The most accurate measures of body fat are performed directly, using scales, underwater weighing, X-ray bone density scans, air displacement machines or calipers that pinch the skin. Estimating your body fat using a calculator found online is less precise, but can give you an idea if you carry too much fat and are therefore at increased risk of chronic disease. Even though a calculator doesn't directly measure your fat, it could help you determine if you need to visit your doctor for a thorough medical workup.

Why a Woman Should Care About Body Fat Percentage

The weight of your body doesn't always provide a good indication of your health because it fails to tell you how much of your body is made up of fat or lean tissue, which includes your muscle, connective tissue, bones and organs.

The average healthy body fat level for a woman is between 20 and 25 percent. Some athletes may have lower levels due to their exercise regimen and the performance demands of their sport. A woman with a body fat percentage below 14 percent is also at increased risk of health complications, such as loss of menses and poor bone health.

Body Mass Index as a Body Fat Calculator

One of the most common calculations of your level of body fatness is the body mass index, or BMI. Many online calculators exist on health websites, or you can figure it on your own by taking your weight in kilograms and dividing it by the square of your height in meters. Your age and gender don't factor into the equation.

A conversion factor can be applied to the equation for American measurements. This changes the formula to:

BMI = weight in pounds / (height in inches x height in inches) x 703.

A BMI of 18.5 to 24.9 is considered normal, which means you have a healthy level of body fat. A BMI below 18.5 indicates you may be underweight, while a value above 25 indicates overweight, and 30 or greater is classified as obese. BMI doesn't directly measure your body fat, but correlates moderately with more direct measures, including body-fat scales and bone density scans. BMI seems most reliable in those that register with a high BMI -- such as at 25 and above.

BMI has limitations, however, and it doesn't provide an exact measure of your body fat percentage. BMI can overestimate body fat in people who are extremely muscular, for example. Muscle is more dense than fat, so an athletic person may seem to weigh a lot for her size, but not have an abundance of fat -- just a lot of muscle, which doesn't pose a health risk.

BMI can also be inaccurate for women who are sedentary or older. Their weight may put them at a normal BMI for their height, while they still have too much fat tissue. A doctor may order further analysis, such as blood tests, to determine whether your BMI is providing an accurate picture of your health.

Body Fat Calculators Using Girth

Some online calculators provide an estimation of your actual body fat percentage using the circumference, or girth, of specific parts of your body. The calculator then plugs these numbers into a complex formula to estimate fat percentage.

Although these formulas are used by the Army, Navy and Marines, they can be inaccurate, especially for women who are overly thin or fat and for people who are extremely athletic. The tape measure measurements can't distinguish between muscle and fat. It's also relatively easy to pull the tape measure too tight or let it sag during measuring, which may further skew results.

Note that these circumference measurements differ from skinfold caliper measurements that actually pinch the skin and are plugged into an entirely different formula. Consult a trained fitness or healthcare professional for an accurate skinfold caliper method of body fat analysis.

More Accurate Measures of Body Fat

While an online calculator may be one of the easiest ways to estimate your body fat percentage, seeking out a more direct method will provide you with better information. The most reputable methods of body fat measurement include hydrostatic -- or underwater -- weighing, bone density scans, MRIs and air displacment technology -- such as that found in the Bod Pod, but they may not be readily available or affordable. Other direct body fat measuring methods are easier to come by at fitness or health centers.

If an online calculator suggests your body fat is in an unhealthy range, seek out a more precise measurement to get a better understanding of your body composition 5. Specific scales measure body fat via electrical impedance, which involves shooting a slight electrical current through your feet and legs, and sometimes your hands, to determine your body composition 15. Professional caliber scales are most accurate, but home versions are available. Skinfold caliper tests are also available for minimal cost by many fitness trainers and healthcare providers.