Body mass index (BMI) is a measure of body fat based on height and weight that applies to adult men and women.Understanding your BMI resultUnderweightBeing underweight could be a sign youre not eating enough or you may be ill. If youre underweight, a GP can help.Healthy weightKeep up the good work! For tips on maintaining a healthy weight, check out the food and diet, and fitness sections.OverweightThe best way to lose weight if youre overweight is through a combination of diet and exercise.The BMI calculator will give you a personal calorie allowance to help you achieve a healthy weight safely.