If you have ever looked at a water test report or a handheld TDS meter, you have seen the letters “TDS” followed by a number. Many people assume a lower number always means better water, or that a high number means contamination. The reality is more nuanced, and this guide explains what TDS does and does not tell you.
What is TDS?
TDS stands for total dissolved solids. It is the combined amount of everything dissolved in water that can pass through a fine filter, measured in milligrams per litre (mg/L) or, equivalently, parts per million (ppm). Most of it is dissolved minerals and salts such as calcium, magnesium, sodium, potassium, bicarbonates, chlorides and sulphates.
How is TDS measured?
A laboratory can measure TDS precisely. A handheld TDS meter is simpler: it measures how well the water conducts electricity and converts that into an estimated TDS figure. Pure water conducts almost no electricity, while water with more dissolved salts conducts more. This is useful as a quick check but it only reports a total, not what the dissolved substances actually are.
What TDS levels are considered acceptable?
The World Health Organization does not set a health-based limit for TDS. It publishes taste-based guidance instead, where lower values are generally more palatable:
| TDS (mg/L) | General taste rating (WHO guidance) |
|---|---|
| Below 300 | Excellent |
| 300 to 600 | Good |
| 600 to 900 | Fair |
| 900 to 1,200 | Poor |
| Above 1,200 | Unacceptable |
These are taste ratings, not safety limits. Local standards may also set their own acceptable ranges.
Does low TDS mean the water is safe?
Not by itself. TDS says nothing about bacteria, viruses, pesticides, or many harmful chemicals such as arsenic, which can be present at tiny concentrations that barely move the TDS number. Safe water needs microbiological and chemical testing, not just a TDS reading.
Does high TDS mean the water is unsafe?
Also not necessarily. Water with a high mineral content, such as hard water, can have a high TDS and still be safe to drink, though it may taste different and leave scale in kettles and dispensers. A very high TDS can, however, signal that further testing is worthwhile, especially if the source is a borewell.
Why do purified waters have low TDS?
Treatment methods like reverse osmosis remove most dissolved solids, so the resulting water has a very low TDS. Some producers then add a small, controlled amount of minerals for taste, which raises the number again. A TDS reading of a purified water therefore reflects the treatment and any added minerals, not whether the water is better or worse.
What to look for when you buy water
- A supplier who can tell you how the water is treated.
- Regular testing by a named laboratory, covering microbiological and chemical parameters, not just TDS.
- Clean, sealed bottles and a production date or batch code.
Our checklist for choosing a water delivery service covers these in more detail.
Frequently asked questions
Is zero TDS water good to drink?
Water with extremely low TDS is safe from a mineral-content standpoint but can taste flat. Many producers add a small amount of minerals for taste.
Can I rely on a cheap TDS meter?
It is useful for a quick comparison between two samples, but it cannot tell you whether water is microbiologically or chemically safe.
Why does my kettle build up white scale?
Scale comes from dissolved calcium and magnesium, which are more common in higher-TDS or harder water.
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