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BMI, or the body mass index, is a measure of a person's body fat based on the relationship between his/her height and weight. It mostly applies to adult men and women and does not really measure a person's body composition but is only an indicator of it.
The classifications under this index are: extremely underweight (<16), underweight (16-18.5), normal weight (18.5-25), overweight (25-30), obese class I (30-35), obese class II (35-40), and morbid obese (40+).