Leash - I don't know about all of them, but I have a set of Tanita ones that do the body fat % (gawd it took me ages to save up the $$ for them!) but they measure it by sending a very tiny miniscule current through your body. The theory behind it is that fat and bones etc don't pick up on the current whereas your muscles and organs etc do....so it works it out from that....I hope that makes sense!?