The Body Mass Index (BMI) is a number calculated from a person’s height and weight. It is fairly reliable as a measure of body fatness. There is a variation in the result depending on age and origin. It is a screening tool to identify possible weight problems in adults. BMI is not a diagnostic tool. If the BMI result is under 18.5 or over 25 then it is wise to see a health specialist to determine if there is indeed a weight problem which needs correcting.