Obesity refers to having a high amount of body fat in relation to lean body mass. Most studies and federal guidelines define an overweight adult as someone with a body mass index of 25 or higher - 30 or higher is considered obese. Body mass index is calculated from weight and height. Many experts say it can indicate the potential for health problems.
Most federal guidelines consider children overweight if their BMI is at the 85th percentile or higher for their age, sex and height. A child is considered obese at higher than the 95th percentile for BMI for children.