Hard Water vs Soft Water: The Differences, and How to Tell Which You Have
Quick answer: hard water has more dissolved calcium and magnesium, and soft water has little. The USGS calls water soft at 60 mg/L (about 3.5 grains per gallon) or less and very hard above 180 mg/L. Hard water leaves white scale on faucets and in water heaters, spots on glasses, and soap scum, since the minerals react with soap. Soft water lathers easily and doesn’t scale, but it can be more corrosive to pipes, and softened water has added sodium. Neither is considered a health risk: the WHO sets no health-based limit for hardness. To know which you have, check the signs, then confirm with a test strip or your utility’s water report.
What makes water hard or soft
Hardness is mostly calcium and magnesium dissolved from rock and soil. Hach notes that hardness comes almost entirely from these two minerals, and NDSU Extension’s definition also counts the usually small amount of iron. It’s measured as calcium carbonate, in mg/L (the same as ppm) or grains per gallon (gpg); 1 gpg is 17.1 mg/L.
Soft water gets that way in one of two ways. Some sources are naturally low in minerals, such as New York City’s upstate reservoirs (see the city table). Or the water is softened: a water softener swaps the calcium and magnesium for sodium using ion-exchange resin. Our guide to how a water softener works explains the process.
| Class | USGS (mg/L) | About gpg | NDSU Extension (gpg) |
|---|---|---|---|
| Soft | 60 or less | 3.5 or less | Under 1 (1–3.5 is “slightly hard”) |
| Moderately hard | 61–120 | 3.6–7 | 3.5–7 |
| Hard | 121–180 | 7.1–10.5 | 7–10.5 |
| Very hard | More than 180 | More than 10.5 | More than 10.5 |
The two scales agree except at the bottom: NDSU’s “soft” is under 1 gpg, the level a water softener aims for, while the USGS counts anything up to 60 mg/L as soft.
Hard water vs soft water at a glance
| Hard water | Soft water | |
|---|---|---|
| Minerals | More calcium and magnesium | Little; softened water has sodium in their place |
| Soap | Forms scum that’s hard to remove, and cuts detergents’ cleaning power (NDSU) | Lathers easily, no scum |
| Scale | White buildup on faucets and showerheads, and inside water heaters, where it slows heating and uses more energy (NDSU) | Little or none |
| Dishes and laundry | Spotted glassware; dingy, scratchy clothes, and whites turn grayish (NDSU) | Cleaner rinse |
| Pipes | Generally less corrosive, but can scale up (Health Canada) | Can corrode pipes and plumbing (Health Canada, Australia’s NHMRC) |
| Drinking | No health-based limit (WHO); some calcium and magnesium in your diet | Fine to drink; softened water adds about 8 mg of sodium per liter for each gpg removed (NDSU) |
| Skin | Can leave soap on skin and irritate it, in one lab study | Feels slippery; softeners didn’t help children’s eczema in a trial |
Details and studies are in the sections below. For whether softening is worth it, see water softener benefits.
Do I have hard water? Signs checker
Tick what you see. The checker separates real hardness signs from look-alikes that point to other problems.
You probably have hard water
You ticked 2 classic hardness signs. A quick test confirms it and gives you a number.
- Check your utility’s annual water quality report first: some list hardness, and it’s free. Or use hardness test strips.
Signs are a hint, not a measurement. Hardness classes follow the USGS scale.
Signs you have hard water
NDSU Extension lists the common household effects of hard water. The more of these you see, the likelier it is:
- White crust on faucets and showerheads. NDSU calls whitish buildup on faucet and showerhead ends probably the most obvious result of hard water.
- Film on shower doors, walls and tubs, the same scale spread thin.
- Soap scum and poor lather. Soap combines with calcium and magnesium to form a scum that’s hard to clean off sinks and appliances.
- Spotted glasses after the dishwasher dries them.
- Dingy laundry. Clothes can look dull and feel harsh, and whites gradually turn grayish.
- Scale in the kettle, coffee maker or water heater. Calcium and magnesium carbonate come out of the water when it’s heated. NDSU notes they coat electric heating elements and can shorten a water heater’s life.
- Clogged showerheads and aerators, as scale builds up in small openings.
To be sure, test: hardness strips take seconds. See our water hardness test kits.
Check price on hardness test stripsStains that look like hard water but aren’t
Some popular guides call reddish-brown stains a hard water sign. They usually aren’t: color points to a different mineral, and each needs a different fix.
| What you see | Likely cause | Where to go next |
|---|---|---|
| White, chalky crust | Hardness (calcium and magnesium) | Water softeners |
| Orange or reddish-brown stains | Iron (Penn State Extension) | Softeners and filters for iron |
| Black or dark brown stains | Manganese (Penn State Extension) | Iron and manganese treatment |
| Blue-green stains | Corrosive water dissolving copper pipes, usually from low pH (Penn State Extension) | Test pH, copper and lead; Penn State says blue-green stains don’t tell you whether lead is present |
| Rotten-egg smell | Hydrogen sulfide | Rotten egg smell guide |
What is limescale?
