Water hardness unit converter

Understanding the units

What each hardness unit actually measures

Every hardness unit reports the same thing — dissolved calcium and magnesium — but each was defined by a different field for a different instrument, so the numbers look nothing alike. The reference unit is mg/L as calcium carbonate, and ppm is the same number under a different name. Everything else on this page is a fixed multiple of that reference. The scale below is the master table above, unpacked one definition at a time.

mg/L and ppm — the reference

Milligrams per litre as calcium carbonate is what laboratories weigh directly: the mass of hardness minerals in one litre of water, restated as if it were all calcium carbonate. In dilute water one litre weighs essentially one kilogram, so a milligram per litre is one part in a million — which is why ppm and mg/L are the identical number for hardness. When a report says 150 ppm, 150 mg/L, and 150 mg/L as CaCO₃, all three are the same measurement. There is nothing to convert between them.

gpg — the US softener unit

Grains per gallon is the unit water softeners are set in, and it is a mass-per-volume figure like mg/L, just in older units. One grain is 64.79891 mg, and a US gallon is 3.785412 L, so one grain per gallon works out to 64.79891 ÷ 3.785412 = 17.118 mg/L. That is the whole conversion: multiply gpg by 17.118 to reach mg/L, or divide mg/L by 17.118 to reach gpg. Charts that round the factor to 17.1 differ by about a tenth of a percent.

°dH — the German degree

The German degree, written °dH and sometimes labelled dGH on aquarium kits, comes from a different chemical basis: one °dH is 10 mg/L of calcium oxide (CaO), not calcium carbonate. Restating that as calcium carbonate scales it by the molar-mass ratio, 100.087 ÷ 56.077, so one °dH equals 17.848 mg/L. Because 17.848 is close to the grain factor of 17.118, a gpg and a °dH are within about four percent of each other — near, but not the same.

°fH — the French degree

The French degree, °fH, is the tidiest of the lot: it is defined as exactly 10 mg/L of calcium carbonate, so one °fH is 10 mg/L with no rounding involved. Multiply °fH by 10 to reach mg/L, divide mg/L by 10 to reach °fH. This clean factor of ten is why French degrees appear so often in European tables next to °dH.

mmol/L — the molar unit

Some laboratories report hardness in millimoles per litre, counting molecules rather than weighing mass. One millimole of calcium carbonate weighs its molar mass, 100.087 mg, so one mmol/L equals 100.087 mg/L. Multiply mmol/L by 100.087 to reach mg/L, and divide the other way. A millimole per litre is also close to ten French degrees, since 100.087 is close to ten times ten.

Both directions

How the units relate — worked both ways

Every conversion on this page is a single multiply or divide, and the reverse conversion uses the same factor, inverted — there is never a second constant to remember. Take grains: 10 gpg × 17.118 = 171 mg/L, and to go back, 171 ÷ 17.118 = 10 gpg. The multiply and the divide are the same 17.118, run in opposite directions. See gpg to ppm and ppm to gpg for that pair worked out across a full range.

German degrees behave identically with their own factor. 10 °dH × 17.848 = 178 mg/L, and 178 ÷ 17.848 ≈ 10 °dH brings it straight back. French degrees are the easy case — ten times to enter mg/L, ten into to leave — and mmol/L uses 100.087 the same way. The dedicated °dH to ppm and ppm to °dH pages show the German factor run in each direction, and mmol/L to ppm covers the molar unit.

Because each pairing is one reversible factor, you can chain them through the reference unit to reach any two units at all. Convert into mg/L, then out of mg/L into the unit you want, and the master factor table above holds every one of those factors in a single grid. The water hardness calculator does the full chain for a single value in one step.

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