A characteristic of seawater is that it is salty. It is usual to measure salinity in parts per thousand and the open ocean has about solids per litre, a salinity of 35ppt. The Mediterranean Sea is a little higher at 37ppt and the Dead Sea has as much as dissolved solids per litre (300ppt). Although sodium chloride is the main salt present, making up about 85% of the solids in solution, there are also per litre of the chlorides of other metals such as potassium and magnesium and of sulphates, carbonates, bromides and other salts. A kilogram (2.2 lb) of salt can therefore be found in 28 litres or one cubic foot of typical ocean water. Despite differences in the levels of salinity in different seas, the relative composition of the dissolved salts is very stable throughout the world's oceans.