What's in my water
TDS in drinking water — what the number actually tells you
Total Dissolved Solids is the combined mass of everything dissolved in water — minerals, salts and metals — expressed in milligrams per litre (mg/L), which handheld meters display as ppm.
Limits and standards
BIS publishes two figures for many parameters: an acceptable limit, and a permissible limit that applies only where no alternate source of water exists. The second is a fallback for communities without a choice — not a target to design a purchase around.
| Standard | Limit | What it means |
|---|---|---|
| BIS IS 10500:2012 — acceptable limit | 500 mg/L | The figure to design for where you have a choice of source. |
| BIS IS 10500:2012 — permissible in absence of alternate source | 2,000 mg/L | A fallback for communities with no other supply, not a target. |
| WHO | No health-based guideline value | WHO does not set a health limit for TDS. Palatability is generally rated good below 600 mg/L and poor above 1,000 mg/L. |
Where it comes from
Geology
Groundwater dissolves whatever rock it moves through. Deccan basalt, alluvial plains and coastal aquifers each produce a characteristic TDS band.
Seawater intrusion
Over-extraction near the coast draws saline water inland, pushing TDS into the thousands within a few kilometres of the shore.
Sewage and agricultural return flow
Raises TDS along with nitrate and chloride — which is why a sudden TDS rise is worth investigating rather than just filtering.
Treatment itself
Coagulants and pH correction chemicals add dissolved solids. Municipal water is often slightly higher in TDS leaving the plant than entering it.
Why it matters
- High TDS water tastes salty, bitter or flat, and that is the reason most households buy a purifier — a palatability problem, not a safety one.
- TDS is a proxy, and a weak one. Two samples at 450 ppm can be a perfectly good municipal supply and a nitrate-contaminated borewell.
- It is genuinely useful for one decision: whether reverse osmosis is warranted at all. Below roughly 200 ppm, RO removes minerals you have no reason to remove and wastes water doing it.
Note
Can you detect it yourself?
This is the section most purifier buying decisions skip. A handheld TDS meter is the instrument almost every Indian household is shown, and for most contaminants that matter it reads exactly the same whether the contaminant is present or not.
| Question | Answer |
|---|---|
| Can you sense it? | Above roughly 1,000 mg/L most people taste it — salty if chloride-dominated, bitter if sulphate-dominated. Below 500 mg/L, taste tells you nothing reliable. |
| Does a TDS meter show it? | This is the one contaminant a TDS meter is actually for — with the caveat that it infers the figure from conductivity rather than measuring dissolved mass. Expect ±10% against a laboratory gravimetric result, and more on unusual water chemistry. |
| Field / home test | A ₹300–800 handheld meter is adequate for tracking your own water over time. Calibrate against a known standard solution, and take readings at the same tap and temperature — conductivity rises about 2% per °C. |
| Laboratory test to ask for | TDS by gravimetric method — APHA 2540 C — evaporation and weighing (indicative ₹150–₹400) |
When to pay for a laboratory test
- Your TDS reading changed by more than 150 ppm without a change of source — something upstream changed and the meter cannot tell you what.
- TDS is normal but the water tastes or smells wrong. That combination points at something the meter cannot see.
- You are on borewell or tanker supply and have never had a full chemical panel done. TDS alone is not a water quality assessment.
What removes it
Effectiveness ratings below describe the technology working correctly, within its service life, on water or air that suits it. “Depends” means there is a condition in the caveat that decides the outcome — read it before buying.
- Reverse osmosis (RO)Effective
85–98% rejection of dissolved salts
The only household technology that meaningfully reduces TDS. Rejection falls as membranes age and as feed pressure drops.
Caveat: Wastes 2–4 litres to make 1 litre on most domestic units. Below 200 ppm feed there is little to remove and the waste is the whole story.
- DistillationEffective
>99%
Complete for dissolved solids, but slow and power-hungry — a laboratory method rather than a household one in India.
- Ion exchange softenerNo effect
Swaps calcium and magnesium for sodium. The hardness goes; the TDS stays roughly the same or rises slightly.
Caveat: Frequently mis-sold as a TDS solution. It is not one.
What does not work
Each of these is commonly assumed — or actively marketed — to help with total dissolved solids (tds). None of them do.
UV
UV inactivates organisms. It does not remove a single dissolved ion, so the TDS reading is identical before and after.
UF (ultrafiltration)
A UF membrane has pores around 0.01–0.1 µm. Dissolved ions are thousands of times smaller and pass straight through.
Activated carbon
Adsorbs organics, chlorine and taste compounds, not dissolved salts. Carbon can lower TDS by a few ppm and no more.
Boiling
Boiling removes water as vapour and leaves the solids behind, so it raises TDS in what remains.
How a TDS meter actually works
A handheld TDS meter passes a small current between two electrodes and measures electrical conductivity in microsiemens per centimetre. It then multiplies that reading by a conversion factor — usually 0.5, sometimes 0.7 — and displays the result as ppm.
That conversion factor is an assumption about what is dissolved in your water. It is reasonably good for typical municipal supply dominated by calcium bicarbonate. It is poor for water dominated by sodium chloride, and it is meaningless for anything present at trace concentration.
This matters because it sets the limit of what the number can ever tell you. The meter is not identifying substances. It is measuring how easily electricity crosses your water and converting that to a mass estimate.
Important
What TDS is blind to
Read that table as a single sentence: every contaminant on this site with a genuine health basis is either invisible to a TDS meter or indistinguishable from something harmless.
| Contaminant | BIS limit | Effect on a TDS reading at the limit |
|---|---|---|
| Arsenic | 0.01 mg/L | Under 0.01 ppm — undetectable |
| Lead | 0.01 mg/L | Under 0.01 ppm — undetectable |
| Fluoride | 1.0 mg/L | About 1 ppm — lost in the noise |
| Nitrate | 45 mg/L | Roughly 45 ppm — real, but indistinguishable from harmless salts |
| E. coli | Not detectable in 100 mL | Zero. Bacteria are not dissolved solids |
| Pesticide residues | Individually 0.0001–0.003 mg/L | Zero at any realistic concentration |
Can TDS be too low?
This is where the industry argues in both directions, usually depending on what it is selling that week. The honest position is narrower than either side's.
WHO's assessment is that drinking-water is a minor contributor to total dietary calcium and magnesium intake for most people, so demineralised water is not a nutritional problem in a normal diet. What is well established is that very low TDS water tastes flat, and that it is more aggressive towards metal plumbing — which is a corrosion issue, not a health one.
The practical conclusion: if your source is already below about 150 ppm, RO is solving a problem you do not have, and a UV or UF unit is the better purchase.
Frequently asked questions
What is the ideal TDS level for drinking water in India?
Is 40 TDS water safe to drink?
Does high TDS mean my water is unsafe?
Why does my TDS meter read differently from the plumber's?
Sources
Every limit and standard quoted on this page comes from one of the documents below. Where a figure is not in one of these, we have not published it.
Bureau of Indian Standards · published 2012-05-01 · checked 2026-08-01
BIS states two figures for many parameters: an 'acceptable limit' and a 'permissible limit in the absence of an alternate source'. The second is a fallback for communities with no other supply, not a target.
Guidelines for Drinking-water Quality, 4th edition incorporating the 1st addendum
World Health Organization · published 2017-04-01 · checked 2026-08-01
This page states what published standards say and what the physics of each technology allows. It does not state what is in your supply — no page can. Where the two questions meet is a test result, and every entry in this library says which test to ask for.