According to the CDC, a normal BMI range is 18.5 to 24.9 in adults. Below 18.5 is considered underweight; 25-29.9 is overweight, and 30 and above is considered obese. Normal BMIs in children vary by gender and height. A correlation between being fat and having a high BMI is strong.
The CDC notes that when they have the same BMI, women have more body fat then men, and older people have more body fat than younger people. Athletes have higher BMIs without being fat. A better measure for unhealthy fat and weight is a waist measurement. An overweight waist is a predictor of a slew of obesity-related illnesses.