In marine aquariums, it is not the salinity of water which is measured, but the
density (often expressed as specific gravity, S.G.), which can be calculated according
to the following formula:
There are no units of measurements. The saltier the water, the higher its density.
The density also varies according to temperature (it goes down as the temperature
goes up). The table overleaf shows the relationship between salinity and density
with respect to temperature, which is relatively constant (25-26°C) in aquariums.
The density, expressed as specific gravity, a value which is easy to use, is all
that is required to calculate salinity: it must range between 1.022 and 1.024.