Medical experts today often argue that body fat percentage is a better guide to a person's health than other measures such as their body mass index. Body fat is made up of two components: essential fat and storage fat. Because different people have different needs for essential fat, the healthy body fat percentage varies across age and gender.
Body fat percentage is exactly what the name suggests: the percentage of your body weight which is made up of fat. It's usually considered to be a better guide to health than the body mass index, which simply looks at your weight and height. This can be misleading as, for example, an athletic and healthy person may have a high body mass index because they have a muscular physique. Alternatively, somebody with a low body mass index may have a dangerous proportion of their weight made up by fat, which can increase the danger of obesity-related health issues.
There are several ways of measuring to see if you have a healthy body fat percentage. The most common is bioelectrical impedance. This involves sending a small electrical signal through your body. Because fat has a lower water content than other tissue, it is a less effective conductor of electricity and thus slows down the movement of the signal.
Less common methods include using calipers to see how much fat can be pulled away from particular areas of the body. Some researchers use a technique involving infrared light, though this is not particularly reliable. The most effective way to check if somebody has a healthy body fat percentage is to weigh them underwater with their lungs emptied. This works because fat floats in water while bones and muscle sink. Naturally this technique should only ever be carried out by trained professionals.
Calculations to find a healthy body fat percentage have to take into account the age and gender of the person. That's because there's a certain level of fat, known as essential fat, which people need to survive. This level varies: women need more than men as their bodies are designed for pregnancy, while older people normally need more to keep warmer. Fat beyond this level is known as storage fat, some of which is needed as fuel if the body does not have enough food. Too little or too much storage fat can cause health risks.
Exactly what body fat percentage is healthy is a disputed point. There are several online charts which vary in the figures they give, though the general trends are largely the same. To give one example, the World Health Organization lists healthy female ranges as rising from 17-31% for an 18-year-old to 24-36% for somebody aged over 60, while the male ranges rise from 10-20% for an 18-year-old to 13-25% for somebody aged over 60. Because of the differing figures from different sources, it is best to consult your own doctor or other medical professional to determine what is a healthy body fat percentage for you.