Hard Water: What It Does (and Doesn't Do) and How to Fix It

Crusty faucets, spotty glasses, soap that won’t lather, a white rim in the kettle: that’s hard water. It’s mostly a plumbing and cleaning problem, not a health one.

Short answer: hard water is water with a lot of dissolved calcium and magnesium. The USGS calls water above 120 mg/L (about 7 grains per gallon) “hard” and above 180 mg/L “very hard.” It builds scale in water heaters and pipes and makes soap less effective, but the USGS and the WHO don’t treat it as a health concern. In a large trial, a water softener did not improve children’s eczema. If your hardness is low or moderate, you may not need to do anything at all.

What is hard water?

According to the U.S. Geological Survey, “the simple definition of water hardness is the amount of dissolved calcium and magnesium in the water.” Water picks these minerals up as it moves through rock and soil. The World Health Organization’s background document on hardness names limestone and chalk as the most common sources, and notes that calcium usually makes up most of the hardness, with magnesium in smaller amounts.

Hardness is usually reported “as calcium carbonate”: one number that covers both minerals. That’s the number you’ll see on a utility report or a test result.

How hard is “hard”? The scale in mg/L and grains per gallon

The USGS uses this classification, in milligrams per liter (mg/L) as calcium carbonate. Softener sellers and many test kits use grains per gallon (gpg) instead. One grain is 64.8 mg and one gallon is 3.785 liters, so 1 gpg is about 17.1 mg/L: divide mg/L by 17.1 to get gpg.

USGS category	mg/L as calcium carbonate	Grains per gallon (approx.)
Soft	0 to 60	0 to 3.5
Moderately hard	61 to 120	3.5 to 7
Hard	121 to 180	7 to 10.5
Very hard	more than 180	more than 10.5

For a practical threshold, the Minnesota Department of Health says that if your water is harder than 7 grains per gallon or 120 mg/L, “you might need a water softener” to keep appliances running well and improve how the water looks and tastes. It also says plainly that there is no requirement to soften: it’s a personal choice.

Where is hard water common in the US?

Hardness depends on local geology and varies across the country. In a USGS study of about 2,100 domestic wells across the country, hard water was prevalent in the east-central and western United States, linked to carbonate aquifers and aquifers with high dissolved solids. The USGS also publishes an older national map of stream hardness, but it cautions that those data are not meant for local decisions.

The WHO notes that small water supplies using groundwater often have significant hardness (though some large surface-water supplies do too), and that calcium levels up to and above 100 mg/L are common in natural water, particularly groundwater. So well owners in particular should check, and only a test will tell you.

Your own water matters more than any map. If you’re on city water, check your utility’s annual report (here’s how to read it). If you’re on a well, include hardness in your next well water test.

What hard water does in your home

The effects are real, and they’re mostly about heat and soap.

• Scale in water heaters and pipes. When hard water is heated, calcium carbonate comes out as solid scale. The USGS says this scale can shorten equipment life, raise water-heating costs, lower the efficiency of electric water heaters and clog pipes. The WHO adds that soft or softened water scales very little, which allows more efficient heat transfer and “probably” a longer life for water heaters.
• Soap and detergent. Soap reacts with calcium to form soap scum, so you need more soap or detergent to get things clean (USGS). That’s also why hard water feels like it doesn’t lather.
• Spots and films. The Minnesota Department of Health lists fewer mineral spots on glassware and less soap film in sinks, tubs and washing machines among the advantages of softening.

None of these sources puts a reliable dollar figure on it, so be wary of exact savings claims in softener ads.

Is hard water bad for your health?

No, not on the current evidence. The USGS describes hardness as “a property of water that is not a health concern, but it can be a nuisance.”

The WHO reviewed the question for its drinking-water guidelines and did not set a guideline value for hardness. Its conclusion: there are “insufficient data to suggest either minimum or maximum concentrations” of these minerals. Some population studies have suggested harder water, or magnesium in water, might be linked to lower cardiovascular deaths, but the WHO says that evidence “is being debated and does not prove causality.” It also notes that drinking water can be a meaningful source of calcium and magnesium for people whose diets are low in them.

As of October 2026, the EPA has no enforceable drinking-water limit for hardness. Its secondary standards, which are non-enforceable guidelines for taste, appearance and plumbing effects, don’t list hardness on its own; hardness and deposits appear as noticeable effects of high total dissolved solids (guideline: 500 mg/L).

