That body fat machine used in the gyms...?

This technique is called bioelectrical impedance. The other two methods for measuring body compositon are hydrostatic & skin fold measurements. The bioelectrical is typically the least accurate because so many factors need to be accounted for, but if you are doing it consistently the same way, you will know your personal change.

These work by sending an electrical signal through your body. The machine measures the amount of time the signal takes to circulate; higher body fat = slower signal. The catch with these is that your hydration level can significantly (2-3% or more) affect the result. Think of the hairdryer in the bathtub: signal travels faster in water right? Same with your body. If you're dehydrated, you will register as higher body fat. I

have a scale that does the same thing but mine also measures your hydration so I can tell if my BF% increases & I'm dehydrated, or vice versa. While most bioelectrical are off a bit from your true BF%, you can use it as a baseline. If you are150lbs & 20% then go back and are 152lbs & 19%, you are ultimately changing the composition of you body! This can be really helpful if the scale doesn't seem to want to cooperate :flowerforyou:

Hope this helps!
Arianne