A TDS meter estimates total dissolved solids by measuring how well your water conducts electricity, since dissolved minerals and salts carry current. It is a genuinely useful tool for exactly one household job, checking that a reverse osmosis membrane still works, and a misleading one when treated as a general water-quality score.
What the number actually is
The meter passes a tiny current between electrodes and converts conductivity into an estimate of everything dissolved in the water: calcium, magnesium, sodium, bicarbonates, chlorides and whatever else is ionized. It cannot tell any of them apart. A high reading might be harmless minerals from limestone; a low reading might still carry things the meter cannot see. Hardness minerals, the dissolved calcium and magnesium the US Geological Survey defines and bands, are typically a large share of a household TDS reading, which is why hard-water regions see high numbers that signal nuisance rather than danger.
The job a TDS meter does brilliantly
Reverse osmosis verification. An RO membrane rejects most dissolved solids, so comparing a reading at the RO faucet against one from the untreated tap tells you in seconds whether the membrane is still performing or quietly exhausted. That before-and-after ratio is the single most practical measurement a homeowner with an RO system can take, it costs nothing per test, and it turns membrane replacement from guesswork into evidence. This is the use case that justifies owning the meter, and our TDS meter roundup compares models on exactly this duty.
What a TDS reading cannot tell you
Almost everything that makes water unsafe. Lead, arsenic and PFAS matter at concentrations far too small to move a conductivity reading. Bacteria and viruses are not dissolved solids and register nothing. Chlorine taste, pesticides and most organic chemicals are invisible to it too. So a low TDS number is not a safety certificate, and a high one is not an alarm: mineral-rich spring water reads high, while contaminated water can read low. Judging water safety by TDS is reading the wrong gauge with great precision.
Using one without fooling yourself
Rinse the probe, let readings stabilize, and test the same points consistently: raw tap, post-filter, post-RO. Track the trend rather than the absolute number, because temperature and meter calibration shift readings more than people expect. Use it to watch equipment: a creeping post-RO number means the membrane is aging, and a sudden jump usually means a blown seal or exhausted prefilters. For actual safety questions, a lab test answers what the meter cannot; our well water testing guide covers when that is the right spend.
Questions people ask about tds meter
What is a good TDS reading for drinking water?
There is no single safe number: TDS is mostly a taste and mineral measure, not a safety one. The useful comparisons are your RO output against your raw tap, and your own trend over time.
Can a TDS meter detect lead or bacteria?
No. Lead matters at levels far below what conductivity can register, and microorganisms are not dissolved solids at all. Only lab testing answers those questions.
Why did my TDS reading go up after my water softener?
Softening exchanges calcium and magnesium for sodium rather than removing solids, so conductivity stays similar or rises slightly. That is normal and not a malfunction.