What water hardness is
Hard water contains dissolved calcium and magnesium, picked up as water flows through limestone, chalk and other minerals. It is safe to drink, and can even supply some of these minerals, but it leaves scale in kettles, water heaters and pipes, spots on dishes, and makes soap lather poorly.
How it is calculated
Total hardness is expressed as the equivalent amount of calcium carbonate (CaCO₃):
Hardness (mg/L as CaCO₃) = 2.497 × calcium (mg/L) + 4.118 × magnesium (mg/L)
The factors are the ratios of the molar mass of CaCO₃ (100.09 g/mol) to the molar masses of calcium (40.08) and magnesium (24.31).
Example
Water with 80 mg/L of calcium and 10 mg/L of magnesium:
2.497 × 80 + 4.118 × 10 ≈ 241 mg/L as CaCO₃, or about 14.1 grains per gallon: very hard.
Units and conversions
| Unit | Equals, in mg/L as CaCO₃ |
|---|---|
| 1 grain per gallon (gpg) | 17.1 |
| 1 French degree (°f) | 10 |
| 1 German degree (°dH) | 17.8 |
| 1 mmol/L | 100.1 |
| 1 ppm | 1 |
Water softeners in the United States are usually sized in grains per gallon; test strips often show ppm.
How hard is hard?
The U.S. Geological Survey uses these classes:
| mg/L as CaCO₃ | gpg | Class |
|---|---|---|
| 0 to 60 | 0 to 3.5 | Soft |
| 61 to 120 | 3.5 to 7 | Moderately hard |
| 121 to 180 | 7 to 10.5 | Hard |
| over 180 | over 10.5 | Very hard |
Much of the Midwest, Texas and the Southwest has hard or very hard water, while New England, the Pacific Northwest and parts of the Southeast are mostly soft.
Finding your numbers
Your water utility publishes calcium, magnesium or hardness in its annual Consumer Confidence Report. If you have a private well, a lab test or a home test kit gives the result.
What to do about hard water
- Descale kettles and coffee makers with vinegar or citric acid.
- Set the water heater no hotter than needed: scale forms faster at high temperatures.
- A water softener exchanges calcium and magnesium for sodium or potassium. Many people keep a hard-water tap for drinking, especially if they watch their sodium intake.
- There is no federal health limit for hardness; it is an aesthetic and maintenance issue.