Water Hardness Converter (dGH, dKH, ppm)

The conversion factors

Water hardness is the amount of dissolved calcium and magnesium, usually expressed as the equivalent amount of calcium carbonate (CaCO3). The common units relate like this:

• 1 °dH (dGH or dKH) = 17.848 mg/L (ppm) CaCO3 (defined as 10 mg/L of calcium oxide)
• 1 mmol/L = 100.09 mg/L CaCO3 = 5.60 °dH
• 1 °fH (French) = 10 mg/L CaCO3
• 1 °e (English / Clark) = 14.25 mg/L CaCO3 (one grain per imperial gallon)

dGH and dKH use the same unit, a German degree. The names only say what was measured: GH is general (total) hardness, calcium and magnesium, and KH is carbonate hardness (alkalinity), which buffers pH.

Example

Water with 6 dGH has 6 × 17.848 = 107.1 ppm as CaCO3, which is 1.07 mmol/L, 10.7 °fH or 7.5 °e.

Watch out

• Some test kits report KH in ppm, others in dKH. Check which one your kit uses before comparing with a target.
• The word "ppm" in aquarium use almost always means mg/L as CaCO3. A reading in ppm of calcium alone is a different number.
• Different sources round the factor to 17.86 or 17.9; the difference is well under 1%.

Frequently asked questions

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