Hard water is water carrying a lot of dissolved calcium and magnesium, picked up as it moves through rock. It is not a health risk, but it leaves white scale in kettles, geysers and shower heads and makes soap harder to lather. Most Cape Town municipal water is soft, Johannesburg's is moderately hard, and genuinely hard water in South Africa usually comes out of boreholes, so a test rather than a guess should decide whether you need a softener.
What is hard water?
Hardness is a measure of the calcium and magnesium dissolved in water, mostly picked up as groundwater seeps through limestone and other sedimentary rock. Rain starts out with very few minerals, which is why a rainwater tank gives you soft water while a borehole on the same property may not.
Lab results give hardness in milligrams per litre as calcium carbonate (mg/L as CaCO3), the same as parts per million. Softener specifications often use grains per gallon (1 gpg is about 17.1 mg/L), and some test kits use German degrees (1 °dH is about 17.8 mg/L).
Temporary and permanent hardness
The carbonate or "temporary" hardness comes out of the water when it is heated and settles as scale: the crust in your kettle and on a geyser element. The "permanent" hardness stays dissolved even after boiling. Both count towards the total on a test report.
How hard is hard?
These are the USGS bands, and the WHO's background document on hardness gives almost identical ranges:
| Total hardness (mg/L as CaCO3) | Approx. gpg | Classification | What it usually means |
|---|---|---|---|
| 0-60 | 0-3.5 | Soft | Scale is rarely an issue |
| 61-120 | 3.6-7 | Moderately hard | Light scale in kettles over time |
| 121-180 | 7.1-10.5 | Hard | Regular scale on kettles, taps and shower glass |
| Above 180 | Above 10.5 | Very hard | Heavy scale on anything that heats water |
The UK drinking water regulator sums up the WHO position: water at 200 mg/L or more will produce scale, while water at 100 mg/L or less is more corrosive to pipework.
Signs of hard water at home
- Kettle scale: a white or cream crust on the element that comes back after cleaning.
- Crusty shower heads and taps: white deposits around aerators and nozzles, and spray holes that slowly block.
- Spots on glass: shower doors and drinking glasses that dry with white marks.
- Poor lather: calcium reacts with soap to form soap scum, so you need more soap and detergent.
- Geyser trouble: scale on the element lowers the efficiency of an electric geyser, raises the cost of heating water and shortens the element's life.
Not every mark is hardness. Orange-brown or black staining comes from iron or manganese and needs a different fix, covered in our borehole water treatment guide. Water that tastes salty may have high total dissolved solids (TDS) rather than, or as well as, high hardness.
How hard is the water where you live?
Cape Town
Most of Cape Town's supply comes from dams and is soft. In the City's published analysis for July 2016 to June 2017, the five large treatment works (Blackheath, Faure, Steenbras, Voëlvlei and Wemmershoek) supplied water with a typical hardness of 26 to 50 mg/L. Two smaller supplies stood out: Witzands, serving Atlantis, at 82 mg/L, and the Brooklands works in Simon's Town at 179 mg/L. Blending and seasons move these figures, so treat them as a guide, not a reading for your street.
Johannesburg and Gauteng
Rand Water's 12-month SANS 241 report for Johannesburg (August 2024 to August 2025) recorded an average total hardness of about 73 mg/L, with 324 samples ranging from 11 to 106 mg/L. That is moderately hard: think kettle scale rather than a geyser emergency. For other towns, look for your municipality's published results; our guide to tap water quality in South Africa explains where to find them.
Boreholes, wellpoints and rainwater
Groundwater is where genuinely hard water usually shows up. The WHO notes that calcium levels of 100 mg/L and more, before magnesium is even counted, are common in natural sources, particularly groundwater. Hardness depends on the rock the water passes through, so a neighbour's result is no substitute for your own test. Rainwater is the opposite: very low in minerals and slightly acidic, so a rainwater filtration system deals with sediment and microbes, never hardness.
How to test your water hardness
- Test strips and drop kits: cheap, quick and good enough to tell soft water from very hard. Check the units, as many kits report in °dH or gpg.
- A TDS meter: useful, but it does not measure hardness. TDS covers all dissolved salts, including sodium, chloride and sulphate, so salty water with little calcium can still read high.
- A laboratory test: the reliable option, and the right one for a borehole. Ask the lab to include total hardness, calcium and magnesium alongside the SANS 241 parameters. SANS 241:2015 sets no hardness limit, so it is an extra you request rather than a pass or fail line.
Once you have a number, the free TDS and hardness checker converts between mg/L, °dH, gpg and mmol/L and shows which band your reading falls in.
Is hard water bad for you?
No. The USGS calls hardness a nuisance rather than a health concern, and the WHO has set no health-based guideline value for it. Neither SANS 241 nor the UK regulations set a hardness limit. Hard water can even add a little calcium and magnesium to your diet, although food supplies most of both.
