Ion Concentration as CaCO₃ Equivalent

C_{\mathrm{CaCO_3}} = C_{\mathrm{ion}} \times \frac{50.04}{\mathrm{EW}}

sig figs

Worked example: 40 mg/L Ca2+ at EW 20.04 → 99.88 mg/L as CaCO3. Press Try an example to run it live, then adjust anything.

Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value. Change any unit and the answer follows.

Where Ion Concentration as CaCO₃ Equivalent fits

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EWEquivalent Weight from Molar Mass and Valence

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Water hardness and softening8 formulas

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Ion Concentration as CaCO₃ Equivalent explained

A water analysis lists a dozen ions with a dozen different atomic weights, and you cannot add or subtract them directly: 20 mg/L of calcium and 20 mg/L of sodium are not the same amount of chemistry. Expressing everything "as CaCO₃" fixes that by converting each ion to the mass of calcium carbonate that carries the same number of charge equivalents. Calcium carbonate has a molar mass of 100.09 and a valence of 2, so its equivalent weight is 50.04 g/eq; divide that by the ion's own equivalent weight and you have the factor. Calcium (EW 20.04) gets 2.497, magnesium (12.15) gets 4.118, sodium (23.0) gets 2.18, bicarbonate (61.0) gets 0.82, and sulphate (48.03) gets 1.04. Forty mg/L of Ca²⁺ becomes 40 × 2.497 = 99.9 mg/L as CaCO₃.

Once every ion is on the CaCO₃ scale you can do the things that make an analysis useful: check that cations balance anions to within a few percent (the classic sanity test on any lab report), split total hardness into carbonate and non-carbonate fractions by comparing hardness to alkalinity, and size a softener or a dealkalizer directly from the numbers. The trap is the word "equivalent": the divisor is the equivalent weight, molar mass divided by charge, not the molar mass. Using 40.08 for calcium instead of 20.04 halves every hardness figure you produce.

Ion Concentration as CaCO₃ Equivalent formula

C_{\mathrm{CaCO_3}} = C_{\mathrm{ion}} \times \frac{50.04}{\mathrm{EW}}

Where

$C_{\mathrm{CaCO_3}}$ = Concentration as CaCO₃ (mg/L)
$C_{\mathrm{ion}}$ = Concentration as the ion (mg/L)
$\mathrm{EW}$ = Equivalent weight of the ion (g/mol)

Where this comes from

Worked from first principles: a definition, a balance or unit arithmetic, so it needs no outside source.

In this toolkit

Water hardness and softening

8 formulasHardness as CaCO₃, grains per gallon, resin volume, salt dose and days between regenerations — sizing and running a softener.

Revised 16 October 2026

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formula.expert. (2026, October 16). Ion Concentration as CaCO₃ Equivalent Formula & Calculator. https://formula.expert/solvers/caco3-equivalent-concentration

https://formula.expert/solvers/caco3-equivalent-concentration