Water hardness unit converter
Enter the total hardness from your supplier report or test kit and choose its unit. You will see the same hardness in every common unit, plus the Drinking Water Inspectorate (DWI) description.
Enter your result (0 or more) to see the conversions.
These are equivalent units, not new tests. Do not enter calcium alone, conductivity or a TDS-meter reading. Rounded conversions can differ slightly from supplier tables.
What the hard and soft bands mean
DWI describes hardness in mg/l as calcium carbonate. The table shows where each band starts and ends in the other units. The bands describe limescale and soap lather. They are not safety grades, and a hardness band says nothing about whether water meets other drinking-water standards.
| DWI band | mg/l | mmol/l | °Clark | °dH | °fH | gpg |
|---|---|---|---|---|---|---|
| Soft | up to 100 | up to 1.00 | up to 7.00 | up to 5.60 | up to 10 | up to 5.84 |
| Slightly hard | 100–150 | 1.00–1.50 | 7.00–10.50 | 5.60–8.40 | 10–15 | 5.84–8.76 |
| Moderately hard | 150–200 | 1.50–2.00 | 10.50–14.00 | 8.40–11.20 | 15–20 | 8.76–11.68 |
| Hard | 200–300 | 2.00–3.00 | 14.00–21.00 | 11.20–16.80 | 20–30 | 11.68–17.53 |
| Very hard | over 300 | over 3.00 | over 21.00 | over 16.80 | over 30 | over 17.53 |
WaterGrade counts each band’s upper value in that band; for example, exactly 200 mg/l is moderately hard. Band limits from DWI: water hardness. DWI also says to ask your water company for the hardness in your area.
Worked example
264 mg/l as CaCO₃ is about 2.64 mmol/l, 18.48 Clark degrees, 14.78 German degrees, 26.40 French degrees or 15.42 US grains per gallon. These all describe the same water: hard, on the DWI scale.
Check your unit before converting
- mg/l and ppm: for drinking water, ppm as CaCO₃ is the same number as mg/l as CaCO₃.
- mg Ca/l (calcium): some reports give total hardness expressed as calcium. That is a different basis: multiply by 2.5 to get mg/l as CaCO₃. Do not enter a separate calcium result as total hardness.
- Grains per gallon: US grains per gallon (gpg) are used by many softener and appliance makers. A grain per imperial gallon is the Clark degree.
- Test strips: if your strip shows a range, keep the range rather than turning it into one precise number.
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Conversion basis
1 mmol/l = 100 mg/l as CaCO₃ and 1 °fH = 10 mg/l. Clark and German degrees use rounded household factors, the same ones used across WaterGrade: 1 mg/l = 0.07 °Clark = 0.056 °dH. By definition, 1 °Clark is one grain per imperial gallon (about 14.25 mg/l) and 1 °dH is 10 mg/l of calcium oxide (about 17.85 mg/l as CaCO₃), so other tables can differ in the second decimal place. 1 US grain per gallon = 17.12 mg/l, from the definitions of the grain (64.79891 mg) and the US gallon (3.785 litres). This tool does not choose a model-specific appliance setting. Prepared by WaterGrade; sources checked 11 September 2026.