Water Hardness Calculator (mg/L, °dH, °f, °Clark, gpg)
Total hardness of tap water from its calcium and magnesium, in mg/L of CaCO₃ and in German, French, English and American degrees, with its class.
Results
What this tool does
Hardness is mostly dissolved calcium and magnesium, and the convention is to express both as the equivalent amount of calcium carbonate: each mg/L of calcium counts for 2.497 and each mg/L of magnesium for 4.118, because magnesium is lighter and so carries more charge per milligram. The national degrees are just different units for the same thing, from 10 mg/L per French degree to 17.848 mg/L per German degree. Your water supplier's analysis normally lists calcium and magnesium; the USGS bands run from soft below 60 mg/L to very hard above 180.
Formula
hardness (mg/L CaCO₃) = 2.497 × Ca + 4.118 × Mg · °dH = ÷ 17.848 · °f = ÷ 10 · °Clark = ÷ 14.254 · gpg = ÷ 17.118
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ca | Calcium (Ca²⁺) | mg/L |
mg | Magnesium (Mg²⁺) | mg/L |
H | Total hardness as CaCO₃ | mg/L |
DH | German degrees (°dH) | °dH |
FH | French degrees (°f) | °f |
CL | English degrees (°Clark) | °Clark |
GP | US grains per gallon | gpg |
CS | Hardness class (USGS scale) | — |
Worked example
- Calcium (Ca²⁺)30 mg/L
- Magnesium (Mg²⁺)8 mg/L
- Total hardness as CaCO₃107.9 mg/L
- German degrees (°dH)6.0 °dH
- French degrees (°f)10.8 °f
- English degrees (°Clark)7.6 °Clark
- US grains per gallon6.3 gpg
- Hardness class (USGS scale)Moderately hard
Limitations
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.