The healthy body mass index range for adults is from 18.5 to 24.9, according to WebMD. A BMI of less than 18.5 is considered underweight for an adult, while a BMI of between 25 and 29.9 is overweight. A BMI of more than 30 is considered obese.
These BMI ranges apply to adults aged 20 or over, states WebMD. Having a BMI above the healthy range could increase an adult's risk of blood vessel disease. Having a normal BMI doesn't necessarily mean that an adult has a healthy amount of body fat, as inactive people sometimes have too much body fat, even if they have a healthy BMI.