Total Hardness
Total hardness measures all divalent mineral ions in water (calcium and magnesium), while pool chemistry specifically uses only calcium hardness.
In Plain Language
Total hardness (as used in household water softening) includes both calcium and magnesium hardness. In pool chemistry, only calcium hardness is relevant because calcium is the ion that participates in calcium carbonate equilibrium (and therefore the LSI). Magnesium does not contribute to pool scaling in the same way. When comparing tap water hardness reports (which give total hardness) to pool targets (calcium hardness), note that calcium typically represents 60–70% of total hardness in most water supplies.
Why It Matters
Distinguishing total hardness from calcium hardness prevents miscalculation when using municipal water quality reports for pool chemistry.
Typical Values
Last reviewed: 2026-06-01