BMI Weight Status Less than 18.5 Underweight 18.5-24.9 Normal 25-29.9 Overweight More than 30 ObeseBMI, or Body Mass Index (the formula is your weight in kilos divided by the square of your height in metres), is a commonly used measure of obesity. The table above shows what your BMI says about you. If, for example, you are a 157.5 cm (5"2") tall woman, the "normal" range of 18.5-24.9 would imply a weight between 46 and 61 kg.
In an elite athlete like Anju Bobby George (177 cm, 63 kg, BMI of 20) the figure overestimates the actual amount of body fat, because heavy training burns off so much, and adds muscle mass. But in people who have lost muscle mass—like crash dieters—it actually underestimates the fat level. Which means that a coffee and cigarettes diet is even worse than you thought.