A TDS tester passes a small current between two probes and converts the water's conductivity into an estimate of total dissolved solids. It tells you roughly how much is dissolved in the water and nothing whatsoever about what. That distinction is the whole meaning of the number, and it is why a low reading is not a clean bill of health.
What the meter is really measuring
Dissolved ions carry current, so the more of them there are the higher the conductivity, and the pen applies a conversion factor to report a dissolved solids figure. Calcium, magnesium, sodium, chloride, sulfate and nitrate all contribute. Two waters with identical readings can have completely different compositions, one harmless and one not, because the meter cannot tell a mineral from a contaminant.
What it cannot see at all
Anything that does not conduct is invisible to it: bacteria, viruses, most pesticides, many organic compounds, and trace contaminants present at tiny concentrations. The EPA's 2024 rule set maximum contaminant levels of 4.0 parts per trillion for PFOA and PFOS and 10 parts per trillion for PFHxS, PFNA and GenX. Nothing at those concentrations moves a conductivity reading, so a TDS pen is silent on the contaminants people most often worry about.
The one job it does well
Use it as a performance check on a reverse osmosis system. Measure the feed water and the product water at the same time, and the ratio between them tells you whether the membrane is still rejecting as it should. Log it when the system is new so you have a baseline, then repeat every few months. A steadily rising product reading is the earliest warning that the membrane is failing, well before taste changes.
Where it gets used dishonestly
The classic doorstep demonstration dips a pen in tap water, reports a high number, and implies danger. All it has shown is that the water contains dissolved minerals, which is normal and is what makes water taste like water. Note too that softening does not lower TDS: it exchanges calcium and magnesium for sodium, so the reading stays similar while the water genuinely is soft. Test for what you care about instead.
Questions people ask about tds tester meaning
What is a good TDS reading for drinking water?
There is no safety threshold in that number, because the meter cannot identify what is dissolved. Low simply means little is dissolved.
Does a TDS meter detect lead or bacteria?
No. Bacteria do not conduct, and trace metals are far too dilute to move the reading. Those require a laboratory test to detect.
Will a water softener lower my TDS?
No. It swaps calcium and magnesium for sodium, so the dissolved load stays roughly the same even though the water is genuinely softened.
How do I use a TDS pen on an RO system?
Measure the feed and the product water together, compare the two, and track the ratio over time against the baseline you took when new.