The density of liquid water is 1,000 kg/m3 (62.43 lb/cu ft) at 4 °C. Ice has a density of 917 kg/m3 (57.25 lb/cu ft).

The maximum density of water occurs at 3.98 °C (39.16 °F).[13] Most known pure substances become more dense as they cool, however water has the anomalous property of becoming less dense when it is cooled to its solid form, ice. During cooling water becomes more dense until reaching 3.98 °C. Below this temperature, the open structure of ice is gradually formed in the low temperature water; the random orientations of the water molecules in the liquid are maintained by the thermal motion, and below 3.98 °C there is not enough thermal energy to maintain this randomness. As water is cooled there are two competing effects: 1) decreasing volume, and 2) increase overall volume of the liquid as the molecules begin to orient into the organized structure of ice. Between 3.98 °C and 0 °C, the second effect will cancel the first effect so the net effect is an increase of volume with decreasing temperature.[14] Water expands to occupy a 9% greater volume as ice, which accounts for the fact that ice floats on liquid water, as in icebergs.