Water hardness levels

Best for

Anyone holding a well-water lab report who wants to know what the hardness number means and whether it is worth softening.

Wrong fit

If the same report shows iron, manganese or a sulfur smell, read those results first, because they change what equipment you need.

Tradeoff

Hardness classes are conventions, not standards. Home strips and drop kits are fine for spot checks, but buy equipment from a lab result.

The two hardness scales

There is no federal drinking water limit for hardness. Instead, two descriptive scales are in common use. The water treatment industry uses the Water Quality Association (WQA) scale in grains per gallon. Government and scientific reports often use the USGS scale in mg/L. They line up closely at the top end and differ at the soft end.

WQA scale (grains per gallon)

Penn State Extension and NDSU Extension publish the same bands.

USGS scale (mg/L as calcium carbonate)

The practical difference: water at 2 gpg (34 mg/L) is “slightly hard” to WQA and “soft” to the USGS. Either way, that is not water that needs a softener. Both scales agree that anything over about 7 gpg (120 mg/L) is hard, and that is the point where the Minnesota Department of Health says a softener may help.

Converting gpg, mg/L and ppm

WQA defines 1 grain per gallon as 17.1 mg/L (ppm) of hardness as calcium carbonate. Some extension publications round this to 17.

• gpg = mg/L ÷ 17.1
• mg/L = gpg × 17.1
• For hardness in drinking water, ppm and mg/L are the same number.

Softener settings and sizing almost always use gpg, so convert your lab result before you program a control valve or use the sizing calculator.

What each level means at home

The 30 gpg line comes from WQA’s best-practice booklet, which says that even at efficient salt settings, some hardness may leak through when water has more than 30 gpg of hardness, TDS above 750 ppm or iron above 2 ppm. That does not mean a softener cannot handle it; it means the salt setting and size need more attention. See water softener settings.

Why well water is often hard

USGS explains that as water moves through soil and rock, it dissolves small amounts of naturally occurring minerals; if those minerals are in the ground around a well, hard water is delivered to the home. Its national map shows hard water most common in the east-central and western United States, which USGS links to carbonate aquifers and aquifers with relatively high concentrations of dissolved solids. USGS also cautions that the map uses 1975 data and is not designed for local decisions, so the only way to know your number is to test your own well.

Reading hardness on a well-water report

Look for a line called total hardness, reported “as CaCO3”. That means the calcium, magnesium and other hardness minerals are expressed as an equivalent amount of calcium carbonate, which is the same basis that grains per gallon use.

A few things to check:

• Units. Most labs report mg/L. A few report gpg. If the number is in the hundreds, it is almost certainly mg/L.
• Calcium and magnesium listed separately. If you only see those two lines, ask the lab for total hardness as CaCO3 rather than adding them up yourself, because the raw numbers are not on the same basis.
• Iron and manganese. These also use up softener capacity. WQA’s term for the adjusted figure is “compensated hardness,” which accounts for iron, manganese and some other ions. WQA notes that no single way of calculating it has proven uniformly accurate. A common rule of thumb adds about 4 gpg for each 1 mg/L of iron; the sizing guide explains where that comes from.
• pH. Softeners for well water generally want pH in a normal range. Penn State recommends softeners for iron removal only above pH 6.7.

If iron is above about 0.3 mg/L, read iron and water softeners before you choose equipment.

Testing hardness yourself

Hardness test strips and drop-count kits are cheap and fast. They are good for two jobs:

1. A quick first look before you pay for a lab test.
2. Checking softened water at a tap to confirm the softener is working.

They are less precise than a lab, and they do not tell you iron, manganese, pH or bacteria, all of which matter for a well. Use a certified lab result to choose and size equipment. Your state health department or county extension office can point you to a certified lab.

For checking a working softener, the target is under 1 gpg (17.1 mg/L), the NSF/ANSI definition of soft water that WQA cites. If you see several gpg at the kitchen tap, the softener is out of salt, set wrong, fouled or undersized.

Why the number on the scale is not a health limit

The USGS calls hardness a nuisance, not a health concern. It also cites the World Health Organization’s note that drinking water may contribute calcium and magnesium to the diet, which could matter for people whose intake of those minerals is marginal. The reason to care is cost: scale insulates water heater surfaces, narrows pipes and clogs small passages, and hard water takes more soap to clean. WQA notes that “antiscaling” devices that do not remove calcium and magnesium are not proven to produce soft water, which matters if you are comparing a softener with a salt-free unit.

Decide based on how hard the water is, what the scale is costing you and what else is in the well. For a step-by-step decision, see do you need a water softener for well water.