Grains per Gallon vs mg/L: Reading Any Hardness Number

One conversion

One grain per US gallon is 17.1185 mg/L of calcium carbonate. Divide mg/L (or ppm, which is the same thing for water) by 17.1 to get grains per gallon. Utility reports and the USGS use mg/L; softener sellers, dishwasher manuals and most test kits use gpg.

The two scales do not line up

The USGS calls water hard above 120 mg/L (7 gpg) and very hard above 180 mg/L (10.5 gpg). The Water Quality Association scale used by the softener trade calls anything above 7 gpg hard and above 10.5 gpg very hard. HardnessCheck draws its verdict on the gpg scale: under 3.5 gpg you do not need a softener, 3.5 to 7 is optional, 7 to 10.5 is worth it, and above 10.5 it pays its way.

Where the country falls

Of 3,035 counties: 740 (24%) are under 3.5 gpg, 434 (14%) are optional, 431 (14%) are worth it, and 1,430 (47%) are above 10.5 gpg.

Reading a test strip or utility report

Test strips report in steps (often 0, 1, 3, 7, 10, 15, 25 gpg), so read them as a band, not an exact figure. A utility's annual water quality report may list hardness as a range across its sources; use the upper figure if your utility blends wells and surface water, because that is what your heater sees on its worst week.

Where to go next

See the range across the country: Moore County, NC (0.8 gpg), Warren County, KY (9.7 gpg), Clay County, MO (31 gpg).

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