Density of Ice at 0 °C

Freezing water expands by about 9 % of its liquid volume, because the hydrogen bonds in hexagonal ice hold the molecules in an open cage that is less efficiently packed than the jostling liquid. The density ratio 917/1000 also fixes the iceberg: about 92 % of floating ice sits below the waterline, since a floating body displaces its own mass.

Engineering-wise, this is the number behind every burst pipe and spalled slab. Water confined while freezing generates hydrostatic pressures in the tens of megapascals, far beyond copper tube or concrete tensile capacity, which is why freeze protection is a containment problem rather than an insulation problem: insulation only delays it. Ice density falls slightly as it cools further (about 919 kg/m³ at −20 °C) and glacier ice with trapped air can be well under 900 kg/m³, so treat 917 as clean, bubble-free ice at the melting point.