Normally, a BMI of 18.5 or less is considered underweight, while a BMI of 25 or more is considered overweight. Less than 16.5 is described as "starvation", and over 35 is clinically obese. For children, these values are different, and depend on age. Different ethnic body types are also sometimes assessed differently.
Occasionally "BMI prime" is used (also given here), which is BMI/25, thus is greater than 1 if the person is overweight.