TDS in Water: What the Number Means

Pick up any water-filter marketing brochure and you will almost certainly see a TDS meter being waved around as proof of how dramatically a product improves your water. The number drops, and you are supposed to feel reassured. The reality is more nuanced than that — and understanding what TDS actually tells you, and what it decidedly does not, will help you make smarter decisions about your household water.

What TDS actually measures

Total dissolved solids is exactly what it sounds like: the total mass of all substances dissolved in a given volume of water, measured in milligrams per litre (mg/L). Those substances can include calcium, magnesium, sodium, potassium, bicarbonates, sulphates, chlorides, nitrates, heavy metals, and traces of organic compounds — essentially anything small enough to pass through a 2-micron filter.

A TDS meter does not identify which substances are present. It measures electrical conductivity and converts that reading into an estimated dissolved-solids figure. The device has no way of distinguishing between calcium (broadly harmless and nutritionally useful) and lead (a serious health hazard). Two water samples could show identical TDS readings and be completely different in terms of safety and taste.

What the numbers tend to mean in practice

SANS 241 (2024 edition), South Africa’s drinking-water quality standard, sets an aesthetic guideline of 1 000 mg/L for TDS in drinking water — above that level water may taste unpleasant. There is a separate, lower threshold for TDS from specific concerning ions rather than TDS as a blanket measure.

As a rough guide, here is how TDS ranges are commonly discussed:

TDS range (mg/L)	General character
Below 50	Very low mineral content; can taste flat; may lack beneficial minerals
50–300	Typical of good quality municipal or filtered water
300–600	Moderate mineral content; acceptable for most uses
600–1 000	Higher mineralisation; still within SANS 241 aesthetic guideline
Above 1 000	May taste salty or bitter; warrants investigation into specific ions

These ranges are illustrative. Whether a specific reading is acceptable for your household depends on what is actually driving that number — confirm via a full water test rather than treating the table as a rulebook.

Why TDS alone is not a safety measure

This is the point most filter marketing glosses over. A water sample with a TDS of 150 mg/L could still contain unsafe levels of nitrates from agricultural runoff, E. coli bacteria, or trace heavy metals — none of which a conductivity-based TDS meter will flag. Conversely, water from a Johannesburg borehole might show a TDS of 600 mg/L largely because of naturally occurring calcium and magnesium (hardness), which is not a health concern at moderate concentrations.

For a meaningful picture of your water quality, you need a proper laboratory analysis that tests for specific contaminants: microbiological load, heavy metals, nitrates, and other parameters covered under SANS 241. If you are reading a water test report for the first time, our guide to reading a water test report walks you through each section and explains which figures actually matter for health.

Johannesburg’s municipal water is treated to comply with SANS 241, but local pipe condition, pressure fluctuations, and the city’s reported non-revenue water figure of 44.8% (2024/25 audited figures) — which reflects significant system losses — mean there is genuine variability in what reaches your tap. A single TDS reading taken at your tap tells you almost nothing about that variability.

When TDS does matter

There are specific situations where tracking TDS is genuinely useful:

Monitoring filter performance. If you have a water filtration system — particularly a reverse osmosis unit — TDS readings taken before and after the membrane give you a practical indication of whether the membrane is still functioning. A significant rise in post-filter TDS over time usually signals that the membrane needs replacing. You can read more about how RO systems handle dissolved solids in our reverse osmosis explainer.

Borehole water assessment. Johannesburg has many residential boreholes, and borehole water chemistry varies considerably by area. A rising TDS reading from your borehole over successive months can indicate contamination ingress or aquifer changes worth investigating further.

Aquarium, hydroponics and appliance care. For these applications, TDS is a directly relevant parameter, and the target ranges are quite specific to the use case.

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What actually determines whether your water is good

Safe, pleasant drinking water is determined by a combination of factors that TDS cannot capture on its own:

• Microbiological safety — presence or absence of bacteria, viruses and parasites
• Specific chemical parameters — nitrates, fluoride, arsenic, lead, and other regulated substances under SANS 241
• pH — affects taste and pipe corrosion
• Turbidity — cloudiness, which can indicate biological or sediment contamination
• Hardness — calcium and magnesium levels, relevant for appliances and skin feel but not a health risk at typical Johannesburg municipal levels

A TDS meter costs very little and gives you a result in seconds, which is why it is popular. But spending R200 on a meter and checking your tap daily is not a substitute for a laboratory water test, which covers the parameters that SANS 241 actually regulates.

Choosing a filter based on TDS

If your TDS reading is high and a full water test has identified specific dissolved contaminants, then the type of filtration you need depends on what those contaminants are. Reverse osmosis is highly effective at reducing dissolved solids broadly but wastes some water in the process and removes beneficial minerals along with harmful ones. Activated carbon reduces chlorine, some organics and taste compounds but does little for dissolved salts. Ion exchange softens water by replacing calcium and magnesium ions but does not make water safer in the microbiological sense.

No single filter type suits every situation. The correct approach is to test first, identify what is actually in your water, and then match the treatment method to the problem.

Ready to get the right filtration solution for your specific water quality? Request free quotes from qualified installers — no obligation, no hard sell.

The bottom line

TDS is a quick, cheap, and genuinely useful indicator in the right context — particularly for monitoring existing filtration equipment or tracking borehole water changes over time. It is not a water safety test, and a low TDS reading is not evidence that your water is clean. For Johannesburg homeowners, especially those relying on borehole water or older properties with ageing plumbing, a full laboratory analysis against SANS 241 parameters is the only way to know what you are actually drinking. Use TDS as one data point among several, not as a verdict.