Water hardness converter
Your utility report might say ppm or mg/L, a test strip might say gpg, and a European dishwasher might want °dH. Enter any one value to see all of them.
Conversion factors
| 1 unit of… | equals mg/L (ppm) as CaCO₃ | Definition |
|---|---|---|
| gpg | 17.118 | 1 grain (64.8 mg) CaCO₃ per US gallon |
| mmol/L | 100.09 | Molar mass of CaCO₃ |
| °dH (German) | 17.848 | 10 mg/L CaO |
| °e (English/Clark) | 14.254 | 1 grain CaCO₃ per Imperial gallon |
| °fH (French) | 10 | 10 mg/L CaCO₃ |
Factors: Wikipedia, Hard water: unit conversion table and definitions (checked 2026-10-27); the gpg factor matches Maryland Geological Survey: 1 gpg = 17.118 mg/L. ppm and mg/L are treated as equal, which holds for fresh water.
Hardness scale
| USGS class | mg/L (ppm) as CaCO₃ | Grains per gallon (÷ 17.1) |
|---|---|---|
| Soft | 0–60 | 0.0–3.5 |
| Moderately hard | 61–120 | 3.6–7.0 |
| Hard | 121–180 | 7.1–10.5 |
| Very hard | over 180 | over 10.5 |
Classes: U.S. Geological Survey, Hardness of Water (checked 2026-10-27). Conversion: Maryland Geological Survey: 1 gpg = 17.118 mg/L.
Size a softener for this hardness → · Size chart · Salt cost calculator
FAQ
How many ppm is 1 grain per gallon?
1 gpg is 17.118 mg/L (ppm) as calcium carbonate. Our sizing calculator rounds this to 17.1, as most softener makers do.
What is a German degree of hardness?
1 °dH is 10 mg/L of calcium oxide, which equals 17.848 mg/L as calcium carbonate. It's common on European appliances and aquarium test kits.
What hardness is considered hard water?
The USGS calls 121–180 mg/L hard and over 180 mg/L very hard, which is about 7.1–10.5 gpg and over 10.5 gpg.