Millimole per Litre to Degree of Hardness Converter
Water chemistry results are often reported in millimoles per litre, the SI-based unit for the amount of calcium carbonate dissolved in water, while many European water reports and appliance settings use the German degree of hardness, written dH. This converter changes millimoles per litre into German degrees so you can compare a laboratory chemistry result against a hardness guideline or a softener specification. It works in two steps through a common reference concentration. First the calculator multiplies the millimoles per litre figure by 100.0869, the molar mass of calcium carbonate in grams per mole, to get the concentration in milligrams per litre. Then it divides that milligrams per litre figure by 17.848, the number of milligrams per litre of calcium carbonate that defines one German degree of hardness. Because the molar mass and the degree definition are both fixed standard constants, the conversion is exact rather than an estimate, and the same input always returns the same output. Enter your own millimoles per litre figure to read the equivalent hardness in German degrees. The complete formula and a worked example that reconciles exactly to the calculator default are shown below so you can verify every step.
To convert, multiply millimoles per litre by 100.0869 to get mg/L of CaCO3, then divide by 17.848: dH = mmol/L x 100.0869 / 17.848. A reading of 1 mmol/L equals 100.09 mg/L, which is 5.61 dH.
Formula
mg/L = mmol/L x 100.0869
dH = mg/L / 17.848
100.0869 = molar mass of CaCO3 in g/mol
17.848 = mg/L of CaCO3 per German degree
The amount of substance per litre is first restated as a mass concentration in milligrams per litre using the molar mass, then divided by 17.848 to give German degrees.
Worked example
A chemistry result shows total hardness of 1 millimole per litre of calcium carbonate.
- mg/L = 1 x 100.0869 = 100.0869 mg/L of CaCO3
- dH = 100.0869 / 17.848 = 5.6077
- Rounded to two decimal places, the result is 5.61 dH
These are the calculator's default inputs, so the result above matches the widget exactly.
Common hardness conversions
| mmol/L | mg/L as CaCO3 | German degrees (dH) |
|---|---|---|
| 0.5 | 50.04 | 2.80 |
| 1 | 100.09 | 5.61 |
| 2 | 200.17 | 11.22 |
| 3 | 300.26 | 16.82 |
| 5 | 500.43 | 28.04 |
Molar mass and unit definitions follow US National Institute of Standards and Technology reference values.
Millimole per Litre to Degree of Hardness Converter: frequently asked questions
What is the molar mass of calcium carbonate?
The molar mass of calcium carbonate (CaCO3) is 100.0869 grams per mole, the sum of the atomic masses of one calcium, one carbon and three oxygen atoms. It is the conversion factor between millimoles per litre and milligrams per litre.
What is a German degree of hardness?
A German degree of hardness (dH) is a water hardness unit where one degree equals 17.848 milligrams per litre of calcium carbonate. It is common on European water reports and appliance specifications.
How many German degrees is 1 mmol/L?
One millimole per litre of calcium carbonate is 100.09 mg/L, which divided by 17.848 gives 5.61 German degrees of hardness.
Can I convert back to grains per gallon?
Yes. Convert your millimoles per litre to milligrams per litre using the molar mass, then divide by 17.118 to get grains per US gallon.
Are these figures exact?
Yes, to the precision shown. The molar mass and the degree definition are fixed constants, so the calculator computes the conversion deterministically rather than estimating it.
Official sources
- Molar mass and unit reference values: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.