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How can I calculate my BF%?

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one word of advice from personal experience...do not go by one of thoese "free body fat estimate" at your local gym. –  kjy112 Mar 2 '11 at 14:08
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There are several ways to measure your body fat percentage.

Sportsmedicine.About.com has a nice description of the most important ones, in order of their reliability

Underwater Weighing - Hydrostatic Weighing

One method of body composition analysis in which a person is weighed while submerged in a large tank of water is called underwater or hydrostatic weighing This method of determining body composition relies on Archimedes' Principle of displacement which states:

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The density of fat mass and fat-free mass are constant Lean tissue is more dense than water Fat tissue is less dense than water. Therefore person with more body fat will weigh less underwater and be more buoyant.

Skinfold Thickness Measurements

Because underwater weighting it is complicated and cumbersome and requires special equipment, most exercise physiologists use simple skinfold measurements to determine body fat percent. The American College of Sports Medicine says that when performed by a trained, skilled, tester, they are up to 98% accurate.

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Bioelectrical Impedance

Bioelectrical Impedance is another method of assessing body fat percentage. There are a variety of body composition and body fat analyzers and scales available for home use that provide more than just total weight measurements. These devices determine total weight, the percent and amount of body fat, muscle mass, water, and even bone mass. While the readings can be affected by hydration levels, food intake, skin temperature, and other factors, if you follow the directions and take the reading under similar conditions, you will obtain the best results.

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Other methods such as BMI are such a rough estimate, that you might as well follow @Sparafusile's advise and look in the mirror –  Ivo Flipse Mar 2 '11 at 12:59
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Scales utilizing bioelectrical impedance measurements are worthless. Worst of all, they usually need your age and height so that they can statistically compensate and give you adjusted BF values that are expected to be close to average values according to your age/gender/height. Try changing your age on such a scale, and you can divert results by 5-10%. –  ldx Jun 22 '11 at 21:00
    
I pointed out the same thing in a different answer, but it might be good to have it emphasized here. Thanks @Idx –  Ivo Flipse Jun 22 '11 at 21:08
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We have a nice reference page on how to calculate your body fat percentage and the pluses and minuses of each method. Here is a summary.

BIA - The simplest method is hopping on a BIA, bioimpedance analysis, monitor. The limitations: you need a monitor that measures both the upper and lower body, not just one or the other. The readings vary by hydration and water retention levels. To get the most accurate readings take them at the same time of day before eating and after voiding. Because they send a weak current thru you, people with cardiac pacemakers or who are pregnant should not use them.

Skinfold - Another easy method is using the Accumeasure, an inexpensive skin caliper that you use on yourself. With practice you can get reproducible readings at the supra-iliac site and find your body fat percentage by looking up the corresponding mm on Accumeasures chart that takes age and gender into account.

Alternatively, you can have a trained tester take skin fold measurements at several body sites to calculate your body fat percentage.

The Formula Method involves taking circumference measurements at various body sites depending on your gender and applying a formula. Different formulas involve different measurement requirements.

The above methods are the easiest and most affordable ways to monitor your body fat percentage on an ongoing basis. The following are highly accurate, but more expensive and require finding a facility that offers them making them less suitable for ongoing monitoring your progress of your body composition.

The Dexa Dual Energy X-Ray Absorptiometry Test is probably the most accurate, but it does use X-rays even if radiation exposure is low. It is costly and not practical for repeat measurement to determine your progress.

Underwater Weighing, although highly accurate, depends on ones ability to expel air from the lungs while submerged under water. Results also depend on hydration levels. This method can be expensive and cumbersome to use for ongoing body fat monitoring.

The newer Bod Pod method is an air chamber and is similar to the underwater weighing method, however, it uses air displacement rather than water displacement making it more convenient. The same factors apply as far as hydration levels affecting the results. As the popularity of this method increases, it is easier to find a facility in your area.

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Here's how the navy does it:

Basically, use your height and measurements of a couple body parts in a formula to arrive at a estimate.

  1. Measure your height (without shoes).
  2. Measure the circumference around your abdomen, at a horizontal level around the navel for men, and at the level with the least width for women. Don't pull your stomach in.
  3. Measure the circumference of the neck, below the larynx with the tape sloping slightly downward to the front.
  4. For women only: Measure the circumference of the hips, at the largest horizontal measure.

Use the following formulas to calculate your estimated body fat percentage:

For Men (Measurements in Inches):
%Fat = (86.010*LOG(abdomen - neck) - 70.041*LOG(height) ) + 36.76

For Women (Measurements in Inches):
%Fat = 163.205*LOG(abdomen + hip - neck) - 97.684*LOG(height) - 78.38

You can buy some calipers and measure it yourself. There a sophisticated machines and even bathroom scales that claim to be able to measure body fat. The most accurate would probably be the hydrostatic submersion test.

My favorite way is looking in a mirror.

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You should summarize what the link states, makes for a better answer ;) –  KronoS Mar 2 '11 at 13:05
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