What is the approximate average salinity of open-ocean seawater?

The approximate average salinity of open-ocean seawater is 35 parts per thousand.

This means about 35 grams of dissolved salts occur in one kilogram of seawater, often written as 35‰ or expressed as roughly 35 practical salinity units. The value is an average rather than a constant: evaporation raises salinity, while rainfall, river runoff, and melting ice lower it. Enclosed seas can be substantially saltier or fresher than the open ocean.

Most of seawater’s dissolved salts come from weathering of rocks on land, volcanic and hydrothermal processes, and chemical exchange between seawater and ocean-floor minerals. Sodium and chloride are the dominant ions, together making up most of the dissolved salt by mass. Salinity affects seawater density and therefore helps drive ocean circulation when combined with temperature differences.