211.6 mg/L as CaCO₃ — Very hard
- mmol/L
- 2.114
- German °dH
- 11.86
- French °f
- 21.16
- Grains/US gal
- 12.36
- Calcium as CaCO₃ = 60 × (100.09 ÷ 40.08) =
149.8 mg/L - Magnesium as CaCO₃ = 15 × (100.09 ÷ 24.31) =
61.77 mg/L
Classification (USGS): 0–60 soft, 61–120 moderately hard, 121–180 hard, above 180 very hard. Hard water forms limescale (CaCO₃) in kettles, boilers and pipes and reduces how well soap lathers.
How it works
Hardness is expressed as the mass of calcium carbonate that would contain the same number of moles of Ca²⁺ and Mg²⁺. Each mg/L of calcium corresponds to 100.09 ÷ 40.08 = 2.497 mg/L of CaCO₃, and each mg/L of magnesium to 100.09 ÷ 24.31 = 4.118 mg/L.
One German degree (°dH) is 10 mg of CaO per litre (17.85 mg/L as CaCO₃); one French degree (°f) is 10 mg/L as CaCO₃; one grain per US gallon is 17.12 mg/L. The classification follows the US Geological Survey scale.
Frequently asked questions
- What makes water hard?
- Dissolved calcium and magnesium ions, picked up as water flows through limestone, chalk and dolomite. On heating, calcium hydrogencarbonate decomposes and deposits limescale.
- How do I convert °dH to ppm?
- Multiply by 17.85. For example, 10 °dH is about 178 mg/L (ppm) as CaCO₃ — hard water.
- How is hardness measured in the lab?
- Usually by titration with EDTA using an indicator such as Eriochrome Black T, which changes colour when all the Ca²⁺ and Mg²⁺ have been complexed.
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