Water Education
Hard Water Basics: What It Is and Why It Matters
What hard water is, how it is measured, and why it causes scale, spots, and soap scum. Sourced from USGS and WQA.
If your glasses come out of the dishwasher spotted, your soap never seems to lather, and your showerhead slowly clogs with white crust, you are probably living with hard water. Here is what that means, how it is measured, and what you can do about it.
What hard water is
The U.S. Geological Survey defines water hardness simply as the amount of dissolved calcium and magnesium in water (USGS, Hardness of Water). As groundwater moves through soil and rock, it dissolves small amounts of naturally occurring minerals and carries them along. Where calcium and magnesium are common in the ground, the water that reaches homes is hard.
The Water Quality Association describes hard water the same way: water that contains dissolved compounds of calcium and magnesium and sometimes other metals. The term originally referred to water that was hard to wash in (WQA FAQs).
How hardness is measured
Hardness is reported as calcium carbonate equivalent, in milligrams per liter (mg/L) or grains per gallon (gpg). USGS uses these general classes (USGS, Hardness of Water):
0 to 60 mg/L: soft. 61 to 120 mg/L: moderately hard. 121 to 180 mg/L: hard. More than 180 mg/L: very hard.
For example, water at 250 mg/L is very hard by this scale. Divided by 17.1, that is about 14.6 gpg.
Where hard water is common
Hardness varies across the United States. A USGS study of about 2,100 domestic wells found hard water was prevalent in the east-central and western United States, reflecting where carbonate aquifers and aquifers with high dissolved solids occur (USGS). USGS also notes that regional maps are not meant for decisions at a single home. For your own water, a test from your utility, health department, or a water treatment professional is the right source.
What hard water does in a home
USGS lists the everyday effects. In hard water, soap reacts with calcium to form soap scum, so you need more soap or detergent to get hands, hair, and laundry clean. Spots and film on dishes are hard water residue. When hard water is heated, calcium carbonate deposits form. That scale can shorten equipment life, raise water heating costs, lower the efficiency of electric water heaters, and clog pipes. Over years, scale inside pipes narrows the opening and can lower water pressure (USGS, Hardness of Water).
WQA adds that dissolved calcium and magnesium salts are primarily responsible for most scaling in pipes and water heaters, and cause problems in laundry, kitchen, and bath (WQA FAQs).
Is hard water a health concern?
According to USGS, hardness is not a health concern, but it can be a nuisance. USGS also notes that the World Health Organization states drinking water may contribute calcium and magnesium to the diet. If you have a specific health question about your water, talk to your doctor or local health department.
What you can do about it
The most common way to reduce hardness for a whole home is an ion exchange water softener, which swaps calcium and magnesium for sodium or potassium ions (WQA). Our article How Water Softeners Work explains the process and how to care for one.
For small problems, vinegar dissolves scale on coffee makers and showerheads. USGS notes that the acidity of vinegar helps dissolve mineral buildup.
First step: know your number
Before choosing any treatment, get your hardness tested. If you are on a public water system, your utility can often tell you. If you are on a private well, read our guide to well water testing. Once you know your number in mg/L or gpg, you and your installer can size and set your system correctly.
Sources
USGS Water Science School, Hardness of Water (www.usgs.gov/special-topics/water-science-school/science/hardness-water)
Water Quality Association, FAQs (find.wqa.org/learn-about-water/faqs)
Water Quality Association, Point-of-Entry Solutions (find.wqa.org/improve-your-water/solutions/for-the-entire-building)
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