TDS levels explained: what's safe, what's too low

Most people's first encounter with TDS is a small number on an RO purifier's display, or a two-pronged meter dipped into a glass at a water shop. The reading climbs or drops and gets treated as a verdict — good water or bad water — even though TDS answers a narrower question than that.

The Bureau of Indian Standards (BIS) and the World Health Organization (WHO) both publish figures for it, and both are more specific than the popular "under 500 is safe" shorthand suggests. There's also a genuine, unresolved question at the other end of the scale: whether water stripped almost entirely of dissolved solids is doing you any favours, or just tastes flat while quietly costing you minerals you'd otherwise get from your water.

What TDS actually measures

Total dissolved solids is the combined weight of everything dissolved in water that isn't water itself — calcium, magnesium, sodium and potassium salts, bicarbonates, chlorides, sulfates, nitrates, and trace organic matter, folded into one number and reported in milligrams per litre (mg/L), which for practical purposes equals parts per million (ppm).

Labs measure it two ways: evaporate a filtered sample and weigh the residue, or measure electrical conductivity and estimate TDS using a conversion factor, since dissolved ions carry current. Nearly every handheld meter — including the ones built into RO purifiers — uses the second method. It's an estimate, not a direct weight, and it shifts with temperature and with which ions happen to dominate the sample.

The number also doesn't say what's dissolved. A reading of 400 mg/L could be mostly calcium and bicarbonate — unremarkable, arguably useful — or it could include nitrate or fluoride at levels worth worrying about. TDS is a bulk quantity, not a composition.

The BIS and WHO limits

BIS sets an acceptable limit of 500 mg/L for TDS under IS 10500:2012, India's drinking-water quality standard, and a separate permissible limit of 2000 mg/L "in the absence of an alternate source." That second figure isn't a target: the standard frames it as tolerable only when nothing better exists, and its own note says values above the acceptable limit render water unsuitable, tolerable only up to the permissible ceiling before a source has to be rejected outright.

WHO has never set a health-based guideline value for TDS, because there isn't enough evidence tying TDS itself — as opposed to specific dissolved substances — to disease outcomes. What it has published instead is a palatability scale built from taste-panel studies: excellent below 300 mg/L, good between 300 and 600, fair between 600 and 900, poor between 900 and 1200, and unacceptable above 1200. That's a taste rating, not a safety threshold.

The two don't measure the same thing. BIS's 500 mg/L is a regulatory ceiling meant to flag water for treatment or rejection; WHO's bands describe how water is likely to taste to a panel of testers. Neither substitutes for testing individual contaminants — IS 10500 itself sets close to 50 other separate limits alongside TDS for exactly that reason.

Why lower isn't automatically better

Water-treatment engineers commonly target RO output in the 50–150 mg/L range — low enough to taste clean, high enough to avoid the flat, insipid taste of near-zero-TDS water. That complaint isn't folklore: WHO's own background document on TDS notes that extremely low concentrations can be unacceptable to tasters for the same reason very high ones are, just at the opposite end.

The lower-limit debate gets more contentious once TDS approaches zero, which aggressive RO or full demineralisation can do. A WHO-commissioned review of demineralised water raised several concerns, without WHO going so far as to set a formal minimum:

• Very low-mineral water tends to leach minerals out of food during cooking rather than add any, and increases urinary loss of sodium, potassium, calcium and magnesium.
• For people whose diets are already light on calcium and magnesium, water is a small but real dietary contributor — one that near-zero-TDS water doesn't provide.
• Demineralised water is more chemically aggressive, drawing metals out of pipes and fittings more readily than mineralised water.

The counterweight, which WHO states plainly, is that no country has adopted a formal minimum TDS or hardness standard, and the long-term controlled human data needed to isolate low TDS from diet as a cause of harm doesn't exist. Treat "very low TDS for years is bad for you" as a reasoned position, not settled science — one that depends heavily on what the rest of your diet already supplies.

What TDS doesn't tell you

It says nothing about microbial safety. Bacteria, viruses and protozoa like cryptosporidium contribute effectively zero dissolved solids. A meter dipped into a clear, chilled, biologically contaminated sample returns a perfectly reassuring number. TDS and pathogen load are unrelated measurements, full stop.

It says nothing about which contaminant is present. Fluoride, nitrate, arsenic and iron all add to the TDS total — but so do harmless calcium and bicarbonate. IS 10500 sets fluoride at 1.0 mg/L, nitrate at 45 mg/L and arsenic at 0.01 mg/L as separate, individually tested limits precisely because the aggregate figure can't stand in for any one of them.

This split matters more in India than in most places, because groundwater TDS varies enormously by geology. Arid and semi-arid belts — parts of Rajasthan, Gujarat and Haryana among them — sit over saline aquifers where evaporation and mineral-rich rock push natural TDS high, independent of any contamination. A hill-fed or shallow surface source elsewhere can show a reassuringly low TDS reading and still carry coliform bacteria if it isn't disinfected. High TDS there is usually a geology story; low TDS here says nothing about disinfection. The two problems call for different tests, not the same meter.

So what should you actually aim for

For municipal or borewell water, use BIS's 500 mg/L acceptable limit as the working baseline, and treat readings consistently above it as a reason to filter or look for another source — not the 2000 mg/L permissible figure, which is a last-resort allowance, not something to aim for.

For RO or purified water, the same 50–150 mg/L range engineers design for works as a household target too; anything up to 300 mg/L is still perfectly fine on taste and mineral grounds. A reading near zero is worth correcting — by adjusting the purifier's TDS controller or blending in a little untreated water — rather than reading as the machine doing an especially thorough job.

Past that, use TDS as a first-pass indicator, not a full answer. If a number is unusually high or low compared to what a given source normally reads, that's the moment to test for what TDS can't see: hardness, fluoride, nitrate, and, wherever there's any doubt at all, bacteriological contamination.