a better estimate of total body fat than weight alone. 

In adults, BMI does not correlate to age, however, certain calculators may give you the average weight for a set of age ranges.

BMI is calculated from the weight and square of the height:

• BMI = body weight (in kg)  ÷  height (in meters) squared

• BMI less than 18.5: Below normal weight
• BMI 18.5 to under 24.9: Normal weight
• BMI 25 to under 29.9: Overweight
• BMI 30 to under 34.9: Class I Obesity
• BMI 35 to under 39.9: Class II Obesity
• BMI 40 or greater: Class III Obesity

BMI is used slightly differently in children, in that it is calculated the same way as it is for adults, but then the ranges are compared to percentiles for children of the same age and sex. 

In patients who are very muscular, such as bodybuilders or professional athletes, the BMI may overestimate their body fat. 

Conversely, older patients who have loss of muscle mass due to aging may have their body fat percentage underestimated.