Does hard water cause eczema or dry skin?

This is a common worry about hard water, and it has actually been studied. The short version: there’s a possible link between hard water and eczema in young children, but installing a softener didn’t make existing eczema better in the best trial so far.

The association. A 2021 systematic review and meta-analysis pooled seven observational studies with 385,901 participants. Children living with harder water had higher odds of eczema than those with softer water (odds ratio 1.28). The reviewers rated the certainty of that evidence as very low, because observational studies can’t rule out other differences between areas. The same review found two studies in people showing more of the detergent sodium lauryl sulfate left on the skin after washing in harder water, a plausible mechanism but not proof of harm.

The treatment trial (SWET). The Softened Water Eczema Trial, published in 2011, randomized 336 children aged 6 months to 16 years with moderate or severe eczema, all living in hard-water areas of England (at least 200 mg/L). Half got an ion-exchange softener installed at home plus their usual eczema care; half got usual care alone. After 12 weeks, nurses who didn’t know which group each child was in scored eczema severity.

Both groups improved by about the same amount: 20% with a softener and 22% without. The difference was not statistically significant, and there was no difference in how much eczema cream the children used. Parents in the softener group did report small improvements on some questionnaires, but they knew they had a softener, and the authors think those results most likely reflect response bias. A few other points worth knowing:

• Families couldn’t be blinded, because softened water produces noticeably more suds. That’s why the trial relied on blinded nurse assessments.
• It ran for 12 weeks, so it can’t speak to longer-term effects.
• It was funded by the UK’s National Institute for Health Research. A consortium of water-treatment companies and the Water Quality Association supplied technical advice, softeners and salt, and one author worked for a UK water-treatment trade association.

The 2021 meta-analysis combined SWET with a second trial and found no meaningful effect of softeners on eczema severity, rating that evidence as moderate certainty.

Prevention is still an open question. A 2022 pilot trial (SOFTER) in London installed softeners before birth in 39 homes, compared with 39 without, in families with a history of allergic disease and very hard water (above 250 mg/L). At 6 months, visible eczema was seen in 33% of babies in softened homes versus 48% in the comparison group. That looks promising, but it was a small feasibility study, the range of plausible results included no difference, and the authors ran no statistical test on it. Its conclusion was only that a full-size trial is feasible.

If you or your child has eczema, a softener isn’t a substitute for treatment. Talk to your doctor or a dermatologist about skin care, and don’t stop prescribed treatments because of a water change.

What about hair?

The evidence here is thin. The few published studies wash cut strands of hair in a lab, not hair on people’s heads, and they disagree:

• A 2013 study soaked shed hair from 15 volunteers in hard water (212.5 ppm) or distilled water every other day for 30 days and found no difference in strength or elasticity.
• A 2018 study of hair from 70 young men in Pakistan found lower tensile strength in strands treated with hard water than with deionized water.
• A 2017 study of hair from 20 volunteers in Saudi Arabia found more magnesium deposited on strands washed in hard water for three weeks, but no visible structural damage under an electron microscope.

None measured hair loss or followed real people over time, so claims that hard water causes hair loss, or that a filter fixes it, aren’t supported by these studies.

Do water softeners add sodium?

Yes, salt-based softeners swap calcium and magnesium for sodium (or potassium, if you use potassium chloride), so softened water contains more sodium. How much depends on how hard your water was to begin with. The EPA’s 2003 drinking water advisory on sodium notes that home softeners can raise sodium in drinking water to more than 300 mg/L.

For most people, that’s not significant: the same advisory says drinking water “does not play a significant role in sodium exposure for most individuals.” Two EPA reference points, neither of them enforceable limits:

• 20 mg/L is a guidance level for people on a very low sodium diet (500 mg a day), and EPA says it should not be applied to the whole population.
• 30 to 60 mg/L is a recommended range based on taste.

If you have high blood pressure or are on a sodium-restricted diet, ask your doctor about softened water. The Minnesota Department of Health suggests keeping an unsoftened tap for drinking and cooking, or regenerating the softener with potassium chloride instead of salt.

