TDS in Water: What Is a Good Level, and What the Meter Can't Tell You
Someone hands you a little pen, you dip it in a glass, and a number flashes: 340. Is that good? Bad? The salesperson standing next to the demo tank certainly has an opinion, usually one that ends with you buying whatever they're selling. The truth about that number is both simpler and more limited than either the pen or the pitch suggests.
What TDS actually is
Total dissolved solids is the combined weight of everything dissolved in your water: minerals like calcium and magnesium, salts, bicarbonates, chlorides, sulfates, and traces of metals. A meter doesn't weigh these directly — it measures electrical conductivity, because dissolved ions carry current, then converts that into an estimated ppm. It's a fast, cheap proxy, and understanding that it's a proxy is the whole game.
The reference scale people actually want
Here's the taste-and-aesthetic scale most guides are gesturing at, drawn from long-standing WHO palatability ranges and the EPA's secondary standard.
| TDS (ppm) | General rating | What it often reflects |
|---|---|---|
| 0–50 | Very low | RO or distilled; can taste flat |
| 50–300 | Excellent (taste) | Typical good tap or spring water |
| 300–500 | Good | Common, within EPA aesthetic guideline |
| 500–900 | Fair | Above EPA's 500 ppm secondary standard; taste declines |
| 900–1,200 | Poor | Noticeably mineral or salty |
| Over 1,200 | Unacceptable (taste) | Often unpalatable |
Read that "rating" column narrowly. It describes palatability and the EPA's aesthetic guideline — cloudiness, taste, staining — not health. The 500 ppm figure is a "secondary" standard for exactly that reason: it's about whether water is pleasant, not whether it's dangerous.
The two myths this number feeds
Myth one: high TDS means unsafe water. A well full of dissolved calcium and bicarbonate can read 600 ppm and be perfectly fine to drink — hard, mineral-tasting, but not hazardous. Plenty of celebrated mineral waters read higher than your tap. High TDS is a flag to investigate what's dissolved, not a verdict.
Myth two: zero TDS is the healthiest water. This is the demo-tank favorite, where a meter reads 000 on the RO side and everyone nods. But a meter can't see the things that actually threaten safety. Some volatile organic compounds, certain bacteria, and pesticides contribute little or nothing to the conductivity reading, so a glass could show a low TDS number and still carry something a lab would flag. Low TDS means low dissolved mineral content — nothing more.
What the meter can and can't do
| A TDS meter can tell you | It cannot tell you |
|---|---|
| Roughly how much is dissolved | What specifically is dissolved |
| Whether RO is reducing minerals | Whether lead or arsenic is present |
| When to change certain filters | If bacteria or VOCs are in the water |
| Relative changes over time | Whether the water is "safe" |
Where the pen genuinely shines is tracking change. A rising number on the clean side of an RO system is a real signal the membrane is wearing out. That's a legitimate, useful job — see how it applies in the remineralization guide, where the same meter tells you whether your calcite stage is adding minerals back.
What we won't pretend to do
We don't run a lab, so we can't tell you what's inside your specific water from a TDS reading — no one can, because the reading doesn't contain that information. For the "what," you need a certified water test that identifies individual contaminants. A meter is a thermometer for dissolved solids: useful, cheap, and completely silent on the questions that decide safety.
Common mistakes
- Treating a TDS number as a safety score. It measures quantity dissolved, not danger. Safety questions need a contaminant-specific lab test.
- Chasing zero. Very low TDS tastes flat and proves nothing about the contaminants a meter can't see.
- Panicking at a hardness reading. High TDS from calcium and magnesium is a taste-and-scale issue, addressed by softening, not an emergency.
- Trusting a demo instead of your own water. The tank at the store isn't your well. Test your actual source before buying anything based on a pen.
FAQ
What is a good TDS level for drinking water?
By taste, under 300 ppm is generally excellent and the EPA's aesthetic guideline sits at 500 ppm. These are palatability benchmarks, not safety thresholds, so higher isn't automatically unsafe.
Does a low TDS number mean my water is safe?
No. TDS measures dissolved mineral content only. Bacteria, some pesticides, and certain volatile compounds barely register, so low TDS says nothing about those hazards.
Is high TDS water bad for you?
Not necessarily. High readings often come from harmless calcium and bicarbonate. It's a cue to find out what's dissolved through a proper test, not proof of harm.
What is a TDS meter actually good for?
Tracking change. It's excellent at showing when an RO membrane is failing or whether a filter is working, since a rising clean-side number is a clear signal.
Related:
General information, not medical advice. Test your water first. Prices and specifications vary by model and region.