Soft water is defined by American National Standards NSF/ANSI 44 and NSF/ANSI 330 as water containing <1 grain of hardness per gallon (or <17.1 mg/L hardness).
Water hardness is demonstrated by scale in water heaters or on plumbing fixtures, by soap deposits on dishes and fabrics, and by soap scum in sinks and bathtubs.
Water can become “hard” as water passes through the atmosphere in the form of rain, snow, sleet, hail, dew, or fog, it picks up minerals along with gaseous and bacterial impurities. And, because water is the universal solvent, it picks up even more impurities in ponds, lakes, and rivers, as it percolates into the underground water table. Water hardness is measured in grains per gallon (gpg). Your water utility company will tell you the hardness of your water supply, or your water quality improvement professional can perform a simple test for that information.
The most commonly used method is ion exchange softening which is relatively inexpensive and provides the luxury of using more natural types of cleaning products for household chores and personal care.