BMI is equal to or less than 18.5 (Underweight)
A lean BMI can indicate that your weight maybe too low. You should consult your physician to determine if you should gain weight, as low body mass can decrease your body's immune system, which could lead to ilness such as disappearance of periods (women), bone loss, malnutrition and other conditions. The lower your BMI the greater these risks become.
Read our Underweight Treatment article for some measures you can take.
Note: Since Body Fat Percentage calculations use total body weight and not estimates of lean muscle mass and fat, BMI can not determine between the overweight and the more muscular. Use our Body Fat Calculator and Waist to Hip Ratio Calculator in combination with our BMI Calculator for a more accurate view of your body fat.