What Is Hard Water? Causes, Effects and Testing

Hard water is water with a high concentration of dissolved calcium and magnesium. It forms when rain and groundwater pass through limestone, chalk or gypsum and pick up those minerals along the way. Hard water is not considered a health hazard, but it leaves scale in pipes and kettles, makes soap lather poorly, and can shorten the working life of water heaters.

How water becomes hard

Rain is almost mineral-free, but slightly acidic from dissolved carbon dioxide. As it seeps through soil and rock, it dissolves calcium carbonate and magnesium compounds. Areas built on limestone or chalk, such as much of southern and eastern England or large parts of the American Midwest and Southwest, tend to have harder water. Regions on granite or other hard, slow-dissolving rock usually have soft water.

Hardness is expressed in a few different units, which causes confusion:

Milligrams per litre (mg/L) as calcium carbonate, also written as parts per million
Grains per gallon (gpg), where 1 gpg is about 17.1 mg/L
Degrees of German, French or English hardness, used in other countries

How hard is hard?

The U.S. Geological Survey uses these classes, measured as calcium carbonate:

Class	mg/L
Soft	0 to 60
Moderately hard	61 to 120
Hard	121 to 180
Very hard	more than 180

Other agencies and countries draw the lines a little differently, so treat the bands as a rough guide.

Temporary versus permanent hardness

Some hardness comes from dissolved bicarbonates. Heating the water drives out carbon dioxide and leaves solid calcium carbonate behind, which is the chalky deposit you see inside a kettle. This is called temporary (or carbonate) hardness. Hardness from other dissolved salts, such as sulphates and chlorides, does not precipitate on boiling and is called permanent hardness. Most supplies contain both.

What hard water does around the home

Scale. Deposits build up inside pipes, shower heads, kettles, dishwashers and washing machines. In water heaters, scale acts as insulation on the heating element, which wastes energy and can shorten the unit’s life.
Soap. Calcium and magnesium react with soap to form a sticky scum, so you may need more cleaning product to get the same lather.
Spots and film. Dried drops leave white marks on glasses, taps and shower doors.
Laundry. Fabrics can feel stiff, and detergents may work less well.

Many people say hard water makes skin feel dry or hair feel dull. The research on this is limited and mixed, so it is better described as a common complaint than a proven effect. Anyone with persistent skin problems should ask a doctor or dermatologist rather than assume water is the cause.

Is hard water bad for your health?

The World Health Organization says there is no health-based guideline value for hardness, and the calcium and magnesium in drinking water can contribute a small share of daily intake. Some studies have looked at links between water hardness and conditions such as heart disease, but the findings are not consistent enough to draw firm conclusions. If you have a medical condition, or are on a restricted-sodium diet and thinking about a water softener, discuss it with your healthcare provider.

A quick conversion example

Suppose a utility report lists hardness as 9 grains per gallon. Multiplying by 17.1 gives roughly 154 mg/L, which falls in the “hard” band of the USGS table above. If the same report gives 250 mg/L, that is already about 14.6 gpg and sits in the “very hard” class. Doing this conversion first avoids comparing numbers that use different units, a mistake that is easy to make when reading product labels and water reports side by side.

Also remember that hardness can vary through the year, especially where a utility blends water from several sources, so one reading is a snapshot, not a permanent label.

How to test your water

Check your utility’s report. In the United States, public water systems publish an annual water quality report. Many include hardness figures. UK water companies publish hardness by postcode area.
Use test strips. Cheap and fast; compare the colour against the chart. Results are approximate.
Try a titration kit. You add drops of reagent until the sample changes colour, and the number of drops gives a more precise reading.
Send a sample to a certified lab. Best for private wells or when you need dependable numbers.
Try the soap shake test. Put some water and a few drops of plain liquid soap in a clean bottle and shake it. A thick lather suggests soft water; cloudy water with little foam suggests hard water. It is only a rough indication.

Common mistake: testing after the water has passed through a softener, which will naturally give a low reading. Sample from a tap that bypasses it.

Ways households deal with hardness

Ion-exchange softeners swap calcium and magnesium for sodium or potassium. Descaling products dissolve deposits in kettles and machines. Boiling removes some temporary hardness but not the permanent kind. Ordinary carbon filters do not remove hardness minerals. The right option depends on the hardness level, plumbing and budget, and a licensed plumber or water treatment professional can advise.

Frequently asked questions

Can you drink hard water?

Yes. It is generally safe to drink, and many people prefer its taste. The main issues are scale and soap performance rather than safety.

Does hard water cause kidney stones?

Evidence is unclear. Kidney stones have many causes, and studies of water hardness have not produced a consistent answer. A doctor can give personal advice.

Does a water filter remove hardness?

Most jug and carbon filters do not. Softeners and some reverse osmosis systems do.