Water Hardness Units: ppm, mg/L, dH and gpg

The same water can show up as a very different number depending on the unit. Each one can be correct. You just need to know which unit you're looking at.

The units you'll meet

ppm and mg/L. Parts per million of calcium carbonate is equal to milligrams per liter.¹ Wikipedia says ppm is equivalent to mg/L without chemical compound specified.² For hardness, you can treat them as the same number.

gpg. Grains per gallon. A gpg is used only for hardness and equals approximately 17 mg/L or ppm.¹ Wikipedia gives a more exact figure, 17.118 ppm.²

dH. German degrees, written °dH or dGH. One degree is defined as 10 mg/L of calcium oxide, or 17.848 ppm.² Some manuals and test strips use it, so check the label.

Wikipedia also lists English, or Clark, degrees and French degrees.² You may see those on older or imported products.

A conversion table

To get from mg/L to gpg, divide by about 17. To go from gpg to mg/L, multiply.¹ The small differences in the table won't matter much when you're only trying to place your water in a band.

Say a report gives a figure in mg/L and your manual wants gpg. Divide, round it, and you'll land close enough to pick a setting. If you're near the edge of a band, go back to the manual and see whether it gives a tolerance.

Reading the bands

The USGS calls water soft from 0 to 60 mg/L as calcium carbonate, and hard from 121 to 180 mg/L.³ Penn State's classification, in gpg, calls water soft when it's less than 1.0.¹

Bands differ slightly between sources. Penn State, for example, adds a "slightly hard" step.¹ Don't worry about the exact edge. If your water sits close to a boundary, treat it as being in between.

Your manual may use levels instead

Many machines don't ask for a number at all. De'Longhi's fully automatic machines use levels, and the maker describes them as running from level 1 for soft up to level 4 for hard.⁴ Manuals vary, so check the scale in your own manual.

That means there are two translations. First turn the number from your report into a plain mg/L or gpg value. Then match it to the table in your own manual. Each maker draws its own lines, so use the maker's table and nobody else's.

A quick checklist

• Write down the unit next to every number you collect.
• Convert only when two numbers use different units.
• Keep the original number as well as the converted one.
• When the manual and a report disagree, follow the manual's scale for the machine.

No number yet? You can look one up from your supplier, or measure it at home.