Water Hardness Scale: gpg, ppm and mg/L Explained

Water hardness is measured as calcium carbonate, in grains per gallon (gpg) or milligrams per liter (mg/L, the same as ppm). On the scale Penn State Extension publishes, under 1 gpg (17 mg/L) is soft, 1–3.5 gpg slightly hard, 3.5–7 gpg moderately hard, 7–10.5 gpg hard, and over 10.5 gpg (180 mg/L) very hard. USGS uses a simpler mg/L scale: 0–60 soft, 61–120 moderately hard, 121–180 hard, over 180 very hard. To convert, 1 gpg is about 17.1 mg/L.

The scale at a glance

Each scale sits in its own columns because the two don’t define the low end the same way.

Nebraska Extension prints the same five classes and boundaries in its softening guide and says its table is “adapted from the Water Quality Association (WQA).” The Water Check classifies your number on the Penn State scale.

gpg, ppm and mg/L

• mg/L and ppm are the same number. Penn State: “Parts per million is also equal to milligrams/liter (mg/l).”
• gpg is a hardness-only unit. Penn State: “A gpg is used exclusively as a hardness unit and equals approximately 17 mg/l or ppm.” Hach’s product page for its Model 5-B drop-count kit uses a more precise figure: “1 drop = 1 gpg (or 17.1 mg/L as CaCO₃).” This site uses 17.1.
• “As calcium carbonate” (CaCO₃). Hardness comes from calcium and magnesium, but labs report it as one number. Nebraska Extension: “For consistency, concentrations are generally converted to the equivalent concentration as calcium carbonate (CaCO3) and expressed in terms of hardness as calcium carbonate.” If a report lists calcium and magnesium separately but no total hardness, ask the lab for total hardness as CaCO₃ rather than adding the two numbers yourself.

Where classifications differ

• The soft end. USGS calls everything up to 60 mg/L soft. Penn State and the WQA-based table split that range into soft (under 17 mg/L) and slightly hard (17–60 mg/L). So 45 mg/L is “soft” to USGS and “slightly hard” to Penn State. Neither label is wrong; they’re different scales.
• Shared boundary numbers. Penn State’s ranges share endpoints (“1.0 to 3.5”, “3.5 to 7.0”), so a reading of exactly 3.5 or 7.0 gpg fits two rows. Penn State’s mg/L edges are rounded (60, 120, 180), so they don’t line up exactly with the gpg edges. At 17.1, 120 mg/L works out to 7.02 gpg. If your number sits right on a line, the class matters less than the number itself.
• The USGS well study. The USGS domestic-well study calls water “soft” below 60 mg/L and “hard” above 120 mg/L.

Converting your number

• mg/L (ppm) to gpg: divide by 17.1. 200 mg/L ÷ 17.1 = 11.7 gpg.
• gpg to mg/L (ppm): multiply by 17.1. 10 gpg × 17.1 = 171 mg/L.

Conversions are ours, at 17.1 mg/L per gpg. As a cross-check, Hach lists its kit’s range as “1-30 gpg (17-513 mg/L as CaCO₃),” the same factor. The Water Check accepts either unit and converts for you. If you don’t have a number yet, see how to test water hardness.

What each level means for pipes and appliances

The class name is a label. What sources say about the number is more useful:

Two practical notes. First, the number matters after you buy, too. MDH says to “Make sure the softener is set to the hardness of your water supply,” because “If the hardness is set too high, the softener will cost more to operate and waste water.” Second, hardness alone doesn’t size a softener. Nebraska Extension: “The appropriate size of water softener depends on several factors including the water hardness level, daily water use and water flow rate.” See what size water softener do I need. For what hardness looks like around the house, see what is hard water.