TDS measures total dissolved solids in milligrams per litre — the minerals and salts dissolved in water. Under BIS IS 10500:2012, the acceptable limit for drinking water is 500 mg/l, with 2000 mg/l permissible where no alternate source exists. WHO describes palatability by band: under 300 is excellent, 300–600 good, 600–900 fair, 900–1200 poor. Critically, TDS is a taste and mineral-content measure, not a safety measure — it says nothing about bacteria, pesticides or heavy metals.
Key Facts
- TDS is the total mass of dissolved solids in water, expressed in milligrams per litre (mg/l), and is measured by a meter reading electrical conductivity.
- BIS IS 10500:2012 sets an acceptable limit of 500 mg/l for drinking water, with 2000 mg/l permissible in the absence of an alternate source.
- WHO palatability bands: below 300 excellent, 300–600 good, 600–900 fair, 900–1200 poor, above 1200 generally unacceptable in taste.
- TDS does not detect bacteria, viruses, pesticides or most heavy metals — a low reading is not a safety certificate.
- Very low TDS water tastes flat, because the minerals that give water its taste have been removed.
- A TDS meter measures conductivity and infers dissolved solids; it does not identify which solids are present.
- Storing water in a clean vessel does not change its TDS.
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TDS is the number everyone quotes and few people define. It is useful, it is easy to measure, and it is routinely asked to answer a question it cannot answer.
What TDS is
Total Dissolved Solids: the combined mass of everything dissolved in the water — calcium, magnesium, sodium, potassium, bicarbonates, chlorides, sulphates — expressed in milligrams per litre. A handheld TDS meter does not weigh these. It passes a current through the water, measures the conductivity, and converts that to an estimated dissolved-solids figure.
That conversion is the crux of what the number can and cannot tell you.
The chart
| TDS (mg/l) | WHO palatability | In practice |
|---|---|---|
| Below 300 | Excellent | Clean tasting; very low readings taste flat |
| 300–600 | Good | Where most people find water tastes best |
| 600–900 | Fair | Noticeably mineral; above the BIS acceptable limit |
| 900–1200 | Poor | Distinctly salty or bitter to most people |
| Above 1200 | Unacceptable | Approaching the 2000 mg/l BIS permissible ceiling |
Read the two scales correctly. The WHO bands describe palatability — how the water tastes. The BIS figures are regulatory limits: 500 mg/l acceptable, and 2000 mg/l permissible in the absence of an alternate source, which is a concession for places without a better supply rather than a target.
What TDS does not tell you
This matters more than the chart.
- It does not detect bacteria or viruses. Contaminated water can read a perfectly comfortable 200 mg/l. A low TDS number is not a safety certificate and should never be treated as one.
- It does not identify what is dissolved. 400 mg/l of harmless calcium and magnesium and 400 mg/l of something you would rather not drink give the same reading. The meter measures conductivity, not composition.
- It misses most pesticides and organic contaminants, which are often poorly conductive.
- It is unreliable for heavy metals, which matter at concentrations far too small to move a TDS reading.
If you want to know whether water is safe, that is a laboratory test against the full IS 10500 parameter list. TDS is one line on that report, not a substitute for it.
The over-purification problem
RO systems are often sold on how low they can push TDS, which gets the goal backwards. Stripping water to a very low reading removes the calcium and magnesium that give it taste, and the result is noticeably flat — this is why many RO units add a remineralisation stage back in. If your supply is already in the 150–300 range, aggressive RO is solving a problem you do not have.
The useful question is not "how low can I get it" but "what is actually in my water", and that is answered by a test report rather than a meter.
Bottled water in India
Packaged drinking water sold in India is regulated and typically sits comfortably within the BIS acceptable range; packaged natural mineral water is a separate category and may read higher because the minerals are the point. Brands print composition on the label, which is more informative than a single TDS figure — see packaged water brands in India for how to read one.
One thing worth stating because it comes up: the vessel does not change TDS. Storing water in a steel, glass or copper bottle does not alter its dissolved-solids content in any way a meter would show. What a clean bottle does affect is taste and hygiene, which is a different question and a real one.
Frequently Asked Questions
What is a good TDS level for drinking water?
Under BIS IS 10500:2012 the acceptable limit is 500 mg/l, with 2000 mg/l permissible only where no alternate source exists. On the WHO palatability scale, most people find 300–600 mg/l tastes best, and below 300 is rated excellent though very low readings taste flat.
Does low TDS mean water is safe?
No, and this is the most important thing on the page. TDS measures dissolved solids by conductivity; it does not detect bacteria, viruses, most pesticides, or heavy metals at the concentrations that matter. Contaminated water can produce a perfectly comfortable reading.
What is the BIS limit for TDS in India?
IS 10500:2012 sets 500 mg/l as the acceptable limit and 2000 mg/l as the permissible limit in the absence of an alternate source. The second figure is a concession for places without a better supply, not a target.
Why does RO water taste flat?
Because it has had the calcium and magnesium removed along with everything else, and those minerals are what give water its taste. Many RO systems add a remineralisation stage specifically to correct this.
Does storing water in a steel or copper bottle change its TDS?
No. The vessel does not alter the dissolved-solids content in any way a TDS meter would register. A clean bottle affects taste and hygiene, which is a separate and more practical concern.
Related guides
Testing whether insulation actually works
Fill with ice water, leave four hours in a warm room, then touch the outside. A working vacuum bottle stays at room temperature and never sweats. Cold or condensation on the outside means the vacuum has failed.
This matters because vacuum failure is silent: the bottle still holds water and looks perfect, but retention drops from about 24 hours to four or six. Worth running inside the return window on anything new.
Care that decides whether it lasts five years or one
- Rinse daily with hot water; do not leave water standing for days.
- Wash weekly with dish soap and a bottle brush — plus a straw brush if it has a straw, which is the step people skip and then wonder about the smell.
- Deep clean monthly — two tablespoons of baking soda in warm water, 30 minutes, then a white vinegar rinse for odour.
- Replace gaskets and straws every 6-12 months. A ₹100 seal saves a ₹1,900 bottle.
- Never freeze an insulated steel bottle — water expands about 9% and can crack the seal weld, permanently destroying the insulation.
- Add a silicone base boot. Most vacuum failures start with a vertical drop onto the base.
The four checks worth making before you buy
- Steel grade stated explicitly — 304, 18/8 or SUS 304. "SS" or "food-grade steel" without a number usually means grade 201, which pits in water contact.
- Double-wall vacuum construction if you want any temperature retention. Single-wall insulates nothing and sweats in humidity.
- BIS IS 17526:2021 certification with a CM/L number — the only retention claim in India that has actually been tested.
- Replaceable silicone gasket. Gaskets perish within 6-12 months of daily use; a sealed lid assembly means the bottle dies when the seal does.
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