Grains per Gallon to Millimole per Litre Converter
Water hardness is reported in several different units depending on the country and the laboratory, and grains per gallon is the unit you will most often see on US water reports and softener settings. This converter turns grains per gallon (gpg) into millimoles per litre of calcium carbonate, the SI-based unit used in chemistry and in much of the rest of the world. It works in two clear steps. First it multiplies the grains per gallon figure by 17.118 to get the concentration in milligrams per litre of calcium carbonate, using the fixed definitions that one grain is 64.79891 milligrams and one US gallon is 3.785411784 litres. Then it divides that milligrams per litre figure by the molar mass of calcium carbonate, 100.0869 grams per mole, to give millimoles per litre. Because every constant in this chain is a standard fixed value, the result is exact rather than an estimate, and the same input always gives the same output. Enter the hardness from your own water report to compare it against a chemistry result, a softener specification or an international guideline. The full formula and a worked example that matches the calculator default appear below so you can follow every step.
To convert, multiply grains per gallon by 17.118 to get mg/L of CaCO3, then divide by 100.0869: mmol/L = gpg x 17.118 / 100.0869. A reading of 10 gpg equals 171.18 mg/L, which is 1.71 mmol/L of calcium carbonate.
Conversion formula
mg/L = gpg x 17.118
mmol/L = mg/L / 100.0869
17.118 = 64.79891 mg per grain / 3.785411784 L per US gallon
100.0869 = molar mass of CaCO3 in g/mol
Grains per gallon is first restated as milligrams of calcium carbonate per litre, then divided by the molar mass of calcium carbonate to give an amount of substance per litre in millimoles.
Worked example
A US water report shows total hardness of 10 grains per gallon.
- mg/L = 10 x 17.118 = 171.18 mg/L of CaCO3
- mmol/L = 171.18 / 100.0869 = 1.7103
- Rounded to two decimal places, the result is 1.71 mmol/L
These are the calculator's default inputs, so the result above matches the widget exactly.
Common hardness conversions
| Grains per gallon | mg/L as CaCO3 | mmol/L |
|---|---|---|
| 1 | 17.12 | 0.17 |
| 5 | 85.59 | 0.86 |
| 10 | 171.18 | 1.71 |
| 15 | 256.77 | 2.57 |
| 20 | 342.36 | 3.42 |
Definitions of the grain, the US gallon and water hardness reporting follow US federal standards.
Grains per gallon to millimole per litre: frequently asked questions
What is one grain per gallon in mg/L?
One grain per US gallon equals 17.118 milligrams per litre of calcium carbonate (CaCO3). This follows directly from the standard definitions: one grain is 64.79891 milligrams and one US liquid gallon is 3.785411784 litres, so 64.79891 divided by 3.785411784 gives 17.118 mg/L per grain per gallon.
How do I convert mg/L of CaCO3 to millimoles per litre?
Divide the concentration in milligrams per litre by the molar mass of calcium carbonate, which is 100.0869 grams per mole. For example, 171.18 mg/L divided by 100.0869 gives 1.71 millimoles per litre.
Why is water hardness expressed as calcium carbonate?
Calcium carbonate is the conventional reference compound for total hardness because it captures the combined effect of calcium and magnesium ions on a single comparable scale. Most US laboratories and utilities report hardness as mg/L as CaCO3, which makes grains per gallon and millimoles per litre directly interchangeable through its molar mass.
Is this the same as German degrees of hardness?
No. Millimoles per litre is an SI-based unit, while German degrees (dH) are a separate scale where one degree equals 17.848 mg/L of CaCO3. To move between gpg, mmol/L and degrees you convert through the common mg/L figure.
Are these figures exact?
The grain, the US gallon and the molar mass are fixed standard constants, so the conversion is exact to the precision shown. The calculator computes each figure deterministically from those constants, never from an estimate.
Official sources
- Water quality and hardness reporting guidance: US National Oceanic and Atmospheric Administration (NOAA). 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.