A pycnometer is a glass flask of a known weight. To determine its exact volume it is filled with a calibration liquid (e.g. water) of a known density. By weighing the contained water, one can calculate the exact volume as the water’s density is a factor we know. In a third step, a simple calculation provides information about the exact volume (volume = weight / density). When determining the unknown density of a sample afterwards, the same formula is just used in a different way (density = weight / volume).