TDS for drinking water: what the number means, and what to do about it.

What TDS is, and what a meter actually measures

TDS stands for total dissolved solids. It is the weight of everything dissolved in a litre of water — calcium, magnesium, sodium, potassium, chlorides, sulphates, bicarbonates, a little silica and a trace of everything else — expressed in milligrams per litre. For water, a milligram per litre is the same thing as a part per million, so mg/L and ppm are interchangeable. This site uses ppm because that is what the meter shows.

A TDS meter does not weigh anything. It passes a tiny current between two electrodes, measures how easily the water carries it, and converts that conductivity into an estimate of dissolved solids using a factor set at the factory. Dissolved salts conduct; pure water barely does. The estimate lands within a few percent of what a laboratory would report by evaporating the sample and weighing the residue, and that is plenty for choosing a purifier. The guide to measuring TDS covers the meter itself: where to test, how to calibrate, and why two taps in the same house read differently.

What TDS does not measure

Because the meter reads conductivity, it only sees things that carry a charge, and it sees all of them the same way. It cannot tell a harmless milligram of calcium from a harmful milligram of nitrate; both raise the reading by one. It cannot see bacteria at all, because bacteria do not conduct. It cannot see the heavy metals that matter, because the few micrograms per litre that count for health are far too small to move the display. It says nothing about chlorine, pesticides, colour or smell.

So a TDS reading answers one question well — how much mineral is in this water — and no other. A low reading is not proof that water is safe, and a high reading is not proof that it is dangerous. The guide on what TDS does not tell you lists the four things that need a laboratory and how often to test for them. Read it once, especially if you draw from a borewell.

The IS 10500:2012 limits

The Indian drinking water standard, IS 10500:2012, gives two figures for most parameters. The first is the acceptable limit. The second is the permissible limit in the absence of an alternate source — the level the standard tolerates only when nothing better can be had. For TDS the figures are 500 mg/L and 2000 mg/L. There is no lower limit. The standard sets a ceiling on dissolved solids, not a floor, which matters for the very-low-TDS question answered further down.

Note the last row. Nitrate has no relaxation because the health risk is direct, particularly for infants. Fluoride above 1.5 is a long-term bone and dental risk. Both are common in Indian groundwater, and neither shows up on a TDS meter as anything more than a number. Hardness gets its own row because it is not the same thing as TDS, even though the two rise together; the TDS vs hardness guide explains why a softener does not lower one and RO does not target only the other.

The chart: what each TDS range needs

When people ask for the TDS range for drinking water they usually mean one of two different ranges. The first is the standard's: up to 500 is acceptable, up to 2000 is tolerated when there is no alternative. The second is the range a purifier should aim for, which is narrower and mostly about taste. The safe TDS level guide gives one answer to both, and to the best, ideal and minimum phrasings of the same question. The World Health Organization, for what it is worth, sets no health-based guideline for TDS at all; it treats the figure as a matter of taste and acceptability, which is a useful reminder of what the number is for.

If you already own an RO purifier and your feed is under 300, you have not made a mistake. Where the purifier has a TDS controller, set it so the output sits comfortably above flat-tasting and leave it there. The membrane will simply last a very long time.

Why the same number means different things

Three hundred ppm from a treated municipal supply, 300 from a borewell in January, and 300 from an RO purifier with its controller wide open are three different waters that happen to share a reading. The municipal water has been settled, filtered and chlorinated; its 300 is mostly harmless mineral and the reading will hold all year. The borewell's 300 is a winter number that may be 500 by May, and the water has never been disinfected. The RO output's 300 is mostly bypass water the controller let past the membrane, which is fine on a clean supply and a bad idea where the feed carries fluoride or nitrate.

That is why this site keeps asking where your water comes from before it says what a number means. Borewell owners in particular should read the borewell TDS guide: it explains why groundwater carries more mineral than surface water, why the reading climbs as the water table drops, and which two tests to get done at a lab before trusting any purifier with the result.

TDS after the purifier

An RO membrane rejects most of the dissolved solids that reach it. The molded membrane in the LUXGEN range is rated at 96–98% salt rejection, which means a 1000 ppm feed leaves the membrane at roughly 20 to 40 ppm. Whatever you then read at the tap is that figure plus whatever a TDS controller or mineral stage adds back. On a purifier without a controller, output is a small fraction of feed; if it is not, something has changed. The RO water TDS guide says what a healthy output looks like, how to check it, and what a slow upward creep means for the membrane.

Very low output is the opposite worry, and a common one. Water that has had nearly everything taken out tastes flat, and some people are uneasy drinking it. The low TDS guide separates the taste question, which is real, from the health question, which is less settled than the internet suggests, and explains what a TDS controller and a mineral cartridge each do about it.

UV or RO: the reading decides

The single most useful thing a TDS reading does is settle whether you need reverse osmosis. Under 300, with a clean lab report, you do not; a UV purifier disinfects and leaves the mineral alone. Above 500 you do, because nothing but a membrane removes dissolved solids. Between the two, most households in India buy RO for the summer headroom, and that is a reasonable call. The UV vs RO guide sets out when each is enough, what each cannot do, and where the running costs actually differ.

Which LUXGEN parts fit each band

For high-TDS supply — anything past 500 — the purifier to look at is the LUXGEN Royale Copper (LUX-ROY-8). It runs RO + UV + UF with a copper cartridge and an adjustable TDS controller, so you set the output taste instead of drinking whatever the membrane produces. 8 L storage, 15 L/h flow, wall-mounted. Compare it with the Aura and the Curve on the purifier comparison.

Above 2000 ppm the membrane matters more than the machine. Fit the 100 GPD Molded Membrane (LUX-RO-100M). It is rated for feed up to 3000 ppm with 96–98% salt rejection, and its fully molded body has no glued seams to fail under the higher pressure such water needs. ₹900. Pair it with a pump that can drive it — the LUX-PUMP-100 at 140 PSI — and read the 80 vs 100 GPD guide before ordering.

Between 500 and 2000, the standard 80 GPD (LUX-RO-80, ₹650) or 100 GPD (LUX-RO-100, ₹800) 1812 membrane is the right buy. Between 150 and 300, do not spend on RO at all. Any UV purifier will do, and the money is better spent on a sediment cartridge changed on time. The full range lists every part with its specification, and the filter change schedule tells you when each one is due.

The TDS guides: nine questions, one page each

Everything above is the short version. Each of the questions below has its own page, written to be read on its own and to send you back here when the wider picture matters.

• What is a safe TDS level for drinking water? — best, ideal, safe and minimum are one question. One answer, with the BIS figures and why "best" depends on your source.
• What TDS should RO water have? — what a healthy output looks like, how to check it, and what a creeping reading says about the membrane.
• How do I measure TDS properly? — using the meter, where to test, calibration, and why readings differ between taps and seasons.
• Is low TDS water bad for you? — RO stripping minerals, flat taste, and what a TDS controller and a mineral cartridge each do.
• What causes high TDS water? — what is actually dissolved in it, what it does to health and appliances, and when RO stops being optional.
• Why is borewell water TDS so high in India? — groundwater, the seasonal rise, what the south typically sees, and what to test for.
• TDS vs hardness: what is the difference? — two measurements that travel together but are not the same; why a softener leaves TDS alone.
• UV or RO: which does my reading call for? — when UV is enough, when RO is required, and where the running costs differ.
• What does TDS not tell you? — bacteria, nitrate, fluoride and heavy metals: the honest limits of the number.