The value of the EC [mS/cm] is determined by 3 anions: Cl (chloride), HCO3 (bicarbonate) en SO4 (sulphate).
For EC values below roughly 2 mS/cm the relation between EC and chloride is not linear due to the possible presence of bicarbonate and suphate. Above values of roughly 2 mS/cm chloride is dominant and the following relation can be used:
Cl (mg/l) = 360 * EC (mS/cm) – 450
for water of 25 °C. This formula is based on groundwater analysis in the Netherlands and is described in De Louw 2013.
A useful rule of thumb for EC>2 mS/cm is:
Cl (g/l) = EC (mS/cm) / 3