Rafaella -
There are variety of ways to measure body fat. A calculator like that one, which only takes into account your weight and height, is going to be the least reliable option. Other online calculators take into account measurements from many different areas on your body (thigh, calf, waist, hips, neck, wrist, etc), and they are somewhat more accurate. They will still, though, only be an estimate of your body fat.
There are several ways body fat can be directly measured. Again, there are different levels of accuracy with each method. There are scales that send a small electric signal through your feet, measure the amount of time it takes for the signal to return, and uses that to calculate your body fat percentage. These will give you varying measurements depending on your hydration level, time of day, etc, but will probably be more accurate than an online calculator and can be done at home, or as part of your weigh-in process.
The most accurate methods for measuring must be done by a trained professional. A pro can either measure the width of various skinfolds in specific locations on your body with calipers and enter those numbers into a calculation to come up with a percentage. If the readings are taken accurately by someone well-trained, this can be very accurate. The most accurate (and most expensive) is hydrostatic weighing, in which you are weighed underwater, again, by a professional. Because fat and bone/muscle have different densities, your % body fat can be calculated by your underwater weight results.
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