Water Hardness Converter
This water hardness converter turns a value in one unit into all others, mmol/l, °dH, °fH, Clark degrees, grains per gallon and ppm, and shows the hardness class from soft to very hard.
Example: Water hardness 12 · Unit: °dH (German degrees) → Water hardness: 12 °dH = 2.14 mmol/l, very hard (over 180 mg/l). Source: Waschmittelgesetz (WRMG) § 9 – Härtebereiche weich/mittel/hart in mmol/l. Updated: .
How it is calculated
12 °dH equal 2.14 mmol/l, 21.4 °fH and about 214 mg/l CaCO₃, which is very hard by the USGS scale. Water hardness says how much dissolved calcium and magnesium the water contains.
How it is calculated
All values are converted via the amount of substance in mmol/l. 1 °dH equals 10 mg of calcium oxide per litre, which is 0.1783 mmol/l. 1 °fH is 10 mg of calcium carbonate per litre (0.1 mmol/l), and 1 ppm equals 1 mg/l CaCO₃. The English (Clark) degree and the US grains per gallon refer to 1 grain of calcium carbonate per imperial or US gallon. The class follows the USGS scale.
Why hardness matters
- Dose detergent and dishwasher salt correctly.
- Judge limescale build-up in coffee machines, kettles and boilers and plan descaling.
- Set water softeners and filters to the right value.
Frequently asked questions
How do you convert °dH to mmol/l?
Multiply by 0.1783: 12 °dH is 2.14 mmol/l. Conversely 1 mmol/l is about 5.6 °dH.
How do you convert ppm to grains per gallon?
Divide ppm (mg/l CaCO₃) by 17.12 for US grains per gallon. 100 ppm is 5.84 gpg.
When is water hard?
By the USGS scale above 120 mg/l CaCO₃ (hard) and above 180 mg/l (very hard). Soft water is up to 60 mg/l.
What is the difference between total and carbonate hardness?
Total hardness covers all calcium and magnesium salts; carbonate hardness is only the part present as bicarbonate that precipitates as limescale when water is heated.
Sources and legal basis
- Waschmittelgesetz (WRMG) § 9 – Härtebereiche weich/mittel/hart in mmol/l
- USGS Water Science School – Hardness of Water (classes by mg/l CaCO₃)
- Wikipedia – Wasserhärte (Umrechnung °dH, °fH, °e, gpg, mmol/l)
As of: · Editorial policy
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