Conversion factors
| Unit | Equals |
|---|---|
| 1 mmol/L | 5.61 °dH = 10.0 °fH = 7.02 °e = 100.1 mg/L CaCO₃ |
| 1 °dH | 0.178 mmol/L = 1.78 °fH = 17.8 mg/L CaCO₃ |
| 1 °fH | 0.1 mmol/L = 0.56 °dH = 10.0 mg/L CaCO₃ |
| 1 gpg (US) | 17.1 mg/L CaCO₃ = 0.96 °dH |
Hardness ranges
The German detergent act (WRMG) defines three ranges: soft below 1.5 mmol/L (below 8.4 °dH), medium from 1.5 to 2.5 mmol/L (8.4–14 °dH) and hard above 2.5 mmol/L (above 14 °dH).
Conductivity and dissolved solids
The dissolved solids content (TDS) can be estimated from the electrical conductivity: TDS in mg/L ≈ conductivity in µS/cm × factor. The factor is about 0.5 for sodium chloride solutions and 0.64–0.7 for natural water. For an exact value a laboratory analysis is needed.
Why hardness matters in industry
Hard water forms scale in boilers, heat exchangers, cooling circuits and on reverse osmosis membranes. Softening systems exchange calcium and magnesium for sodium; reverse osmosis removes hardness together with the other salts.
Frequently asked questions
How do I convert °dH to mmol/L?
Multiply the value in °dH by 0.1783. 14 °dH is 2.5 mmol/L.
How do I convert ppm to °dH?
Divide the value in mg/L CaCO₃ (ppm) by 17.85. 250 ppm is 14 °dH.
What is the difference between °dH and °fH?
One German degree (°dH) corresponds to 10 mg/L of calcium oxide, one French degree (°fH) to 10 mg/L of calcium carbonate. 1 °dH equals 1.78 °fH.