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Water Hardness Calculator

Calculate the total hardness of water from its calcium and magnesium content and classify it as soft, moderately hard, hard, or very hard using WHO thresholds.

Input

Enter the calcium and magnesium concentrations (mg/L) shown on a water-quality report or mineral-water label to calculate the total hardness as calcium carbonate and its WHO classification.

mg/L
mg/L

Result

Total hardness (as CaCO₃)

70.5

Unit: mg/L ⁄ WHO classification:Moderately hard

WHO classification

Moderately hard

From calcium

50.0 mg/L

From magnesium

20.5 mg/L

Hardness classification (WHO)

ClassificationHardness range (mg/L)
Soft0 to under 60
Moderately hard (this water)60 to under 120
Hard120 to under 180
Very hard180 and above

How it works

  • Total hardness (mg/L) is expressed as calcium carbonate. The formula is: hardness = calcium (mg/L) × 2.5 + magnesium (mg/L) × 4.1, where each factor converts Ca and Mg into the equivalent amount of calcium carbonate.
  • Classification follows the WHO scale: under 60 mg/L is soft, 60 to under 120 mg/L is moderately hard, 120 to under 180 mg/L is hard, and 180 mg/L and above is very hard.
  • You can use the calcium and magnesium values (in mg/L) printed on a mineral-water label or in a municipal water-quality report exactly as they appear.
  • Hardness affects taste and mouthfeel, the flavor of cooking, coffee, and tea, and how well soap lathers. Soft water tends to feel mild, while hard water has a more pronounced mineral character.
  • How hardness is perceived and which uses it suits varies from person to person, and the figures can change with the labeled values and the season of collection. Use the result as a rough guide only.
  • This tool provides reference values for water quality and does not assess or diagnose drinking suitability or health effects. For an official evaluation, refer to certified water-quality test results.