As it is the precise mixture of minerals dissolved in the water, together with water's pH and temperature, that determine the behaviour of the hardness, a single-number scale does not adequately describe hardness. However, the United States Geological Survey uses the following classification for hard and soft water:
Seawater is considered to be very hard due to various dissolved salts. Typically seawater's hardness is in the area of 6,570; ppm (6.57 grams per litre). In contrast, freshwater has a hardness in the range of 15 to 375 ppm; generally around 600 mg/L.