Density of water at 20 °C

Twenty degrees is the conventional laboratory reference temperature, and 998.207 kg/m³ is the value glassware calibration, pipette gravimetry and specific gravity all rest on. Specific gravity is a ratio of densities, so it depends on which reference temperature the number was taken at — the common conventions are 20/20 °C, 20/4 °C and, in petroleum work, 60/60 °F.

Water expands with temperature above 4 °C, and by more than instinct suggests in heating systems: from 999.97 kg/m³ at 4 °C to 971.8 at 80 °C is a 2.9 % volume increase, which is precisely what an expansion tank has to absorb. A hydronic loop holding 2000 L at fill temperature will push roughly 50 L into the tank when it comes up to design temperature.