How accurate is the Body Mass Index?

Sarah Schenker from the British Nutrition Foundation replies: If you want to check that you are a healthy weight for your height, a simple way is to calculate your Body Mass Index or BMI.
BMI can be calculated as follows:
Weight in kilograms (there are 2.2lbs in 1kg and 14lbs in 1 stone) divided by height in metres2 (there are 2.5cm in 1 inch and 100cm in 1 metre).
Once you have calculated your BMI, you can check to see which weight range you fall into. It's important to remember there is no single perfect weight, rather a weight range, which allows for differences in peoples' build and frame size.
BMI is used by health professionals because it's easy to measure and in most cases it's a good indication of a person's weight, but there are exceptions. For example, heavily muscled athletes, such as weight lifters or boxers, have a high BMI but are not obese. Elderly people have little muscle so may have a normal BMI even if they have a high percentage of body fat. Excess weight does not necessarily mean excess fat.
One good way to get an estimation of body fat is by using skinfold calipers. Measurements are taken at four different sites (biceps, triceps, subscapular - on the back below the shoulder blade, and supra-illiac - on the tummy just up from the hip bone). But the readings are only as good as the calipers (plastic ones are useless) and the person using them. An unskilled user won't get accurate results.
Another popular way of measuring body fat is bio-electrical impedance. This machine is often used in gyms, but is also prone to problems. An electric current is passed between the hand and foot and the impedance reflects the proportion of body fat - the greater the impedance the greater proportion of body fat. But results depend on the person being well hydrated and fluctuate quite considerably throughout the day.
The only truly accurate methods involve X-ray scanning, hardly handy equipment for the gym!