Water hardness calculator explained
Regulatory agencies report hardness in mg/L as CaCO. Calcium and magnesium are the dominant contributors, so the standard conversion is
,
where concentrations are in mg/L. This calculator multiplies the measured {ca} and {mg} values by their equivalence factors and sums them to produce total hardness {hardness}.
Use it to quickly interpret ICP-OES or titration data, classify water (soft, moderate, hard), or size softening equipment.
How the conversion works
The factors 2.497 and 4.118 convert calcium and magnesium mass concentrations to CaCO equivalents by accounting for their molar masses and valence:
For Ca (40.08 g/mol) the factor is 2.497; for Mg (24.31 g/mol) it is 4.118.
Units and conversions
| Quantity | Units | Notes |
|---|---|---|
| mg/L | Calcium concentration from analysis. | |
| mg/L | Magnesium concentration. | |
| Hardness | mg/L as CaCO | Sum of Ca and Mg contributions. |
Worked examples
- Municipal groundwater
, .
At 212 mg/L as CaCO, the water is "hard" (120-180 mg/L is hard, >180 mg/L is very hard).
- Boiler feed sample
, .
This water is very hard and needs softening or chelation before entering a boiler.
Tips and pitfalls
- Include any significant Sr or Fe contributions by converting them to CaCO equivalents when necessary.
- When reporting hardness as grains per gallon, divide mg/L by 17.1.
- Softening ion exchangers are sized based on total hardness; re-run the calculation after blending streams.
- Seasonal variability can swing hardness drastically; build routine sampling into your QA plan.
References and further reading
- USGS - Water hardness conversion table
- U.S. EPA National Secondary Drinking Water Regulations (hardness classification guidance)