liters of the 2.5 to 3 liters of water lost daily is excreted through urine. But there's also the stool, expelled air (half a liter), and breathing: either the water vapor that escapes the skin as a result of temperature fluctuations, or sweating. We lose at least half a liter of water every day via perspiration, which fluctuates depending on the temperature and the amount of physical activity we engage in.