How to test your water’s hardness

• City water: look for hardness in your utility’s annual water quality report, or call the utility and ask. Also ask whether they already soften the water: the Minnesota Department of Health warns that softening water your utility has already softened isn’t needed and may cause corrosion.
• Well water: use a home test kit or an independent lab. Hardness is an easy add-on to a broader well test, which matters more for safety (bacteria, nitrates, arsenic). See what to test a well for.
• Check the units. Results come in mg/L, ppm (the same as mg/L for water) or grains per gallon. Divide mg/L by 17.1 to get gpg.

Write the number down. If you do buy a softener, Minnesota’s guidance is to set it to your actual hardness: set too high, it costs more to run and wastes water. More on testing in Step 1: find out what’s in your water.

How to fix hard water

Option 1: Do nothing

If your water is soft or moderately hard, or the scale doesn’t bother you, there’s no health reason to treat it. Descaling the kettle and showerhead now and then may be all you need.

Option 2: An ion-exchange (salt-based) water softener

This is the standard way to remove hardness from all the water entering your home. Resin beads trade calcium and magnesium for sodium or potassium, and the system regularly flushes the beads with brine to recharge them.

The standard to look for is NSF/ANSI 44. On NSF’s own product listings, this standard is titled “Cation Exchange Water Softeners,” and the claims listed under it are Hardness Reduction, Efficiency Rated, Barium Reduction and Radium 226/228 Reduction. Certification applies to specific models, so check the listing for the exact model number rather than trusting a logo on the box.

The trade-offs, according to the Minnesota Department of Health:

• Sodium in the softened water (see above).
• Salt and wastewater. Salt use has negative impacts on the environment, and the water used to recharge the resin beads goes down the drain as waste.
• Maintenance: adding salt or potassium chloride and periodic testing to make sure it’s working.
• Possible corrosion. Softened water can corrode pipes, which can raise lead and copper levels at the tap. Minnesota advises not running a softener for the first few weeks after new copper plumbing is installed. (The WHO notes that softened water “is not necessarily corrosive,” so this depends on your water and plumbing. If you have older plumbing, see lead in drinking water.)

You also don’t have to soften everything. Minnesota notes that people often soften only showers, sinks and laundry, or only hot water, and leave toilets, outdoor spigots and a cold drinking tap unsoftened.

Option 3: Salt-free “conditioners”

Salt-free systems are sold as a way to deal with scale without salt or wastewater. Be clear on what a product actually claims. Removing hardness is something you can check yourself: test the water before and after the unit. Reducing scale without removing hardness is a different claim, and much harder to check at home.

As for proof: NSF’s listings for home water treatment don’t have a hardness reduction or scale-prevention category for these devices: the Hardness Reduction claim appears only under NSF/ANSI 44, the standard for cation exchange softeners. We also didn’t find a peer-reviewed study of household salt-free conditioners on PubMed. That doesn’t prove they don’t work, but it does mean the claims are mostly the maker’s. If you’re considering one, ask for an independent test report and which test protocol it followed.

Option 4: Reverse osmosis for drinking water only

If your concern is the taste of very hard water or white residue in your kettle, a point-of-use system at the kitchen tap can help. The WHO notes that reverse osmosis membranes deplete the mineral content of water. But it only treats the water at that one faucet, so it does nothing for your water heater, shower or laundry. See how reverse osmosis works.

What won’t soften water: shower filters

Shower filters are sometimes sold as a fix for hard water, but they aren’t softeners. The NSF standard for shower filters is NSF/ANSI 177, and on NSF’s listings the only claim certified under it is free available chlorine reduction. A shower filter may reduce chlorine; it is not certified to remove calcium or magnesium. More in our shower filter section.

The bottom line

• Hard water means calcium and magnesium; above about 120 mg/L (7 gpg) it’s “hard” by the USGS scale.
• It’s a nuisance, not a health hazard: scale in water heaters and pipes, soap scum, spots. The WHO sets no health-based limit for it, and the EPA has no enforceable limit.
• A randomized trial in 336 children found that a softener did not improve existing eczema. Whether softening helps prevent eczema in babies is still being studied. Hair claims rest on a few small, conflicting lab studies.
• Find your number first: your utility report for city water, a test for a well.
• If you decide to treat it, an ion-exchange softener is the option that actually removes hardness. Look for NSF/ANSI 44 on the certifier’s listing for the exact model, and think about sodium and salt use. NSF lists no hardness reduction certification for salt-free conditioners or shower filters.

Ready to compare options? Start at our water softeners hub.