What Is Water Hardness?

Quick answer

Water hardness is the amount of dissolved calcium and magnesium in the water. The U.S. Geological Survey classes water as soft up to 60 mg/L, moderately hard from 61 to 120, hard from 121 to 180, and very hard above 180, all expressed as calcium carbonate. Hard water is not generally a health concern, but it leaves scale and soap scum.

Water hardness: The concentration of dissolved calcium and magnesium in water, usually reported in milligrams per litre as calcium carbonate.

Where hardness comes from

Rain is naturally soft. As water moves through soil and rock, it dissolves minerals. The USGS Water Science School describes hardness as the amount of dissolved calcium and magnesium, with other metals contributing as well. Water that has passed through limestone or chalk picks up calcium readily, which is why hardness varies so much from one region to another, and sometimes between neighbouring towns with different water sources.

The hardness scale

USGS water hardness classification
Hardness (mg/L as calcium carbonate)	Classification
0 to 60	Soft
61 to 120	Moderately hard
121 to 180	Hard
More than 180	Very hard

Different countries report hardness in different units, including degrees German, French or English hardness and grains per gallon. The figure on a dishwasher or kettle descaler may therefore not match the unit on your water report, so check which unit a number uses before comparing. The mg/L scale above is the one the USGS uses.

What hard water does around the home

The USGS notes that calcium in hard water reacts with soap to form soap scum, so more soap or detergent is needed for the same cleaning, and a film can be left on hands, dishes and glassware. When hard water is heated, calcium carbonate can come out of solution and deposit as scale. That scale can reduce the efficiency of water heaters, raise heating costs, shorten the life of equipment, and over time narrow pipes.

Typical effects of soft and hard water
Where you notice it	Soft water	Hard water
Soap and shampoo	Lather easily; rinse can feel slippery	Lather poorly; may leave a film
Kettles and heaters	Little scale	White scale builds up with heating
Glassware after washing	Usually clear	Can dry with a cloudy mineral film
Laundry	Cleans with less detergent	May need more detergent; towels can feel stiff

The glassware effect is the one that most often sends people looking for answers. A cloudy film on glasses from a dishwasher can be dissolved mineral deposit, a diagnosis tested with a small vinegar patch, as covered in Why Glasses Look Cloudy After the Dishwasher.

How to find out how hard your water is

The USGS page recommends contacting your local health agency, water utility, or a water-softening vendor for hardness information for a specific location. Many utilities publish an annual water quality report that includes hardness. Home test strips give a rough reading, and a laboratory test gives a precise one. Because the number can differ between a municipal supply and a private well, check the source that actually feeds your tap.

What can be done about it

Options range from simple adjustments to equipment. Using the amount of detergent recommended for your hardness level and topping up the dishwasher's rinse aid and salt reservoir, if it has one, address the most common household symptoms. Descaling kettles and coffee machines with the descaler or acid the manufacturer recommends removes buildup. A whole-house water softener replaces calcium and magnesium with another ion, and it needs maintenance and has trade-offs, so it is best chosen after you know your actual hardness.

Hard water and health

The USGS discusses hardness in terms of household effects such as scale and soap use. Questions about drinking-water health, or about sodium added by softeners, are better directed to your water utility or a health professional.