There are other methods of calculating how healthy someone's weight is. Measuring body fat percentage is a very accurate measure of health, however it requires specialist equipment. Women should have around 8-12% body fat, and men around 3-5%, in order to be healthy. Health can be more easily estimated from your waist-to-height ratio (under 0.5 is healthy under age 40). This estimate is thought to be more accurate than BMI, but still not as accurate as body fat percentage.
Maintaining a healthy weight is important for your long-term health. The best way to be a healthy weight is to eat a balanced diet with lots of fruit and veg and take plenty of exercise. If you are concerned about your weight, if you think you might be overweight or underweight, speak to your doctor about how to lose or gain weight in a healthy way.