Limescale is the hard, chalky deposit hard water leaves behind, mostly calcium carbonate with some magnesium. Heat speeds it up: NDSU explains that calcium and magnesium carbonate come out of hard water when it’s heated, which is why scale builds fastest in kettles, coffee makers, water heaters and on electric heating elements. Inside a water heater it slows heating and uses more energy. On faucets and showerheads it forms as hard water dries.
Australia’s drinking water guidelines set a hardness limit of 200 mg/L for exactly this reason: above it, scale builds up quickly in hot water pipes and fittings. Limescale itself isn’t a health concern; the problem is what it does to fixtures and appliances.
Is hard water bad for you?
- No health limit. The WHO sets no guideline value for hardness because it’s “not of health concern at levels found in drinking-water,” and it notes that minerals in drinking water contribute to nutrition.
- Heart disease. Australia’s NHMRC says some studies found hard water may benefit heart health, but the data are inadequate to conclude the link is causal.
- Kidney stones. In a 2024 UK Biobank study of 288,041 people followed for up to 18 years, water hardness and calcium showed no significant overall link to kidney stones; higher magnesium was linked to lower risk. The authors did find higher risk with hard water in people over 60 and in women. Separately, Health Canada has proposed a health-based value of 300 mg/L for calcium, citing kidney stone risk from calcium intake, but says the evidence doesn’t support a health-based value for hardness itself.
- Skin. A 2018 study found hard water left more soap residue on skin and caused irritation, while a 2011 trial found home softeners didn’t improve children’s eczema. Details are in our shower filter guide.
- Acne, dandruff and hair. We found no good studies showing hard water causes acne or dandruff, or showing what it does to hair.
Is soft water better?
For your plumbing and cleaning, usually yes: less scale, less soap and fewer spots. But soft water has trade-offs:
- Corrosion. Health Canada says soft water can corrode distribution systems and household plumbing, and that hard water is generally less corrosive. Its guidance cites 80–100 mg/L of hardness as a good balance between corrosion and scale.
- Sodium. Softened water carries about 8 mg of sodium per liter for each grain per gallon removed, per NDSU. See is softened water safe to drink?
- The slippery feel that some people dislike.
Do these cities have hard water?
On city water, your utility’s annual report is the best free source. Here’s what three big-city reports say:
| City | Hardness | USGS class | Source |
|---|---|---|---|
| New York City | 25 mg/L average (16–90), about 1 gpg | Soft | NYC DEP 2024 water quality report |
| Washington, DC | 123 mg/L average (83–173), about 7 gpg | Hard | DC Water 2023 water quality report |
| San Diego | 216–230 mg/L at the city’s three treatment plants, 12.6–13.4 gpg | Very hard | City of San Diego 2024 water quality report |
Hardness can vary across a city and by season, so check your own utility’s report. The EPA’s Consumer Confidence Report search finds it. On a private well there’s no report, so test the water yourself; see our well water test kit guide.
How to find out for sure
- Check your water report if you’re on city water.
- Use hardness test strips for a number in seconds; our test kit guide compares strips and drop kits.
- Get a lab test on a well, which also covers iron, manganese and bacteria.
If the number is high, see the best water softeners and our size calculator.
Frequently asked questions
What is the difference between hard water and soft water?
Hard water has more dissolved calcium and magnesium; soft water has little, naturally or because a softener swapped them for sodium. The USGS calls water soft at 60 mg/L or less and very hard above 180 mg/L.
Is hard water or soft water better to drink?
Both are safe. The WHO sets no health-based limit for hardness, and hard water adds a little calcium and magnesium to your diet. Softened water has added sodium, which matters only if you’re on a strict low-sodium diet.
Does New York City have hard water?
No. NYC DEP’s 2024 report gives an average of 25 mg/L, about 1 grain per gallon, which is soft.
Can hard water cause acne?
We found no good studies showing it does. One lab study found hard water leaves more soap residue on skin and can irritate it, but that’s not the same as causing acne.
Is limescale harmful?
Not to your health; the WHO doesn’t treat hardness as a health concern. It does clog fixtures, slow water heaters and shorten appliance life.
Are orange stains a sign of hard water?
No. Orange or reddish-brown stains usually mean iron, and black stains mean manganese. Hard water leaves white, chalky deposits.
Related guides
- Best water hardness test kits
- Water softener benefits
- How does a water softener work?
- Is softened water safe to drink?
Sources
- U.S. Geological Survey, Water-Resources Investigations Report 02-4045, Table 3 (hardness classes).
- NDSU Extension, Water Softening (Ion Exchange), WQ1031 (revised May 2022).
- Penn State Extension, Iron and Manganese in Private Water Systems and Corrosive Water Problems.
- Hach, Total Hardness Test Kit, Model HA-71A (what causes hardness).
- World Health Organization, Guidelines for Drinking-water Quality: hardness fact sheet and Hardness in Drinking-water (background document).
- NHMRC, Australian Drinking Water Guidelines: Hardness (as calcium carbonate).
- Health Canada, Guidelines for Canadian Drinking Water Quality: Operational Parameters (2024 consultation draft).
- The association between domestic water hardness and kidney stone disease: a prospective cohort study from the UK Biobank, International Journal of Surgery, 2024.
- NYC DEP, 2024 Drinking Water Supply and Quality Report; DC Water, 2023 Drinking Water Quality Report; City of San Diego, 2024 Annual Drinking Water Quality Report.