What water hardness measures
Water hardness reflects the total concentration of dissolved calcium and magnesium ions, which cause scale buildup, reduce soap effectiveness, and affect the taste of water. It is conventionally expressed as an equivalent mass of calcium carbonate (CaCO3), even though the hardness ions are not actually carbonate.
The formula used
Total hardness (mg/L as CaCO3) = (calcium mg/L × 2.497) + (magnesium mg/L × 4.118). These conversion factors come from the ratio of CaCO3’s molar mass to the equivalent weights of calcium and magnesium. The result is also converted to grains per gallon (divide by 17.118) and degrees French (divide by 10), two other units commonly seen on water reports and softener specifications.
- 0-60 mg/L as CaCO3 is generally considered soft water.
- 61-120 mg/L is moderately hard, 121-180 mg/L is hard, and above 180 mg/L is very hard.
This tool is intended for general reference and educational purposes; use certified laboratory results for regulatory or engineering decisions.