What Is TDS in Water? Meter Readings, Good Levels & Common Myths

Point a €10 pen meter at a glass of water and it spits out a number — 320, 87, 12. That's TDS. Understanding what TDS in water actually measures (and crucially, what it can't see) turns that number from marketing theater into a genuinely useful daily diagnostic. It's also the metric modern RO systems display live on their faucets, so let's decode it properly: the scale, the ideal ranges, the myths, and why "lower" isn't automatically "better."

TDS, defined in plain language

Total Dissolved Solids is the combined concentration of everything dissolved in water — minerals (calcium, magnesium, sodium, potassium), salts (chlorides, sulfates, nitrates) and dissolved metals — expressed in ppm (parts per million) or the equivalent mg/L. A reading of 300 ppm means 300 milligrams of dissolved "stuff" per liter.

How the meter works matters: it doesn't count particles. It measures electrical conductivity — dissolved ions conduct electricity, so more ions ≈ higher reading — then converts to an estimated ppm. That shortcut is exactly why TDS has blind spots (next section).

The TDS scale: where does your water sit?

TDS (ppm)	Classification	Typical taste & notes
0–50	Very low / purified	RO, distilled. Crisp but can taste "flat" without minerals
50–150	Low–ideal	Sweet spot for taste — light mineral character
150–300	Moderate	Most EU tap & bottled waters; fine
300–500	Hard / mineral-heavy	Noticeable taste, kettle scale, white spots
500–1,000	High	WHO palatability threshold ~600; many find >500 unpleasant
1,000+	Very high	Unsuitable for regular drinking

European tap water commonly lands between 150 and 450 ppm depending on geology — hard-water regions (much of Spain, southern Germany, parts of France and the UK) sit at the high end, which is why limescale dominates kitchens there (EU tap water deep-dive).

The critical limitation: what TDS cannot tell you

This is the part most articles bury. Because TDS is a conductivity estimate of everything dissolved, it cannot distinguish:

• Healthy calcium from toxic lead. Both raise the number identically. A 200 ppm mineral water and 200 ppm water with heavy-metal contamination read the same.
• It can't see non-ionic contaminants at all: PFAS, pesticides, microplastics, bacteria, viruses and most organic chemicals barely move a TDS meter. Pristine-looking 40 ppm water can still carry them.

The correct mental model: TDS is a quantity gauge, not a quality verdict. Its superpower is different — it's the perfect before/after performance test for an RO membrane, because RO removes ionic solids wholesale. Tap in at 350 ppm, glass out at 15 ppm = your membrane is doing its job. That's why Hydrion builds dual TDS displays into systems like the 600 GPD dual-display RO (€449.99) and the smart tap on the 1000 GPD flagship — live in/out numbers, every pour, no faith required.

What TDS should RO water read?

A healthy membrane rejects 90–99% of dissolved solids. From 300 ppm tap water expect roughly 5–30 ppm at the spout. Two practical notes:

• Rising output TDS over months = the membrane is aging or pre-filters are spent — your cue to change cartridges. This is the everyday value of the display.
• Remineralized RO reads higher on purpose. An alkaline post-filter like the TAM3 (€69.99) adds back calcium and magnesium, lifting output to a tasty 30–80 ppm and pH above 7.5. A "worse" number that's better water — full logic in why remineralize RO water.

Three TDS myths, retired

1. "0 ppm is the goal." Zero just means zero minerals — flat-tasting and unnecessary. 30–100 ppm of re-added minerals is the taste-and-wellness sweet spot.
2. "Low TDS water is automatically safe." No — see the blind spots above. Safety comes from the barrier (0.0001-micron RO + carbon + UV where needed), which the complete RO guide explains.
3. "High TDS means poisoned water." Usually it just means mineral-rich/hard water — a scale and taste problem, not a toxicity verdict. (Famous mineral waters exceed 1,000 ppm.)

Bonus: TDS and hydrogen water bottles

One niche where the number directly matters: electrolysis needs conductive water. Near-zero TDS (pure RO/distilled) makes SPE/PEM hydrogen bottles like the Hydrion Core underperform. Remineralized RO at 30–80 ppm is the ideal input — clean and conductive (usage guide).

FAQ

What's a good TDS level for drinking water?

For taste and everyday use, roughly 30–150 ppm — low enough to avoid scale and off-tastes, high enough to carry pleasant minerality. Regulatory limits in the EU sit far higher.

Are cheap pen TDS meters accurate?

Accurate enough for trends and before/after checks (±2–5%). Calibrated lab analysis is for absolute precision; a pen meter is for monitoring.

Does boiling water reduce TDS?

No — boiling removes nothing dissolved and slightly concentrates TDS as water evaporates. Only membrane filtration (RO) or distillation reduces it.

Why did my RO water TDS creep up?

Spent pre-filters letting chlorine degrade the membrane, an aging membrane, or (on tank systems) "TDS creep" from stagnant storage — one more argument for tankless designs.