Total Hardness as CaCO₃

Hardness is a property, not a substance: it is the combined effect of every divalent cation in the water, and in practice calcium and magnesium account for essentially all of it. Because they have different atomic weights, the trade converts both to a single reference salt, calcium carbonate, so that numbers from different labs, different waters and different treatment steps can be added and subtracted. The factors come straight from equivalent weights: 50.04/20.04 = 2.497 for calcium and 50.04/12.15 = 4.118 for magnesium. A well water reporting 80 mg/L Ca and 25 mg/L Mg therefore has a total hardness of 2.497 × 80 + 4.118 × 25 = 199.8 + 103.0 = 303 mg/L as CaCO₃, or about 17.7 grains per gallon.

The single most common field error is reading a lab sheet that already says "as CaCO₃" and multiplying by 2.497 anyway: instant 2.5× overstatement, an oversized softener and a customer paying for salt they do not need. Check the units column before you touch the calculator. The second trap is the magnesium factor: because Mg²⁺ is light, a modest 25 mg/L of it contributes as much hardness as 41 mg/L of calcium, which is why waters from dolomitic aquifers feel far harder than their calcium number suggests and why magnesium-heavy water scales heat exchangers with a stubborn magnesium silicate rather than easily-acid-cleaned calcite.