Determining the Hardness of Water


It is natural for water to contain minerals. These minerals can include calcium, magnesium and sodium. When water is in contact with soil and rocks it will dissolve the minerals within the soil and the rocks, which in turn harden the the water.

The hardness of water is measured by the concentration of calcium and magnesium in the water  – the level of hardness increases as the concentration of Calcium and Magnesium increases. The concentration of these minerals is a direct result of the types of soil and rock that the water comes in contact with and the length of time that it is in contact. This is because the rocks and soil inherently bear minerals such as Calcium and Magnesium. For instance, ground water that has spent years running through rivers and lakes will generally be quite ‘hard’; however the ‘hardest’ water will often be water that has come from a well. By contrast, fresh rain water will not have had time to absorb such minerals or not have been exposed to landscape that contains high levels of these minerals and will therefore be ‘soft’.

The hardening of water occurs because water, being a solvent, dissolves and retains small amounts of minerals – most notably calcium and magnesium. These natural minerals are unable to be filtered out like suspended solids can, such as sand and grit.

Due to a myriad of factors including temperature, pH levels, etc. being used to determine the hardness of water, there is no numerical or adjective ranges that can accurately classify water hardness. In saying this, the United States Geological Survey (USGS) have put together the following guide to encourage country-wide standardization.

Adjective RatingHardness Range (ppm)
0 – 60Soft
61 – 120Moderately Hard
121 – 180Hard

The map below illustrates the concentration of hard water accorss the United States

Hardness of Water

Mean hardness as calcium carbonate at NASQAN water-monitoring sites during the 1975 water year. Colors represent streamflow from the hydrologic-unit area. Mape edited by USEPA, 2005. Modified from Briggs and others, 1977.