Water hardness converter: dGH, dKH, ppm CaCO3, mmol/L and gpg

The definitions behind the numbers

Hardness units count different things in different ways, so every conversion runs through one common unit, mg/L “as CaCO3”: the mass of calcium carbonate that would carry the same number of hardness ions.

Unit	Definition	mg/L as CaCO3
German degree (°dH, dGH, dKH)	10 mg/L of CaO	17.848
mmol/L	one millimole of Ca²⁺ + Mg²⁺ per liter	100.09
meq/L	one milliequivalent (half a millimole of a 2+ ion)	50.04
grain per US gallon (gpg)	64.79891 mg CaCO3 per 3.785412 L	17.118
French degree (°fH)	10 mg/L as CaCO3	10
English (Clark) degree	64.79891 mg CaCO3 per 4.54609 L	14.254

The German degree is defined as 10 mg/L of calcium oxide ^[1]; aquarium product directions such as Seachem’s use it for both GH and KH ^[6]. Converting it needs the molar masses of calcium oxide and calcium carbonate, which come from the IUPAC atomic weights: CaO is 56.077 g/mol and CaCO3 100.086 g/mol ^{[2, 3]}, so 10 mg of CaO is equivalent to 10 × 100.086 ÷ 56.077 = 17.848 mg of CaCO3. Grains and gallons use the exact values in NIST’s guide to the SI ^[4].

GH and KH share a unit, not a meaning

GH counts the calcium and magnesium in the water. KH counts carbonate alkalinity: bicarbonate and carbonate, the part of the water’s buffering that carbon dioxide reacts with. The two are often close in tap water from limestone areas, because calcium carbonate supplies both, but they can drift apart: adding Epsom salt raises GH without touching KH, and adding baking soda raises KH without touching GH.

For KH, chemists usually express alkalinity in meq/L. One dKH is 17.848 ÷ 50.04 = 0.357 meq/L, so 1 meq/L is 2.80 dKH. Seachem’s Alkaline Buffer directions use that conversion (“2 meq/L is equal to 5.6 dKH”) ^[5], while its Equilibrium directions round 1 meq/L to 3 dH ^[6]. The difference is rounding, not chemistry.

Worked example

A tap water report gives hardness as 150 mg/L CaCO3:

• 150 ÷ 17.848 = 8.4 °dH
• 150 ÷ 100.09 = 1.50 mmol/L
• 150 ÷ 17.118 = 8.8 gpg

If the same report gives alkalinity as 107 mg/L CaCO3, that is 6.0 dKH, or 2.14 meq/L, the number the CO2 calculator needs.

Where to use the result

• KH in dKH feeds the CO2 estimate.
• GH tells you whether calcium and magnesium need topping up; the dosing calculator shows how many degrees a dose of Epsom salt or gypsum adds.
• Test kits read in steps (often one drop per degree), so a converted figure is only as precise as the test it came from.