What’s hard and scaly and dwells in your pipes? No, it’s not the pet alligator your parents flushed down the toilet -- he’s grown up and terrorizing the sewers of Chicago. We’re talking about hard water.
We call water "hard" if it contains a lot of calcium, magnesium or other minerals. Groundwater acquires these metals by dissolving them from surrounding soil and rock. Industry measures water hardness in terms of grains per gallon (GPG) or milligrams per liter (mg/L). A grain is defined as 64.8 milligrams of calcium carbonate [source: Business Dictionary]. If your water tests at 1 GPG (17.1 mg/L) or less, then you have soft water. Water around 1-3.5 GPG (17.1-60 mg/L) occupies a gray zone between soft and slightly hard water and 3.5-7 GPG (60-120 mg/L) is moderately hard. Hard water is around 7-10.5 GPG (120 - 180 mg/L), and very hard water is above that.