Water Hardness

Water often has minerals dissolved in it. The higher the amount of dissolved minerals, the harder the water is.  Water hardness is important because the harder the water is, the more competition the minerals make for the surfactants in detergent.  Detergents lather up with greater difficult in hard water.  Hard water also forms deposits that clog plumbing and build up around taps, which usually look like off-white solids on the inside surfaces of pipes and heat exchanges [1]

The most important minerals in hard water are calcium ions (calcium sulphate and calcium chloride) and magnesium ions (magnesium sulphate and magnesium chloride).  Other minerals eg dissolved bicarbonates) are less important because they dissolve in hotter water. 

Modern detergents contain water softeners eg washing soda (sodium carbonate), this is the compound in powder detergents that make the powder a powder. The precise mixture of minerals dissolved in the water, together with the water’s pH and temperature determine the behaviour of the hardness, a single-number scale does not adequately describe hardness.  However, the United States Geological Survey uses the following classification into hard and soft water[2]:

Classificationhardness in mg of calcium carbonate (CaCO3/L) or parts per millionWhat this means for you
Soft0–60Water is soft and will lather very well.  Suddy detergents in front loaders may cause oversudding in very soft water locations, which means a poorer clean
Moderately hard61–120Most detergents will work well for you, but you may consider adding a booster for poorer quality detergents
Hard121–180Add water softeners routinely
Very hard≥ 181 
Table 1 Classification of water hardness (CaCO3/L), US Geological Survey
Source:  The US Geological Survey

Water hardness in Australia

Analysis of water hardness in major Australian cities by the Australian Water Association shows a range from very soft (Hobart, Melbourne, Darwin) to moderately hard (Brisbane, Adelaide) – see map[3]. Total Hardness levels of calcium carbonate in ppm are: Canberra: 40;[32] Melbourne: 10–26;[33] Sydney: 39.4–60.1;[34] Perth: 29–226;[35] Brisbane: 100;[36] Adelaide: 134–148;[37]Hobart: 5.8–34.4;[38] Darwin: 31.[39] 

Water hardness in New Zealand

New Zealand has almost exclusively soft to very soft water.

Water hardness in the UK

Water hardness varies a lot by region in the UK. Regions with very soft water include parts of Wales and Scotland. See map: https://goo.gl/images/guXrsm

How do I find out how hard my water is?

You can look up the location of your water and the hardness in an internet search engine.  If you have water that is very locally sourced eg a bore, you can use test kits from pool shops.  Rain water is typically very soft.

Water hardness in the United States

Water hardness in North America is typically hard to very hard.  See maps of water hardness in the following link: USGS – U.S. Geological Survey Office of Water Quality. “USGS Water-Quality Information: Water Hardness and Alkalinity”usgs.gov.

When should I use a water softener?

UK households do not need to add water softener eg sodium carbonate (washing soda) or Calgon, unless water hardness is over 300ppm, recommended detergent amounts account for water hardness.

Households in the US may need to use water softeners routinely above 250-300 ppm. Washing soda or borax may be suitable for lower ranges of very hard water, Calgon is another alternative.


[1] “Hard water”.  Wikipedia https://en.wikipedia.org/wiki/Hard_water

[2] USGS – U.S. Geological Survey Office of Water Quality. “USGS Water-Quality Information: Water Hardness”.https://www.usgs.gov/special-topic/water-science-school/science/hardness-water?qt-science_center_objects=0#qt-science_center_objects Accessed 23 April 2020


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