Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have a negative effect on health. People are generally considered obese when their body mass index (BMI), a measurement obtained by dividing a person's weight by the square of the person's height, is over 30 kg/m2, with the reference range 25–30 kg/m2 defined as overweight. Some East Asian countries use lower values as reference. Reference numbers for BMI are 18.5 -24.9 for Normal weight, 25.0 - 29.9 for Overweight, 30.0 - 39.9 for Obese and 40.0 and above as Extreme obesity.