Water hardness shows up in a confusing mix of units. U.S. test strips and softeners use grains per gallon, city water reports use mg/L or ppm, and European reports use mmol/L or German, French or English degrees. Enter the number you have and pick its unit. The converter shows it in every unit at once and tells you where it falls on the U.S. Geological Survey (USGS) hardness scale.
Your water is
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- Grains per gallon (gpg)—
- mg/L or ppm as CaCO3—
- mmol/L—
- German degrees (°dH)—
- French degrees (°fH)—
- English degrees (°e)—
Classes follow the USGS scale: soft 0 to 60 mg/L, moderately hard 61 to 120, hard 121 to 180, very hard above 180.
Conversion factors
Every unit below measures hardness as if it were all calcium carbonate (CaCO3), so we convert through mg/L as CaCO3. For water, 1 ppm is the same as 1 mg/L.
| Unit | 1 unit equals (mg/L as CaCO3) | 1 unit equals (gpg) |
|---|---|---|
| Grains per gallon (gpg) | 17.12 | 1 |
| mg/L or ppm as CaCO3 | 1 | 0.058 |
| mmol/L (calcium plus magnesium) | 100.09 | 5.85 |
| German degree (°dH) | 17.85 | 1.04 |
| French degree (°fH) | 10 | 0.58 |
| English or Clark degree (°e) | 14.25 | 0.83 |
Two quick rules cover most U.S. cases: divide mg/L (ppm) by 17.1 to get grains per gallon, and multiply grains per gallon by 17.1 to get mg/L. For example, 250 mg/L is about 14.6 gpg, and 10 gpg is about 171 mg/L.
USGS hardness classes
The USGS sorts water into four hardness classes by mg/L as calcium carbonate. Here are the same ranges in grains per gallon and German degrees.
| Class | mg/L (ppm) | gpg | °dH |
|---|---|---|---|
| Soft | 0 to 60 | 0 to 3.5 | 0 to 3.4 |
| Moderately hard | 61 to 120 | 3.6 to 7.0 | 3.4 to 6.7 |
| Hard | 121 to 180 | 7.1 to 10.5 | 6.8 to 10.1 |
| Very hard | More than 180 | More than 10.5 | More than 10.1 |
Soft and moderately hard water rarely needs a softener. Hard and very hard water is where scale on fixtures, spotty dishes and shorter water heater life usually start to show up, and where a softener tends to pay off.
What to do next
- Don’t have a number yet? See how to test water hardness with test strips, a drop kit, a lab test or your city’s water quality report.
- Not sure hardness is your problem? Check the signs of hard water around your home.
- If your water is hard, take the gpg number to the water softener size calculator to find the capacity you need, then estimate salt and water costs with the water softener cost calculator.
Sources: USGS Water Science School, hardness of water; USGS, water hardness in the United States. Unit definitions: 1 grain = 64.8 mg; 1 mmol/L of calcium carbonate = 100.09 mg/L; 1 °dH = 10 mg/L of calcium oxide (17.85 mg/L as CaCO3); 1 °fH = 10 mg/L as CaCO3; 1 °e = 1 grain of CaCO3 per imperial gallon (14.25 mg/L).