Water hardness explained: the mg/L scale and what it means for your home

Water hardness is the amount of dissolved calcium and magnesium in water. In Australia it is reported in milligrams per litre (mg/L) as calcium carbonate. The Australian Drinking Water Guidelines set an aesthetic guideline of 200 mg/L, and they group results into four practical bands. Those bands are about soap, scale and plumbing, not about whether the water is safe to drink. There is no health guideline for hardness.

WaterWatch keeps that official number on every suburb page, then describes it in everyday language. We also rank each supply against its region. A suburb can be harder than most of its neighbours and still sit under the 200 mg/L guideline.

The Australian hardness scale

The Water Corporation repeats the guideline table in its Drinking Water Quality Annual Report. The properties below follow that official wording.

• Under 60 mg/L: soft, and possibly corrosive, depending on pH, alkalinity and dissolved oxygen
• 60 to 200 mg/L: good quality for all domestic uses
• 200 to 500 mg/L: will increase scale formation
• Above 500 mg/L: will cause high-level scaling

"Possibly corrosive" does not mean the water is unsafe to drink. It means very soft water can be more aggressive toward some pipes and fittings, depending on those other characteristics. "Will increase scale formation" is the band where kettles, shower screens and hot water systems are more likely to collect mineral deposit. "High-level scaling" is a much higher reading. In 2024–25, no Perth supply had a typical hardness above 500 mg/L. Two Rocks, at 228 mg/L, was the only Perth locality with a typical reading in the 200 to 500 band.

Total dissolved solids (TDS) is a related but different measure. It is the sum of dissolved salts, including hardness minerals, sodium and chloride. The guidelines give TDS an aesthetic value of 600 mg/L, based on taste, and a palatability table: 0 to 600 good, 600 to 900 fair, 900 to 1200 poor, and above 1200 unpalatable. A high TDS reading can taste salty even when hardness is moderate. Mount Hawthorn, for example, had a 2024–25 typical sodium of 127 mg/L and a typical TDS of 572 mg/L, with hardness of 106 mg/L.

Why other charts disagree

Search results and appliance manuals often use a second chart. In that version, under 60 is soft, about 60 to 120 is moderately hard, 120 to 180 is hard, and above 180 is very hard. That is why some summaries call Yanchep (198 mg/L in 2024–25) or Two Rocks (228 mg/L) "very hard", while the Australian guidelines still place 198 in the good-quality band and 228 in "will increase scale formation".

Both charts measure the same thing, in the same units. They cut the bands in different places. The Water Corporation report uses the Australian guideline table. WaterWatch shows the household bands on each suburb page — soft, moderately hard, hard and very hard — and states the 200 mg/L Australian aesthetic guideline beside the number. When you set a dishwasher, coffee machine or softener, use the unit and the chart in that appliance's manual. Some overseas manuals use grains per gallon. Take any conversion from the manual rather than from a generic chart.

How to read a WaterWatch suburb page

On each locality page the typical reading is the year's mean from the report WaterWatch is currently publishing. The page then describes that number in two ways.

First, a household band: soft under 60 mg/L, moderately hard from 60 to 120, hard from 120 to 180, and very hard from 180 up. That is the chart many people meet in a manual or a search result.

Second, a comparison with the rest of the region: softer than most, about average, or harder than most. In Perth, a "harder than most" label can still be well under 200 mg/L. The page also points at the Australian aesthetic guideline of 200 mg/L. Hardness affects scale and taste, not whether the water is safe to drink.

Suburb pages on WaterWatch still show the 2023–24 Water Corporation report. Figures in this guide are from the newer 2024–25 tables. Where the two disagree, the suburb page is the older report and this guide is the newer one.

To see how the 2024–25 readings play out across Perth, read Does Perth have hard water? If scale in a storage tank or continuous-flow unit is the practical problem, continue to hard water and your hot water system.