The body mass index allows to access the body of a person. It is correctly interpretable for adults aged from 18 to 65. The values between 18 and 25 represent a normal BMI. However, BMI is to qualify with the fat mass index which expresses in percentage the proportions between fat mass and muscle mass. For women, the standard is between 25% and 30% while it is between 15% and 20% for men. Above, you have too much fat; under is a deficit.