What Is Water Hardness? From Soft to Very Hard, in mg/L

Hardness is the number behind white crust on faucets, scale in the kettle and soap that won’t lather. The U.S. Geological Survey defines it simply as the amount of dissolved calcium and magnesium in the water. Labs and utilities usually report it “as calcium carbonate”, one number covering both minerals.

What is considered hard water?

The USGS classes water as soft from 0 to 60 mg/L, moderately hard from 61 to 120, hard from 121 to 180, and very hard above 180 mg/L. Softener sellers and many test kits use grains per gallon instead: one gpg is about 17.1 mg/L, which matches the NIST conversion factor, so 120 mg/L is about 7 gpg.

For a practical line, the Minnesota Department of Health says that above 7 grains per gallon (120 mg/L) you might need a softener to keep appliances running well. It also says softening is a personal choice, not a requirement.

Is hard water bad for you?

No. The USGS calls hardness “a property of water that is not a health concern, but it can be a nuisance,” and a University of Nebraska extension guide says the same. The EPA sets no limit for it: hardness isn’t on its list of secondary standards, and appears only as an effect of high total dissolved solids. Our guide to hard water covers what the research says about skin and hair.

How do I find out how hard my water is?

On city water, check your utility’s annual water quality report, which may list hardness, or call and ask; the Nebraska guide suggests both. Our guide on how to read your water quality report shows where to look. On a well, most testing labs offer a hardness test, and home kits and dip strips give a reasonable estimate. Note whether the result is in mg/L or gpg before you compare it with the scale.