Water Hardness Converter
Water hardness is measured as dissolved calcium and magnesium, reported as ppm of calcium carbonate in the United States and as degrees in Europe. One grain per gallon equals 17.1 ppm, one German degree equals 17.85 ppm, and one French degree equals 10 ppm.
Soft water is under about 60 ppm, moderately hard runs to 120, hard to 180, and very hard is above that.
| Parts per million | Grains per gallon | German degrees | French degrees |
|---|---|---|---|
| 17.1 ppm | 1 gpg | 0.9581 °dH | 1.71 °fH |
| 60 ppm | 3.5088 gpg | 3.3617 °dH | 6 °fH |
| 120 ppm | 7.0175 gpg | 6.7234 °dH | 12 °fH |
| 180 ppm | 10.5263 gpg | 10.0852 °dH | 18 °fH |
| 250 ppm | 14.6199 gpg | 14.0072 °dH | 25 °fH |
| 350 ppm | 20.4678 gpg | 19.61 °dH | 35 °fH |
How to convert water hardness units
Pick your two scales
Choose the unit your reading came in and the unit you want, using the pickers under each field.
Enter the reading
Type the number from your water report or test kit into either field and the other updates live.
Check the scale
Read the note under the result to see whether the value counts as soft, moderately hard, hard, or very hard.
Copy the result
Copy the converted value, or use the reference table to size a softener against common values.
Why use this tool
Six scales in one place
ppm, grains per gallon, German, French and Clark degrees, plus millimoles per litre, all convert between each other.
Both directions at once
Edit either field and the other follows, so you never have to work out which way round the factor goes.
A softness reading
The result carries the usual soft to very hard bands, which is what you actually need when sizing a softener.
Reference table included
Common hardness values are listed across the main scales so you can place a reading at a glance.
Runs entirely in your browser
Nothing you type is uploaded or stored.
About this tool
Water hardness measures how much dissolved calcium and magnesium your water carries, and it is reported on a different scale almost everywhere you look. The United States uses parts per million of calcium carbonate, or grains per gallon on softener equipment. Germany uses degrees of hardness written °dH, France uses °fH, and the United Kingdom historically used Clark degrees written °e. A water report and a softener manual will often disagree simply because they picked different scales.
All of them measure the same thing, so they are fixed multiples of each other. One grain per gallon is 17.1 ppm, one German degree is 17.848 ppm, one French degree is exactly 10 ppm, and one Clark degree is 14.254 ppm. This tool holds ppm as the common base and converts through it, so any scale can be turned into any other by editing either field.
The usual bands are: under 60 ppm is soft, 60 to 120 is moderately hard, 120 to 180 is hard, and anything above 180 is very hard. Those thresholds are what matter in practice, because they decide whether you need a softener at all and how much salt it will use. If you are also working out costs or capacity, the percentage calculator and the unit converter cover the arithmetic around it, and volume converter handles tank sizes in the other direction.
Frequently asked questions
- How do I convert ppm to grains per gallon?
- Divide the ppm value by 17.1. Water at 120 ppm is 7 grains per gallon, and 180 ppm is 10.5 grains per gallon. Multiply by 17.1 to go the other way.
- What is a German degree of hardness?
- Written °dH, it was originally defined as 10 mg of calcium oxide per litre, which works out to 17.848 ppm of calcium carbonate. It is the standard scale on German and Central European water reports.
- What counts as hard water?
- Under about 60 ppm is soft, 60 to 120 is moderately hard, 120 to 180 is hard, and above 180 is very hard. In grains per gallon those work out to roughly 3.5, 7 and 10.5.
- Why does my water report disagree with my softener manual?
- Almost always because they use different scales. Reports often quote ppm or mg/L while softeners are sized in grains per gallon, and the two differ by a factor of 17.1.
- Is ppm the same as mg/L?
- For water they are treated as equal, because one litre of water weighs almost exactly one kilogram. A reading of 120 ppm and 120 mg/L of calcium carbonate mean the same thing.
- Is my data uploaded anywhere?
- No. The conversion runs entirely in your browser and nothing you type leaves your device.
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