in 1795 the kilogram was defined to be exactly one litre of water at 4 °c. in 1799 the kilogram was redefined. the new definition said that the kilogram was the mass of the "kilogram des archives". in 1901 scientists measured the volume of one litre of water at 4 °c very carefully. they found that it occupied about  dm3. the bipm redefined the litre as being "exactly" the volume of one kilogram of water at 4 °c.