Body mass index is a standard ratio expressing a person's weight as a function of their height. It is calculated by taking a person's weight in kilograms and dividing it by the square of the person's height in meters.
Adults with a BMI of between 18.5 and 25 are considered to be of "normal weight". Ranges from 16-18.5 and 25-30 are considered "underweight" and "overweight", but do not show any pathology. A BMI of under 16 or over 30 indicates a pathological weight condition and requires medical intervention.
For children, their BMI is compared against the BMI of children of the same age and sex. As long as they fall within the middle 90% of percentiles, their weight is considered "normal". However, children with a BMI in the 5th or lower percentile or the 95th or higher require medical intervention.