The factors for conversion of units relating to different substances are approximate, assuming a distribution of hardness as common in natural waters.
| Quantity | Unit | mmol/l | mval/l | °dH | °eH | °fH | ppm CaCO3 |
| total alkaline earths | 1 mmol/l = | 1 | 2 | 5.6 | 7.02 | 10 | 100 |
| total alkaline earths | 1 mval/l = | 0.5 | 1 | 2.8 | 3.51 | 5 | 50 |
| German hardness | 1 °dH = 10 mg/l CaO = | 0.1783 | 0.357 | 1 | 1.253 | 1.78 | 17.8 |
| English hardness | 1 °eH = 1 grain/gallon CaCO3 = | 0.142 | 0.285 | 0.798 | 1 | 1.43 | 14.3 |
| French hardness | 1 °fH = 10 g/m³ CaCO3 = | 0.1 | 0.2 | 0.560 | 0.702 | 1 | 10 |
| hardness as CaCO3 | 1 ppm CaCO3 = 1 mg/kg CaCO3 = | 0.01 | 0.02 | 0.056 | 0.07 | 0.1 | 1 |
The units ppm (parts per million) = mg/kg and ppb (parts per billion) = μg/kg can be converted to mg/l and μg/l using the factor 1 (approximate, exact at a water temperature of 4 °C).
| Unit | mmol/l | mval/l | mg/l | ppm CaCO3 |
| 1 mmol/l = | 1 | [valency] | [molar mass] | 100.09 |
| 1 mval/l = | 1 / [valency] | 1 | [molar mass] / [valency] | 100.09 / [valency] |
| 1 mg/l = | 1 / [molar mass] | [valency] / [molar mass] | 1 | 100.09 / [molar mass] |
| 1 ppm CaCO3 = | 1 / 100.09 | [valency] / 100.09 | [molar mass] / 100.09 | 1 |
The factors for conversion between conductivity and salt content are approximate, assuming a water temperature of 20 °C and a conductivity of ≥ 1 μS/cm and ≤ 1000 μS/cm.
| Quantity | Unit | μS/cm | MΩ cm | mmol/l NaCl | mg/l NaCl |
| conductivity σ | 1 μS/cm = | 1 | 1 / σ² | 0.009 | 0.53 |
| resistivity ρ | 1 MΩ cm = | 1 / ρ² | 1 | 111 | 1.89 |
| salt content | 1 mmol/l NaCl = | 111 | 0.009 | 1 | 58.44 |
| salt content | 1 mg/l NaCl = | 1.89 | 0.53 | 0.0171 | 1 |