Myth vs fact · Water
The claim: “A TDS meter tells you whether your water is safe to drink”
Does a TDS meter tell you whether water is safe?
The ruling
False, and it is the most consequential false belief in Indian water treatment. A TDS meter passes a current through water and infers dissolved mass from conductivity. At their BIS limits, arsenic contributes 0.01 ppm to that reading and lead contributes 0.01 ppm — both far below the resolution of any handheld meter. Bacteria and viruses are not dissolved solids and contribute nothing at all. The meter's reading is identical whether or not your water carries any of them.
How this was checked
Checked against a published standard
Compare each contaminant's BIS limit against the resolution of a handheld TDS meter. The comparison settles the question without any testing being necessary.
- 1A handheld TDS meter reads in whole ppm and infers mass from conductivity using a fixed conversion factor. Its practical resolution is around 1 ppm at best.
- 2BIS sets arsenic at 0.01 mg/L and lead at 0.01 mg/L. Both are one hundredth of one ppm — two orders of magnitude below what the instrument can resolve. A borewell at ten times the arsenic limit is still contributing 0.1 ppm, which is inside the rounding error.
- 3BIS requires E. coli and thermotolerant coliforms to be undetectable in any 100 mL sample. Bacteria are suspended organisms, not dissolved ions, and contribute nothing measurable to conductivity at any concentration.
- 4Fluoride at its 1.0 mg/L acceptable limit contributes roughly 1 ppm — technically detectable, entirely indistinguishable from 1 ppm of anything else.
- 5Nitrate at 45 mg/L does move the reading meaningfully. It also cannot be told apart from an equal mass of calcium bicarbonate, which is harmless.
What is true in the claim
Every durable myth has a grain of truth, and pretending otherwise teaches nobody anything.
What it means for your purchase
- Use a TDS meter to decide whether you need RO, and a laboratory panel to decide whether your water is safe. They are different questions requiring different instruments.
- A salesman's TDS demonstration has shown you conductivity. Ask which specific substance the reading indicates — there is no answer, because the meter cannot produce one.
- A baseline panel — TDS, hardness, pH, chloride, nitrate, fluoride, iron and coliforms — runs ₹1,200 to ₹3,000 against a purchase of ₹10,000 to ₹25,000.
What this page does not establish
- This page establishes what a TDS meter cannot detect. It does not establish that your water contains any of these contaminants — only a test does that.
- It is not an argument against owning a TDS meter. A ₹300 meter tracking your own water over time is genuinely useful.
- Laboratory TDS by gravimetric method is a real and meaningful measurement. The argument here concerns handheld conductivity meters and what people infer from them.
Frequently asked questions
What TDS level is unsafe to drink?
Why do salesmen use TDS meters then?
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.
IS 10500:2012 Drinking Water — Specification (Second Revision)
Bureau of Indian Standards · published 2012-05-01 · checked 2026-08-01
Guidelines for Drinking-water Quality, 4th edition incorporating the 1st addendum
World Health Organization · published 2017-04-01 · checked 2026-08-01
These pages check claims against published standards, the underlying physics, or our own testing where it exists — and each page says which. We do not emit ClaimReview structured data, because that markup is reserved for accredited fact-checking organisations and we are a commercial publisher. Last reviewed 2026-08-08.