Water Hardness Chart

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Home Hardness Chart

Water Hardness Chart

The US Public Health Service Water Hardness Classification Table

Grains Per Gallon (gpg) Milligrams Per Liter (mg/l)
Parts Per Million (ppm)
Rating Color (chart)
less than 1.0 less than 17.1 Soft not on map
1.0 - 3.5 17.1 - 60 Slightly Hard  
3.5 - 7.0 60 - 120 Moderately Hard  
7.0 - 10.5 120 - 180 Hard  
over 10.5 over 180 Very Hard  


Hardnes Chart

Hardness in water is the most common water quality problem reported by U.S. consumers. In fact, hard water is found in more than 85 percent of the United States. Hard water occurs when excess minerals in the water create certain nuisance problems. While these water problems can be frustrating, water hardness is not a safety issue. Hard water is safe for drinking, cooking, and other household uses.

Hard Water Causes

Hard water can cause several problems for consumers including decreased life of household plumbing and water-using appliances, increased difficulty in cleaning and laundering tasks, decreased efficiency of water heaters, and white/chalky deposits on items such as plumbing, tubs, sinks, and pots and pans. Consequently, it is no surprise that according to the 1997 National Water Quality Survey, one out of five Americans surveyed is dissatisfied with the quality of his/her household water supply.

Approximately 22 percent of the earth's fresh water is ground water, and naturally, as it flows through soil and rock, it picks up minerals. Hard water results when an excessive amount of calcium and magnesium are present. Total hardness is measured in grains per gallon of water (gpg) or milligrams per liter (mg/l). Grains per gallon (gpg) is a unit of weight for a volume of water, as is milligrams per liter (mg/l). Sometimes hardness is measured in parts per million (ppm). Parts per million (ppm) measures the unit(s) of a substance for every one million units of water. Milligrams per liter (mg/l) and parts per million (ppm) are roughly equal in water analysis. One gpg (1gpg) is equivalent to 17.1 ppm or mg/l. When conducting chemical analysis, laboratories usually measure hardness minerals in either grains per gallon (gpg) or milligrams per liter (mg/l). You can evaluate the hardness of your water supply by referring to the chart above.

The best way to determine whether or not your water is hard is to have it tested. However, you can usually detect hard water by the evidence in your home, including:

  • Increased water heating costs due to scale buildup and mineral deposits, and more frequent replacement of hot water heating elements
  • Soap scum on bathtubs, shower tiles, and basins
  • Film left on the body resulting in dry skin and dull, limp hair
  • Decreased sudsing and cleaning capabilities of soaps and detergents, resulting in dingy laundry and reduced life of
  • Increased buildup of scale on plumbing fixtures and cooking utensils such as a tea kettle, coffee maker, pasta pot, and dish
  • Clogged pipes or appliances resulting in reduced water flow and increased repairs


The most common method to treat hard water is through ion exchange water softening. Ion exchange water softening is a process in which the hardness ions, magnesium and calcium, are exchanged with either sodium or occasionally, potassium ions. This is accomplished by directing the flow of hard water over a bed of plastic resin beads. Each bead has a slight electric charge, which holds the sodium on the bead. As the water flows over the beads, the hardness minerals (ions) are attracted to the beads. When the hardness minerals attach themselves to the beads, the sodium ions are displaced. Hence, the hardness ions are replaced by the sodium ions.

As some point the plastic resin beads will be covered with hardness ions and will no longer be able to remove hardness from the water. In order to remove the hardness ions from the beads, a brine or salt (sodium chloride) solution is added to the resin bed. This solution contains a high concentration of sodium ions, which remove the hardness ions from the beads. Next the solution and the hardness ions are flushed out of the resin bed with fresh water, and once again the beads can remove hardness from the water. This process is called regeneration.

Softener Options

There are many different kinds of water softeners available. Before purchasing a water softener you should make sure that the unit has sufficient water softening capacity for your family. A typical person uses 100 gallons of water per day.

Another important feature to consider is how the equipment initiates the regeneration process. Water softening equipment uses three general methods of controlling water softener regeneration:

  1. Manual
  2. Time clock
  3. Demand initiated regeneration - known as Metered

Of these three methods, demand initiated regeneration is considered to be by far the most efficient method of regeneration. The metered method offer savings in salt and water usage over the time clock method by regenerated only after a set amount of water has been use.

The manual regeneration is ideal for remote locations without power. The timed valve is recommended for heavy iron or sulfur areas to ensure the system is able to provide continually clean water.


Soft Water Benefits

Consumers with even slightly hard water can benefit from using a water softening device. In fact, according to a New Mexico State University study, water heating efficiencies for softened water may be increased up to 29 percent when heating with gas and up to 22 percent when heating with electricity. Other possible benefits include:

  • Increased life expectancy and efficiency of the plumbing system due to reduced clogging from scale
  • Increased life expectancy and performance of water-using appliances
  • Reduced soapy residue on clothes, skin and hair
  • Reduced filmy deposits on tubs and shower tiles
  • Reduced scratching of bathroom fixtures and sinks
  • Savings in the amount of soaps and detergents used
  • Decreased spotting, white chalky deposits, of dishes, pots and pans and glassware, etc.


Information on this page is from Household Water Quality - Water Hardness
by Amber Wilson, Kathleen Parrott, and Blake Ross,Virginia Tech June 1999

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