system
Given the following post, respond as if you're commenting on Reddit:
human
How do my scales calculate my body fat percentage? 

I'm 25, weigh 66 kgs and I'm 170cms tall. It said my body fat percentage is 26.7. How does it know I'm not really muscly with no fat, or really fat with no muscle?
gpt
A small and completely harmless electrical current is passed through your body. The electrical current passes more quickly through fat free tissue like muscle than it does through fat or bone tissue. So...

The amount of resistance to the electrical current relates to how much fat-free mass a person has and their body density. Here's the first challenge...

Like all body fat tests, body fat scales don't actually measure your body fat percentage. They determine your body density. The examiner (or the scales) then uses a formula to calculate body fat percentage based on body density.


Bioelectrical Impedance

Bioelectrical impedance methods are basically special body fat scales or hand-held devices that send a safe electrical signal measuring its return through the body. The signal will travel faster with water and since muscle is 70% water and fat contains little water, the body fat scale is able to make a distinction, plug the information into an algorithm and give you a reasonable body fat percentage estimate.

Bioelectrical impedance is simple and quick. The scales are not outrageously expensive, usually $60 to $200, which makes it a practical method. Tanita makes some good ones.

The consistency of these measurements is a bit questionable however. Changes in water retention, food intake, skin temperatures and minor posture changes among other things can affect the readings given by these devices. These variables can leave you with some perplexing numbers.

While the bioelectrical impedance methods won't be as reliable as skinfold measurements methods, they are better than not getting a number at all. So if you're not a very patient person (and learning to get consistent body fat percentages with skinfold measurements will require a little practice and patience), body fat scales can provide serviceable body fat calculations. They will give you something to go on.

For best results, try to keep all factors as close to identical each time you jump on your body fat scales (temp, posture, etc.).