What this calculator does
Water hardness (as CaCO₃) works out total water hardness as calcium carbonate equivalent, from calcium and magnesium ions. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
The inputs explained
| Field | What to enter |
|---|---|
| Calcium concentration [Ca²⁺] (mg/L) | A number, measured in mg/L. Starts at 40. |
| Magnesium concentration [Mg²⁺] (mg/L) | A number, measured in mg/L. Starts at 10. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with calcium concentration [ca²⁺]
Every other input is held at the calculator’s starting values while calcium concentration [ca²⁺] varies. Select any row to load that scenario into the calculator.
| Calcium concentration [Ca²⁺] (mg/L) | Water hardness | Hardness in grains per gallon | USGS classification |
|---|---|---|---|
| 20 | 91.1 mg/L as CaCO₃ | 5.32 | moderately hard |
| 30 | 116.1 mg/L as CaCO₃ | 6.78 | moderately hard |
| 40 | 141.1 mg/L as CaCO₃ | 8.24 | hard |
| 60 | 191.0 mg/L as CaCO₃ | 11.16 | very hard |
| 80 | 240.9 mg/L as CaCO₃ | 14.08 | very hard |
| 120 | 340.8 mg/L as CaCO₃ | 19.91 | very hard |