A TDS tester measures how well water conducts electricity and converts that into an estimate of total dissolved solids. It is a useful trend indicator and a poor safety test. The charts printed on the packaging imply that a low reading means safe water and a high reading means unsafe water, and neither of those statements survives contact with how the meter works.
What the meter is actually doing
The probe passes a small current between two electrodes and measures conductivity, then multiplies by a fixed conversion factor to display a solids figure. Everything charged in the water contributes: calcium and magnesium, sodium, chloride, sulfate, nitrate. The meter cannot distinguish harmless dissolved minerals from a contaminant, and it registers nothing at all for substances that carry no charge, which is a large and important category.
Why the bundled chart misleads
Sellers pair meters with a colour-coded scale suggesting that a lower number is always better. Naturally mineralised spring water reads high and is perfectly good to drink. Water contaminated with an uncharged organic compound can read low and still be a problem. Reverse osmosis and distilled water read very low simply because the minerals have been removed. The number describes mineral content, not quality, and treating the chart as a verdict leads people to buy equipment they do not need.
What the reading is genuinely good for
Trends. Take a baseline from your tap and from the treated outlet on the same day, and the ratio between them tells you how a reverse osmosis membrane is performing. Watch that ratio drift over months and you have an early warning that the membrane is failing, long before taste changes. It is also a quick way to confirm a softener bypass is plumbed the way you think it is, since softened and unsoftened lines read differently.
The test that answers the safety question
For city supplies the answer is already written down: every community water system publishes an annual Consumer Confidence Report, and the EPA maintains a finder to locate yours. It names the regulated contaminants detected in your supply and the levels found, which is exactly what a conductivity reading cannot tell you. Well owners get the equivalent from a certified laboratory panel, and our well water testing guide covers which parameters to order.
Questions people ask about tds tester chart
What is a good TDS reading for drinking water?
There is no meaningful safety cutoff. A high reading usually means dissolved minerals, and mineral-rich water is not unsafe by virtue of the number.
Can a TDS meter detect lead or PFAS?
No. It cannot identify individual substances, and trace contaminants have almost no effect on the conductivity the meter measures.
Should my reverse osmosis water read near zero?
It should read far lower than the feed water. What matters is the gap between them and whether that gap stays consistent over time.