The chemistry in one paragraph
Hardness is the sum of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) in water. Rain picks these up as it percolates through limestone and dolomite, which is why borehole water in dolomitic areas is hard while water off sandstone catchments (like much of the Western Cape) is soft. The ions themselves are harmless to drink — hardness is an equipment problem, not a health problem.
Why it matters to equipment
When hard water is heated or concentrated, calcium carbonate comes out of solution as scale. Scale insulates heating elements (a few millimetres can add double-digit percentages to heating energy), narrows pipes, blinds RO membranes, and coats UV quartz sleeves. Hardness also reacts with soap to form scum, which is why laundries and car washes care about it long before scaling appears.
The units — where everyone gets caught
All hardness units express the same thing; they just use different yardsticks.
| Unit | Meaning | 1 unit equals |
|---|---|---|
| mg/L as CaCO₃ | The SA and international lab standard | 1 ppm CaCO₃ |
| German degrees (°dH) | Common on European equipment | 17.85 mg/L CaCO₃ |
| French degrees (°f) | Common on softener datasheets | 10 mg/L CaCO₃ |
| Grains per gallon (gpg) | US softener capacity ratings | 17.1 mg/L CaCO₃ |
The trap: a lab report might state calcium as mg/L Ca, not mg/L as CaCO₃. Multiply mg/L Ca by 2.5 (and mg/L Mg by 4.1) to convert to the CaCO₃ basis before comparing against any datasheet.
Classifying a water
| mg/L as CaCO₃ | Classification |
|---|---|
| 0 – 60 | Soft |
| 61 – 120 | Moderately hard |
| 121 – 180 | Hard |
| > 180 | Very hard |
As a working trigger: geysers, boilers and steam equipment start complaining above roughly 120 mg/L; RO systems need scale control (softening or antiscalant) evaluated at almost any hardness once recovery is pushed, because the concentrate stream is several times more concentrated than the feed.
Temporary versus permanent
Temporary hardness (carbonate hardness) is the fraction paired with bicarbonate alkalinity — it is the part that precipitates when water is boiled. Permanent hardness is paired with chloride and sulphate and stays in solution through boiling. The distinction matters mainly for chemistry calculations; a softener removes both, because ion exchange does not care which anion the calcium arrived with.
Rule of thumb: get everything onto mg/L as CaCO₃ before you compare numbers, and treat 120 mg/L as the point where heated equipment starts paying for hardness. What to do about it — softening, anti-scale cartridges or antiscalant dosing — is covered in the softening section.