Our body is a combination of lean (muscle), fat, bone and water, each part performing a specific task in our body. Each part of the combination has to be in a certain proportion to that of the others for a normal or healthy functioning of our body.
Fat, as part of the combination, have to be maintained in a certain proportion with the other parts. In order to find the proportion of fat in our body, we need to calculate our body fat percentage which simply is the percentage of storage fat or adipose tissue and essential fat.
Essential body fat is the part of fat in our body that performs tasks like reproductive and other necessary functions in our body. And the functions of storage fat are storing energy, protection, body heat generation and the like.
At different places and for different persons, a certain percentage may denote different fitness levels. For women, a body fat percentage of 14-20% is considered athletic. A BFP of between 21 and 24% is considered under fitness level and 25-31% is the average BFP. Anything over 32% is obese and less than 10 to13% is completely not healthy or the level below which the body functions normally.
Due to some physiologic differences, the amount of body fat in men is a little lower than in women. A BFP measure of 14-18% is considered fit for men and between 6 to13 % is athletic. The average for men is between18 to 24% and 2-5% and below is considered below the normal level. A measure of 25% or more is obese for men.
Because there are different standards for different purposes, it can be a little difficult to determine the normal or healthy percentage of body fat. The normal BFP may vary from person to person depending on age, fitness level, etc. For athletes, for example, a BFP of 5% for men is considered the normal percentage. But most commonly, BFP levels are from 21 - 31% for men and 14-24% for women is considered healthy or normal.
The methods used to measure BFP are many including the skinfold method, Bioelectric Impedance Analysis (BIA) which you can also use at home using a BIA scale, hydrostatic weighing that is tested by immersing the examinee into a tank of water to determine body composition and others.
Out of these measuring techniques, the methods like DEXA scanning and Hydrostatic weighing are considered to be very accurate but they are expensive. On the other hand some of the cheaper options like the anthropometric method and BIA measurement using BIA scales are less accurate or dependable.
Body Fat Percentage has more information on the essentials of body fat.