One liter of water weighs approximately 1 kilogram at room temperature and atmospheric pressure. Water has a high density of 1,000 kg/m³, so one liter, which is equal to 0.001 m³, would weigh 0.001 kg or 1 gram. One kilogram is equal to 1,000 grams, so one liter of water weighs 1,000 grams or 1 kilogram.