Water Hardness as CaCO₃ Calculator
Got Ca and Mg from the lab as mg/L ion? Convert to total hardness as CaCO₃ — the unit every softening and scaling calculation expects — and see where your water lands on the hardness scale.
Total Hardness
| Item | Value |
|---|
How the calculation works
Formulas
Hardness (mg/L as CaCO3) = 2.497 × Ca (mg/L as ion) + 4.118 × Mg (mg/L as ion)
2.497 = 50.04 / 20.04 (equivalent weights CaCO3 / Ca²⁺)
4.118 = 50.04 / 12.15 (equivalent weights CaCO3 / Mg²⁺)
Hardness (mg/L as CaCO3) = 2.497 × Ca (mg/L as ion) + 4.118 × Mg (mg/L as ion)
2.497 = 50.04 / 20.04 (equivalent weights CaCO3 / Ca²⁺)
4.118 = 50.04 / 12.15 (equivalent weights CaCO3 / Mg²⁺)
| USGS classification | Hardness (mg/L as CaCO3) |
|---|---|
| Soft | 0 – 60 |
| Moderately hard | 61 – 120 |
| Hard | 121 – 180 |
| Very hard | over 180 |
Notes & limitations
The 2.497 and 4.118 factors are the ratios of equivalent weights (CaCO3 = 50.04; Ca²⁺ = 20.04; Mg²⁺ = 12.15) — the standard conversion used in Standard Methods (2340 Hardness). The classification bands are the USGS hardness scale.
- Enter Ca and Mg as the ion (mg/L), not as CaCO3 — if your lab already reports them as CaCO3, just add them together.
- This covers calcium + magnesium hardness only. Strontium, iron, or manganese contribute in unusual waters; check the full cation scan if the numbers do not reconcile.
- Hardness as CaCO3 is also the unit used by softening calculators and the Langelier index on this site.
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