There are a number of ways to estimate what percentage of your weight is fat. But measuring body fat is not done routinely, because all of these methods have advantages and disadvantages as well as margins of error.

The easiest and most popular methods for body fat testing are not as accurate as other, more expensive methods. The most popular methods include:

• Skinfold thickness. An instrument called a caliper is used to measure the thickness of fat at one or more sites on the body. Measuring skin-fold thickness gives a reasonable estimate of body fat composition, especially if measurements are taken from several sites. But the results can vary when different people take the measurements or if the person taking the measurements is not well-trained.
• Bioelectrical impedance. A small electrical current is passed through your body, allowing the electrical resistance of your body to be measured. Bioelectrical impedance measurement of body fat is based on the principle that lean body mass conducts electricity better than fat body mass. Along with the electrical resistance, your height and weight are used to calculate your percent of body fat. There are some devices, including body fat scales, that you can buy to calculate your percentage of body fat using this method. Results can vary based on how much water is in your body and where the electrodes are placed.

Other, more accurate methods for estimating body fat are usually done only at universities and research facilities. They include:

• Underwater weighing. This is also called hydrodensitometry and is based on the principle that fat tissue is less dense than muscle and bone.
• DEXA, or dual-energy X-ray absorptiometry. This is a type of X-ray that can tell how much fat tissue you have as well as where it is.

Experts differ in their opinions about what is a healthy percentage of body fat. In general, 10% to 25% body fat is considered healthy in an adult man, and 18% to 32% body fat is considered healthy in an adult woman. Above this level is considered unhealthy.

Percentage of body fat will vary between different population groups. Your age, sex, and activity level will also affect your percentage of body fat. For example, endurance runners have less body fat than swimmers. People living near the North or South Pole generally have more body fat than people living in more moderate temperature zones.

Remember that percentage of body fat, by itself, doesn't tell whether a person is healthy or unhealthy.