What is the difference between hard water and soft water?

The City of Phoenix report says the calcium and magnesium in its water "originate from the soils our source water contacts as it travels to Phoenix's water treatment plants." Those minerals leach into the water on the way. The report adds that "hard water can cause some types of scaling in pipes and water heaters, and on plumbing fixtures such as faucets and showerheads." The same report says that "taste, odor and hardness are not regulated under drinking water health standards," and calls them "harmless, aesthetic aspects that some consumers may notice."

Heat makes it worse. PNNL, a national laboratory, explains that calcium carbonate is "inversely soluble, meaning it precipitates more with increasing temperature." Scale forms fastest where water is hottest, so water heaters are a common place for it to build up.

Hardness is measured in grains per gallon (gpg) or in parts per million (ppm, the same as milligrams per liter). One grain per gallon is about 17 ppm. Penn State Extension uses this scale:

Phoenix's 2025 report lists total hardness of 172 to 302 ppm, or 10 to 17.6 grains per gallon. Most of that range is in the "very hard" band. Penn State says levels below 7.0 gpg "will probably not cause major scaling and soap film," so Valley homes are well above the point where it shows.

### Reading a hardness number

Test results come in two units, and mixing them up is common. Divide ppm by about 17 to get grains per gallon, or multiply grains by 17 to get ppm. The city's own row shows the match: 172 ppm is 10 grains, and 302 ppm is 17.6 grains. So a tap that reads 257 ppm is about 15 grains per gallon, which is more than four grains above the very hard cutoff. Some lab reports and studies give hardness "as calcium carbonate," which is the same ppm figure on the same scale. When you compare a test strip, a lab report and the city's range, make sure all three use the same unit before you decide anything.

"Soft water" means two different things, and the difference matters.

• Naturally soft water. PNNL defines it as water with "a lack of hardness, or a reduced concentration of calcium and magnesium ions (<60 mg/L)." Water from some sources is soft because it never picked up many minerals. It can be low in minerals overall.
• Softened water. This is hard water that has been through an ion-exchange softener. The resin trades the calcium and magnesium for sodium, or potassium if you use potassium chloride. Penn State describes it this way: the resin "releases its sodium ions and readily trades them for the calcium and magnesium ions." The water leaves with less hardness but more sodium. PNNL notes that the softened water "will have the same dissolved mineral content as the previously hard water it was treating," so the total dissolved minerals do not drop.

Softened water carries more sodium. Nebraska Extension says "sodium is added to water at a rate of about 8 mg/L for each grain per gallon (gpg) of hardness." At 10 grains that is about 80 mg/L, and at 17.6 grains about 141 mg/L. People on a sodium-restricted diet "should account for increased intake through softened water and consult their physician." Our page on using potassium instead of salt covers the alternative.

### Is soft water always better?

Not always. Softening removes a nuisance, but it changes the water in other ways. PNNL says soft water "does not appear to have a direct impact on equipment efficiency," yet it "corrodes anode rods." It also warns that "excessively soft water, especially water that is artificially softened from salt-based water softeners, can reduce the life of tanked water heaters by 50 percent or more." Penn State adds that softening means "calcium and magnesium are eliminated from the homeowner's diet." Neither point is a reason to avoid a softener in very hard water. They are reasons to size it right, maintain it and check the water heater anode. Our page on whether salt-based softeners damage a water heater walks through the anode check.

The day-to-day differences show up in soap, scale, your water heater and your diet.

• Soap and lather. Hard water fights soap. Penn State says "hard water leaves scales on pots, soap films on skin, and detergent curds in the washing machine." Soft water does not leave that film.
• Scale on fixtures and heaters. Hard water builds crust. Soft water does not, though PNNL cautions that "soft water corrodes anode rods and will reduce the life of steel-tank storage water heaters unless proper maintenance practices are employed."
• Health. Penn State says softening "eliminates problems that are a nuisance and not a threat to human health," and that the choice to soften is "a matter of personal preference not necessity."
• What a softener changes. The EPA says softeners "reduce hardness, scale formation, and other negative impacts of hard water." They do not filter or purify the water.

You can tell in five steps, from a quick look to a lab number.

1. 1Look at the signs. White crust on faucets, cloudy glasses and stiff laundry point to hard water.
2. 2Read the city report. Phoenix publishes its hardness range every year in its water quality report. Our page on how hard Phoenix water is explains it.
3. 3Test your tap. A hardness test strip or an independent lab test tells you the number at your own faucet. Penn State advises a test "by an independent lab" so you are not oversold.
4. 4Remember the range. Hardness in a city system varies by area. Phoenix's spread from 172 to 302 ppm means two homes in the same city can read very differently, so your own test matters more than the average.
5. 5Match the fix to the problem. If hardness is the issue, whether water softeners work with Arizona hard water and choosing the right size softener are the next steps. For drinking water only, water softener versus reverse osmosis covers the split.

HQ Plumbing & Air is licensed and insured, and can test your water and explain what a softener would change in your home. We offer free estimates and upfront pricing.

If you have seen shower head filters advertised as softeners, whether a shower head water softener helps hair and skin explains what they can and cannot do.