Body fat is a more accurate indication of your health than your weight as it indicates what percentage of your body weight is composed of fat, while if you only use weight and height, it may be misleading because many factors are not taken into account. For example athletes tend to weigh above their "ideal" body weight for their size because muscle weighs more than fat and may be considered overweight according to height weight charts, but they are actually more fit than the majority of the population.
Healthy Body Fat Percentage
There is no universially accepted number for a healthy body fat percentage. A healthy body fat percentage for the average man should be anywhere from 6% to 17%, while a healthy body fat percentage for women should be around 14% - 24%.
At a minimum men should not have less than 5% body fat and women should not have less than 12%, anything less than this is considered dangerous and unhealthy, your body needs fat to function properly. Too much body fat also poses health risks, men with over 25% body fat and women with over 31% body fat are considered obese. Obesity is correlated with an increased chance of getting cardiovascular diseases, diabeties, and cancer.
As most people get older, their body fat percentages usually go up due to less activity and a slower metabolism.
Measuring Body Fat
There are many different methods to measure body fat, the simplest one is through bioelectrical impedance which can be used simply by stepping on a body fat scale. An electric current is sent through the body and based on the speed it passes through the body, body fat percentage is calculated. Electric currents travel fast through lean muscles which contains a higher proportion of water and slower through fat tissues which contain less water. The results of bioelectrical impedance are more prone to inaccuracies due to it's dependcy on the ammount of water in the body at the time of measurement, but is a good tool for measuring progress if you are trying to lower your body fat percentage.
Another method is to use skin-fold measurements to find your body fat percentage. Skin-fold callipers are used to take measurements by pinching the skin in several areas of the body where fat deposits are abundant. The measurments are then put into a formula and a percentage is calculated. If you purchase a calliper yourself you can take measurements but it is recommended that you go to a trained physician to get the most accurate measurements.
Hydrostatic weighing is the most accurate way to measure your body fat percentage but it is also one of the most expensive ways. The method requires the subject to be completely submergered under water and measuring their weight underwater. This method is based on the fact that fat floats in water so a person with more fat will weigh less under water.