How much exactly does one liter of clean water weigh? It can be concluded that one liter of water weighs 1 kilogram. However, this is not entirely true. Originally, in 1765, a unit of 1 kilogram was actually defined by the mass of one cubic decimeter (1 liter) of water at the temperature of melting ice or zero degrees Celsius.
Subsequently, 1 kilogram was redefined based on the IPK (International Prototype of the Kilogram) standard. It is a cylinder made of an alloy of platinum and iridium (in a 90:10 ratio). It was supposed to be exactly 1 liter of water. However, it turned out that the weight of the cylinder corresponds to 1.000028 dm3 of water. So, following the accuracy track, a liter of water weighs less than a kilogram compared to the international standard of a kilogram. However, the difference in value is so small that it is ignored on a daily basis and it is assumed that 1 liter of water weighs 1 kilogram.