Water hardness is the amount of dissolved calcium and magnesium in your water. The more of these minerals present, the “harder” the water. It’s one of the most common water quality characteristics, and it affects everything from how well your soap lathers to how long your water heater lasts.
What Makes Water Hard
As water moves through soil and rock, it picks up naturally occurring minerals along the way. Calcium and magnesium are the two biggest contributors to hardness, though other metals can play a smaller role. Groundwater sources like wells tend to be harder than surface water from lakes or reservoirs, simply because underground water spends more time in contact with mineral-rich rock.
The U.S. Geological Survey classifies water hardness on a simple scale, measured in milligrams per liter (mg/L) of calcium carbonate:
- Soft: 0 to 60 mg/L
- Moderately hard: 61 to 120 mg/L
- Hard: 121 to 180 mg/L
- Very hard: above 180 mg/L
Much of the United States has moderately hard to very hard water, especially in the Midwest and Southwest where limestone and dolomite deposits are common.
Why Hard Water Doesn’t Lather Well
The most noticeable everyday sign of hard water is poor soap performance. When soap meets calcium or magnesium ions in hard water, a chemical reaction occurs. The mineral ions grab onto soap molecules and form an insoluble substance, a solid residue that won’t dissolve. That’s soap scum. It’s not just an annoyance on your shower door. The reaction actually uses up soap, meaning less of it is available to clean. You end up using more soap or detergent to get the same results, and you’re left with a filmy residue on your skin, hair, dishes, and fixtures.
Effects on Skin and Eczema Risk
Hard water can do more than leave your skin feeling dry after a shower. Research involving hundreds of thousands of participants has found that people living in hard water areas have higher rates of eczema. A large UK study found that exposure to domestic hard water above 200 mg/L was associated with 12% higher odds of having eczema compared to soft water areas. In children and infants, the link appears even stronger, with a pooled analysis showing 28% increased odds of eczema in hard water areas.
Several things seem to be going on biologically. Hard water makes it harder to rinse soap and detergent off your skin, leaving residues that strip away protective oils and raise skin pH. Your skin is naturally slightly acidic, and that acidity helps keep its barrier intact. Calcium and magnesium are alkaline metals that push skin pH higher, weakening that barrier. Hard water may also form tiny chalk-like particles when it reacts with soap, which can physically irritate skin. Once the skin barrier is compromised, allergens and bacteria penetrate more easily, setting the stage for inflammation and flare-ups.
What Hard Water Does to Appliances
When hard water is heated, the dissolved minerals come out of solution and form a chalky white deposit called limescale. This builds up inside water heaters, on heating elements, in pipes, and around faucets. The scale acts as insulation between the heating element and the water, forcing the appliance to work harder. Water heaters can lose up to 48% of their efficiency from scale buildup, which translates directly into higher energy bills.
Dishwashers, washing machines, and coffee makers all suffer from the same problem. Scale narrows pipes over time, reducing water flow and shortening appliance lifespan. The white spots you see on glasses and shower heads are the visible version of what’s happening inside your plumbing.
Is Hard Water Safe to Drink?
Hard water is not a health risk. The World Health Organization has reviewed the evidence and found no convincing link between water hardness and adverse health effects. No health-based guideline value for hardness exists. If anything, some epidemiological studies have found an inverse relationship between hard water and cardiovascular disease, meaning people in hard water areas may have slightly lower rates of heart disease. The evidence isn’t strong enough to call it protective, but it consistently points away from harm.
Hard water does contribute some calcium and magnesium to your diet. In hard water areas, drinking two liters a day provides roughly 52 mg of magnesium, compared to about 2.3 mg from soft water. Still, water typically accounts for only 5 to 20% of your total calcium and magnesium intake. The vast majority comes from food.
How to Test Your Water
If you get water from a municipal system, your utility likely publishes an annual water quality report that includes hardness. You can usually find it on their website or request a copy. For well water or a quick check at the tap, you have a few options.
Test strips are the cheapest and easiest method. You dip a strip in water and match the color change to a scale. They’re good for a ballpark number but not precise. Drop-count titration kits, like those made by API or Hach, are more accurate. You add drops of a reagent to a water sample and count how many it takes to trigger a color change, with each drop representing a specific hardness level. These kits are reasonably priced and reliable enough for most household decisions. For the most precise results, you can send a water sample to a testing lab.
How Water Softeners Work
The most common household solution for hard water is an ion exchange water softener. Inside the unit is a bed of tiny resin beads saturated with sodium. As hard water passes through, calcium and magnesium ions stick to the beads, and sodium ions are released into the water in their place. Periodically, the system flushes the beads with a salt brine solution to recharge them, washing the collected calcium and magnesium down the drain.
The tradeoff is sodium. Softening adds about 8 mg of sodium per liter for every grain per gallon of hardness removed. Water that starts at 10 grains per gallon (a common level in hard water areas) will contain roughly 80 mg of sodium per liter after softening. For context, a single slice of bread typically has 100 to 200 mg of sodium, so the amount is modest for most people. If sodium intake is a concern, potassium-based softener salts are an alternative, or you can install a reverse osmosis filter at your kitchen tap for drinking water while softening the rest of the house.
Other options include magnetic or electronic descalers, which claim to alter mineral behavior without removing them, though evidence for their effectiveness is limited. For targeted protection, you can also install scale-inhibiting filters on specific appliances like water heaters or dishwashers.