Hardness gets reported in at least six different units depending on who wrote the document — your utility uses one, an imported softener manual uses another, a European appliance manual uses a third. Enter a reading in any unit below and every other unit updates instantly, with a plain-English reading of what the number means.
Type into any field. All values are expressed as calcium carbonate (CaCO3) equivalent, which is the convention Australian utilities use.
Classification bands follow the commonly cited soft / moderately hard / hard / very hard scale used by water authorities and the water treatment industry. They describe scaling and soap behaviour, not safety — hardness minerals are not a health contaminant at any level found in Australian supply.
Hardness is the combined concentration of dissolved calcium and magnesium. Because those are two different elements with different atomic weights, the convention is to express both as though they were calcium carbonate — the CaCO3 equivalent. Each country then built its own scale on top of that.
| Unit | 1 unit equals | Where you’ll see it |
|---|---|---|
| mg/L as CaCO3 | — | Australian utility water quality reports |
| ppm (parts per million) | 1 mg/L | Used interchangeably with mg/L |
| Grains per gallon (gpg) | 17.118 mg/L | US-made softeners and cartridges |
| German degree (°dH) | 17.848 mg/L | European dishwashers, coffee machines |
| French degree (°fH) | 10 mg/L | French and Italian appliance manuals |
| Clark / English degree (°e) | 14.254 mg/L | Older UK documentation |
| mmol/L | 100.09 mg/L | Laboratory and scientific reporting |
| meq/L | 50.04 mg/L | Water chemistry calculations |
The practical consequence: a European dishwasher manual telling you to set the softener to “12” means 12 °dH, which is about 214 mg/L — not 12 mg/L. Setting it to the wrong scale is one of the more common causes of an appliance softener doing nothing.
Australian supplies span nearly the entire scale, and the variation within a single city can be larger than the difference between cities. Perth is the clearest example: our own zone data shows readings from 29 mg/L in some metro suburbs to 228 mg/L in Two Rocks — soft to very hard within one supply network.
Before sizing a softener or conditioner, look up your zone rather than your city. A city-level average can be out by a factor of five against the water actually arriving at your tap. Our water quality lookup reports hardness by zone from utility annual reports.
Two practical notes on using a converted figure. Softener sizing depends on hardness and daily water use — the capacity figure on a softener is usually expressed in grains, which is why converting to gpg matters even in Australia. And scale formation is driven by hot-water temperature as much as hardness, which is why the same water causes visible scaling in a kettle and none in a cold tap.
If your reading lands in the hard or very hard bands and you are weighing treatment options, our salt-free conditioner review and softener explainer cover what each approach actually does — and our magnetic descaler review covers what the cheapest option can and cannot deliver.
Divide by 17.118. One grain per gallon equals 17.118 mg/L as CaCO3, so 171 mg/L is about 10 gpg. Grains appear mainly on imported softener specifications; Australian utilities report in mg/L.
For practical purposes, yes. A litre of fresh water weighs roughly a million milligrams, so one milligram per litre is one part per million. Reports use the two interchangeably.
On the commonly cited scale: under 60 mg/L is soft, 61–120 moderately hard, 121–180 hard, and above 180 very hard. Australian supplies cover the full range.
No. Calcium and magnesium are dietary minerals, and the Australian Drinking Water Guidelines treat hardness as an aesthetic and operational parameter — scale, soap performance, taste — not a health-based one.