TDS in Drinking Water: What It Measures and What It Doesn't

Dissolve anything in water — mineral salts, a pinch of bicarbonate, organic matter — and it contributes to TDS. In real drinking water, the bulk of TDS is calcium, magnesium, sodium, potassium, bicarbonate, chloride, and sulfate: the ordinary mineral content picked up from rock and soil. Hardness (covered in our hardness guide) is a subset of TDS — just the calcium and magnesium share.

Handheld TDS meters don't even weigh these solids; they measure electrical conductivity and convert it to an estimated mg/L, since dissolved ions conduct electricity. That shortcut matters for what comes later: the meter only sees things that conduct.

This is the crucial part. A TDS reading tells you nothing about:

• Bacteria, viruses, and parasites — microorganisms barely register on conductivity. Water teeming with cholera can read a pristine 50 ppm.
• Lead and arsenic — toxic at microgram-per-liter levels, thousands of times below anything a TDS meter can resolve. Water at 30 ppm TDS can carry a dangerous lead concentration.
• Pesticides, PFAS, and most organic chemicals — many are non-ionic and conduct poorly or not at all.

Conversely, high TDS is usually just minerals. Famous bottled mineral waters run 300 to over 1,000 ppm and are prized for it. TDS answers “how mineral-rich is this water?” — a legitimate question about taste — and nothing more.

Because TDS is about taste, the WHO frames it in palatability terms rather than health limits:

• Under 300 mg/L: excellent palatability
• 300–600 mg/L: good
• 600–900 mg/L: fair
• 900–1,200 mg/L: poor
• Over 1,200 mg/L: unacceptable to most consumers

The WHO sets no health-based guideline for TDS, noting that reliable data on health effects are absent. At the other end, very low TDS (under roughly 50 mg/L) is also just a taste issue — flat and insipid to many palates — though water that low in minerals can be more corrosive to plumbing, which is an indirect concern in some systems.

TDS meters are a staple of door-to-door and social-media filter marketing, and the demonstrations are theater. The classic routine: dip the meter in your tap water (reading, say, 250 ppm), dip it in the salesperson's RO-filtered water (reading 20 ppm), and declare your tap “contaminated.” A flashier version, the “electrolysis precipitator,” runs current through your tap water until dissolved minerals form alarming brown sludge — a reaction driven by the device's own iron electrodes plus ordinary minerals, which happens with any mineral-containing water including premium bottled brands.

What these demos actually prove is that your tap water contains minerals. Which it should. Municipal water at 250 ppm TDS is normal, within WHO's “excellent” palatability band, and says nothing about safety either way. If someone uses a TDS reading to sell you a filter, the number they should be showing you instead is a certified lab test for the contaminants that matter — lead, nitrates, bacteria — none of which a TDS pen can see.

None of this makes the meter useless — it's just a tool for different questions. Legitimate uses include: checking an RO system (a working RO membrane should cut TDS by 90%+, so a creeping reading signals membrane replacement time); monitoring desalinated or remineralized supplies (see our desalination guide); brewing coffee, where target TDS ranges are part of recipe control; and aquariums and hydroponics, where dissolved mineral load genuinely matters.

For judging whether water is safe to drink, though, the tools are different: your utility's water quality report, a certified lab test, or — when traveling — the destination's rating on our world safety map. TDS is a seasoning gauge, not a safety gauge.