A method of estimating body composition, abbreviated BIA, that sends a harmless low-level electrical current through the body and measures how strongly tissues resist it. Lean tissue, rich in water and electrolytes, conducts well, while fat resists, so the impedance reading, combined with height, weight, age, and sex, yields estimates of body fat percentage and muscle mass. It is quick, non-invasive, and affordable, making it popular in gyms, clinics, and research, though accuracy depends on hydration status, recent exercise, and the equations built into the device. It cannot match DEXA scanning for precision, but for tracking changes over time under consistent conditions, it is a practical tool. Like all body composition methods, it informs better than the scale alone.