Skin, hair and eczema
Hard water makes soap harder to rinse off, and the WHO notes it has been suggested as a factor that could aggravate eczema. The Softened Water Eczema Trial tested this with 336 children aged 6 months to 16 years in hard water areas. After 12 weeks, eczema improved by much the same amount whether or not the home had an ion-exchange softener, as judged by nurses who did not know which homes had one. A softener is not an eczema treatment; anyone with a skin condition should ask a doctor.
Your options: softener, scale inhibitor, reverse osmosis or nothing
Ion-exchange water softener
A softener is the only whole-house option that actually removes hardness. Resin beads swap calcium and magnesium for sodium, and the resin is regenerated automatically with salt brine, which goes to drain. North Dakota State University's extension service puts the sodium added at about 8 mg/L per grain per gallon removed, so water at 10 gpg (about 171 mg/L) gains roughly 80 mg/L of sodium.
That is why the UK drinking water regulator advises leaving the kitchen tap used for drinking and cooking unsoftened, noting that extra sodium can be a problem for premature babies and people on a low-salt diet. Garden taps are best left unsoftened too. The regulator also points out that softening only the hot water feed, which here means the geyser, is a more economical way to stop scale than softening the whole supply. Softeners need a drain point, salt top-ups and periodic servicing, the brine can overload a small septic system, and they belong after any iron removal stage because iron fouls the resin. When a test shows the water needs one, a correctly sized water softener is the standard fix.
Scale inhibitors and conditioners
Salt-free devices leave the hardness in the water and try to stop it forming scale. They need no salt or drain, but you do not get the lather and spot-free glass of softened water.
- Polyphosphate dosing: a cartridge of slowly dissolving, food-grade polyphosphate that holds calcium in solution so it does not deposit on heating surfaces. One manufacturer rates its domestic cartridge at 12 months.
- Template-assisted crystallisation (TAC): media that encourages hardness to form microscopic crystals in the water rather than scale on surfaces. In an Arizona State University study for the WateReuse Research Foundation, TAC cut scale formation by more than 88 percent.
- Magnetic and electronic descalers: the weakest evidence. In the same study, electromagnetic and electrically induced precipitation units cut scale by roughly 50 percent, and the UK regulator notes that magnetic devices were designed for continuously flowing industrial systems, not homes where water stands still overnight.
Reverse osmosis for drinking water
An under-sink reverse osmosis system removes most of the hardness, along with other dissolved salts, at one tap. It is a drinking-water solution, not a scale solution for the house, and manufacturers set a maximum feed hardness because scale shortens membrane life, so a hard borehole may need softening first.
What does not remove hardness
Sediment and carbon cartridges do nothing for hardness, and neither does UV. Jug filters with ion-exchange resin reduce the temporary hardness that scales a kettle, but not the permanent hardness.
When is a water softener worth it?
- Soft municipal water (most of Cape Town): a softener is unlikely to earn its place. Descale the kettle now and then; if taste bothers you, a drinking-water filter is the better buy.
- Moderately hard water (about 60 to 120 mg/L, like Johannesburg's average): a judgement call. If kettle scale is the only complaint, a resin jug or simple scale control may be enough; if the geyser is scaling, consider softening its feed.
- Hard or very hard water (above 120 mg/L, common on boreholes): this is where softening, or at least scale control, makes most sense, because scale shortens the life of geyser elements and appliances. If the water also feeds a UV steriliser, keep hardness below about 120 mg/L or the quartz sleeve scales up and the UV dose drops.
On a borehole, hardness is one result among many. The same test decides whether you need iron removal, pH correction or disinfection, and in what order. A borehole water filtration system designed from those results handles all of it together. If you plan to drink treated borehole or wellpoint water, check with the City first if you are inside the City of Cape Town; elsewhere, check your municipality's by-laws.
Frequently asked questions
Is Cape Town water hard or soft?
Mostly soft: the large treatment works supplied 26 to 50 mg/L in the City's 2016/17 analysis, with Atlantis and Simon's Town the harder exceptions. The City notes that groundwater is usually harder, so private boreholes and wellpoints often are.
Can you drink softened water?
For most people, yes, but softening adds sodium, roughly 8 mg/L for every grain per gallon of hardness removed. The usual advice is to keep one kitchen tap unsoftened for drinking and cooking, especially with premature babies or anyone on a low-salt diet in the house.
Does boiling water remove hardness?
Only the temporary part. Boiling drives carbonate hardness out of solution as scale, which is exactly the crust in a kettle. Permanent hardness stays in the water.
Can a TDS meter measure water hardness?
Not directly. A TDS meter estimates all the dissolved salts from the water's conductivity, and hardness is only part of that total. Use a hardness kit or a laboratory test for a true figure.
Not sure whether your water is hard enough to bother with? Send us your water source, property type and area, plus any hardness or lab results you already have, and we will tell you whether softening, scale control or a wider home water filtration system makes sense. When you are ready, request a quote.
