Water hardness converter
Grains per gallon, mg/L, ppm and the European degree scales, all at once.
Every hardness unit measures the same thing, the concentration of calcium and magnesium expressed as calcium carbonate. They differ only in the size of the unit, so every conversion is an exact ratio rather than an approximation.
The ratios
| Unit | Symbol | In mg/L | Where it is used |
|---|---|---|---|
| Milligrams per litre | mg/L | 1 | Laboratory reports worldwide |
| Parts per million | ppm | 1 | The same number as mg/L |
| Grains per gallon | gpg | 17.118 | US water treatment and softener sizing |
| German degrees | °dH | 17.848 | Germany, central Europe |
| French degrees | °fH | 10 | France, defined as 10 mg/L exactly |
| Clark degrees | °e | 14.254 | Older UK reports |
| Millimoles per litre | mmol/L | 100.0869 | Chemistry, the molar mass of CaCO3 |
Why a grain is 17.118 mg/L
A grain is one seven-thousandth of a pound, which is 0.064799 grams. A US gallon is 3.785411784 litres. Divide one by the other and you get 0.017118 grams per litre, which is 17.118 milligrams per litre. It is a definition, not a measurement, so it never changes.
Many sources round it to 17.1, which is fine for household purposes and introduces an error of about one part in a thousand.
Common values
| gpg | mg/L and ppm | °dH | °fH | Class |
|---|---|---|---|---|
| 1 | 17 | 1 | 1.7 | Soft |
| 3 | 51 | 2.9 | 5.1 | Soft |
| 5 | 86 | 4.8 | 8.6 | Moderately hard |
| 7 | 120 | 6.7 | 12 | Moderately hard |
| 10 | 171 | 9.6 | 17.1 | Hard |
| 12 | 205 | 11.5 | 20.5 | Very hard |
| 15 | 257 | 14.4 | 25.7 | Very hard |
| 20 | 342 | 19.2 | 34.2 | Very hard |
| 25 | 428 | 24 | 42.8 | Very hard |
| 30 | 514 | 28.8 | 51.4 | Very hard |
| 40 | 685 | 38.4 | 68.5 | Very hard |
| 50 | 856 | 48 | 85.6 | Very hard |
Frequently asked questions
How many mg/L is 1 grain per gallon?
Exactly 17.118 mg/L, which is the same as 17.118 ppm. The figure comes from the definition of a grain as one seven-thousandth of a pound and a US gallon as 3.785411784 litres, so it is arithmetic rather than measurement.
Is ppm the same as mg/L for water hardness?
Yes. One litre of water weighs one kilogram, so one milligram in one litre is one part per million by mass. Water quality reports use the two interchangeably.
How do I convert ppm to grains per gallon?
Divide by 17.118. So 171 ppm is 10 grains per gallon, 250 ppm is about 14.6, and 300 ppm is about 17.5.
What is a German degree of hardness?
One German degree, written °dH, is 17.848 mg/L as calcium carbonate. It is historically defined as 10 mg of calcium oxide per litre. French degrees are simpler, one °fH is 10 mg/L exactly.