The average salinity of the world's oceans is about 35 parts per thousand. In practical terms, roughly 35 grams of dissolved salts occur in every kilogram of typical seawater, equivalent to about 3.5 percent by mass.
Salinity is not identical everywhere. Evaporation removes water and leaves salts behind, raising salinity, while rainfall, river discharge, and melting ice dilute seawater. The open ocean generally falls near 33 to 37 parts per thousand, although enclosed seas, estuaries, polar waters, and evaporation-heavy regions can differ substantially.
Seawater is not simply table salt dissolved in water. Sodium and chloride are the dominant dissolved ions, but magnesium, sulfate, calcium, potassium, and other substances also contribute. Salinity matters because it affects seawater density, buoyancy, freezing point, and the movement of deep ocean currents. Scientists often report modern measurements in practical salinity units, or PSU, but the familiar quiz answer remains 35 parts per thousand.