It depends on a lot of factors. It measures the resistance to electric current that is passed through the water content in your body. If you are dehydrated, hyperhydrated, if you just drank a lot of water recently, or if you just ate a meal, you can get an inaccurate reading. People with very low bodyfat% or a considerable amount of muscle mass will also get inaccurate readings. For the average guy, it should give you a ballpark range, but don't swear by it.